Chapter 20   The Multiplication of Effects

§156. To the cause of increasing complexity set forth in the last chapter, we have in this chapter to add another. Though secondary in order of time, it is scarcely secondary in order of importance. Even in the absence of the cause already assigned, it would necessitate a change from the homogeneous to the heterogeneous; and joined with it, it makes this change both more rapid and more involved. To come in sight of it we have but to pursue a step further that conflict between force and matter already delineated. Let us do this.

As already shown, when the components of a uniform aggregate are subject to a uniform force, they being differently conditioned, are differently modified. But while we have contemplated the various parts of the aggregate as undergoing unlike changes, we have not yet contemplated the unlike changes simultaneously produced on the various parts of the incident force. These must be as numerous as the others. In differentiating the parts on which it falls in unlike ways, the incident force must itself be correspondingly differentiated. Instead of being as before, a uniform force, it must thereafter be a multiform force -- a group of dissimilar forces. A few illustrations will make this truth manifest.

In the case, lately cited, of a body shattered by violent collision, besides the change of the homogeneous mass into a heterogeneous group of scattered fragments, there is a change of the homogeneous momentum into a group of momenta, heterogeneous in both amounts and directions. Similarly with the forces we know as light and heat. After the dispersion of these by a radiating body towards all points, they are re-dispersed towards all points by the bodies on which they fall. Of the Sun's rays, issuing from him on every side, some few strike the Moon. Reflected at all angles from the Moon's surface, some few of these strike the Earth. By a like process the few which reach the Earth are again diffused: some into space, some from object to object. And on each occasion, such portions of the rays as are transmitted instead of reflected, undergo refractions or other changes which equally destroy their uniformity. More than this is true. By conflict with matter a uniform force is in part changed into forces differing in their kinds. When one body is struck against another, that which we usually regard as the effect, is a change of position or motion in one or both bodies. But this is a very incomplete view of the matter. Besides the visible mechanical result, sound is produced -- a vibration in one or both bodies and in the surrounding air; and under some circumstances we call this the effect. Moreover, the air has not simply been made to vibrate; it has had currents raised in it by the transit of the bodies. Further, if there is not that great structural change which we call fracture, there is a disarrangement of the particles of the two bodies around their point of collision; amounting in some cases to a visible condensation. Yet more, this condensation is accompanied by genesis of heat. In some cases a spark -- that is, light -- results from the incandescence of a portion struck off. Thus by the original mechanical force expended in the collision, at least five kinds of forces have been produced. Take, again, the lighting of a candle. Primarily, this is a chemical change consequent on a rise of temperature. The process of combination having once been set going by extraneous heat, there is a continued formation of carbon dioxide, water, etc. Along with this process of combination there is a production of heat; there is a production of light; there is an attending column of hot gases generated; there are currents caused in the surrounding air. Nor does the decomposition of one force into many forces end here. Each of the several changes worked becomes the parent of further changes. The carbon dioxide formed will eventually combine with some base; or under the influence of sunshine give up its carbon to the leaf of a plant. The water will modify the hygrometric state of the air around; or, if the current of hot gases containing it comes against a cold body, will be condensed: altering the temperature, and perhaps the chemical state, of the surface it covers. The heat given out melts the subjacent tallow and expands whatever else it warms. The light, falling on various substances, calls forth from them reactions by which it is decomposed, and divers colours are thus produced. Similarly with these secondary actions, which may be traced out into ever-multiplying ramifications, until they become too minute to be appreciated. Universally, then, the effect is more complex than the cause. Whether the aggregate on which it falls be homogeneous or otherwise, an incident force is transformed by the conflict into a number of forces that differ in their amounts, or directions, or kinds; or in all these respects. And of this group of variously-modified forces, each ultimately undergoes a like transformation.

Let us now mark how the process of evolution is furthered by this multiplication of effects. An incident force decomposed by the reactions of a body into a group of unlike forces, becomes the cause of a secondary increase of multiformity in the body which decomposes it. By the reactions of the various parts, differently modified as we have seen they must be, the incident force itself must be divided into differently modified parts. Each differentiated division of the aggregate thus becomes a centre from which a differentiated division of the original force is again diffused. And since unlike forces must produce unlike results, each of these differentiated forces must produce, throughout the aggregate, a further series of differentiations. This secondary cause of the change from homogeneity to heterogeneity, obviously becomes more potent in proportion as the heterogeneity increases. When the parts into which any evolving whole has segregated itself, have diverged widely in nature, they will necessarily react very diversely on any incident force -- they will divide an incident force in to so many strongly contrasted groups of forces. And each of them becoming the centre of a quite distinct set of influences, must add to the number of distinct secondary changes wrought throughout the aggregate. Yet another corollary must be added. The number of unlike parts of which an aggregate consists, is an important factor in the process. Every additional specialized division is an additional centre of specialized forces, and must be a further source of complication among the forces at work throughout the mass -- a further source of heterogeneity. The multiplication of effects must proceed in geometrical progression.

§157. The scattered parts of an irregular nebula in course of being drawn together or integrated, cannot display in a definite manner the secondary traits of evolution: these presuppose an aggregate already formed. We can say only that the half-independent components, each attracted by all and all by each, exhibit in their various momenta, different in their amounts and directions, a multiplication of effects produced by a single gravitative force.

But assuming that the integrative process has at length generated a single mass of nebulous matter, then the simultaneous condensation and rotation show us how two effects of the aggregative force, at first but slightly divergent, become at last widely differentiated. An increase of oblateness in this spheroid must take place through the joint action of these two forces, as the bulk diminishes and the rotation grows more rapid; and this we may set down as a third effect. The genesis of heat, accompanying augmentation of density, is a consequence of yet another order -- a consequence by no means simple; since the various parts of the mass, being variously condensed, must be variously heated. Acting throughout a gaseous spheroid, of which the parts are unlike in their temperatures, the forces of aggregation and rotation must work a further series of changes: they must set up circulating currents, both general and local. At a later stage light as well as heat will be generated. Thus without dwelling on the likelihood of chemical combinations and electric disturbances, it is manifest that, supposing matter to have originally existed in a diffused state, the once uniform force which caused its aggregation, must have become gradually divided into different forces; and that each further stage of complication in the resulting aggregate, must have initiated further subdivisions of this force -- a further multiplication of effects, increasing the previous heterogeneity.

This section of the argument may however be adequately sustained without having recourse to any such hypothetical illustrations as the foregoing. The astronomical attributes of the Earth will, even by themselves, suffice for our purpose. Consider first the effects of its rotation. There is the oblateness of its form; there is the alternation of day and night; there are certain constant marine currents; and there are certain constant aerial currents. Consider next the secondary series of consequences due to the divergence of the Earth's plane of rotation from the plane of its orbit. The many variations of the seasons, both simultaneous and successive, which pervade its surface, are thus caused. External attraction of the Moon and Sun acting on the equatorial protuberance of this rotating spheroid with inclined axis, produces the motion called nutation, and that slower and larger one from which follows the precession of the equinoxes, with its several sequences. And then, by this same force, are generated the tides, aqueous and atmospheric.

Perhaps, however, the simplest way of showing the multiplication of effects among phenomena of this order, will be to set down the influences of any member of the Solar System on the rest. A planet directly produces in neighbouring planets certain appreciable perturbations, complicating those otherwise produced in them; and in the remoter planets it directly produces certain less visible perturbations. Here is a first series of effects. But each of the perturbed planets is itself a source of perturbations -- each directly affects all the others. Hence, planet A having drawn planet B out of the position it would have occupied in A's absence, the perturbations which B causes are different from what they would else have been; and similarly with C, D, E, etc. Here then is a secondary series of effects; far more numerous though far smaller in their amounts. As these indirect perturbations must to some extent modify the movements of each planet, there results from them a tertiary series; and so on in ever multiplying and diminishing waves throughout the entire system.

§158. If the Earth was formed by the concentration of diffused matter, it must at first have been incandescent; and whether the nebular hypothesis be accepted or not, this original incandescence of the Earth may now be regarded as inductively established -- or, if not established, at least rendered so probable that it is a generally admitted geological doctrine. Several results of the gradual cooling of the Earth -- as the formation of a crust, the solidification of sublimed elements, the precipitation of water, etc. -- have been already noticed, and I again refer to them merely to point out that they are simultaneous effects of the one cause, diminishing heat. Let us now, however, observe the multiplied changes afterwards arising from the continuance of this one cause. The Earth, falling in temperature, must contract. Hence the solid crust at any time existing is presently too large for the shrinking nucleus, and, being unable to support itself, inevitably follows the nucleus. But a spheroidal envelope cannot sink down into contact with a smaller internal spheroid, without disruption: it will run into wrinkles as the rind of an apple does when the bulk of its interior decreases from evaporation. As the cooling progresses and the envelope thickens, the ridges consequent on these contractions must become greater, rising ultimately into hills and mountains; and the later systems of mountains thus produced must not only be higher, as we find them to be, but must be longer, as we also find them to be. Thus, leaving out of view other modifying forces, we see what immense heterogeneity of surface arises from the one cause, loss of heat -- a heterogeneity which the telescope shows us to be paralleled on the Moon, where aqueous and atmospheric agencies have been absent. But we have yet to notice another kind of heterogeneity of surface, simultaneously caused. While the Earth's crust was thin, the ridges produced by its contractions must not only have been small in height and length, but the tracts between them must have rested with comparative smoothness on the subjacent liquid spheroid; and the water in those arctic and antarctic regions where it first condensed, must have been evenly distributed. But as fast as the crust grew thicker and gained corresponding strength, the lines of fracture from time to time caused in it, occurred at greater distances apart; the intermediate surfaces followed the contracting nucleus with less uniformity; and there consequently resulted larger areas of land and water. If any one, after wrapping an orange in tissue paper and observing both how small are the wrinkles and, how evenly the intervening spaces lie on the surface of the orange, will then wrap it in thick cartridge-paper, and note both the greater height of the ridges and the larger spaces throughout which the paper does not touch the orange, he will see that as the Earth's solid envelope thickened, the areas of elevation and depression became greater. In place of islands more or less homogeneously scattered throughout an all-embracing sea, there must have gradually arisen heterogeneous arrangements of continent and ocean, such as we now know. These simultaneous changes in the ex tent and in the elevation of the lands, involved yet another species of heterogeneity -- that of coast-line. A tolerably even surface raised out of the ocean will have a simple, regular sea-margin; but a surface varied by table-lands and intersected by mountain-chains, will, when raised out of the ocean, have an outline extremely irregular, alike in its leading features and in its details. Thus endless is the accumulation of geological and geographical results brought about by this one cause -- escape of the Earth's primitive heat.

When we pass from the agency which geologists term igneous, to aqueous and atmospheric agencies, we see a like ever-growing complication of effects. The denuding actions of air and water have, from the beginning, been modifying every exposed surface: everywhere working many different changes. As already said (§69) the original source of those gaseous and fluid motions which effect denudation, is the solar heat. The transformation of this into various modes of energy, according to the nature and conditions of the matter on which it falls, is the first stage of complication. The Sun's rays, striking at all angles a sphere that from moment to moment presented and withdrew different parts of its surface, and each of them for a different time daily throughout the year, would produce a considerable variety of changes even were the sphere uniform. But falling as they do on a sphere surrounded by an atmosphere containing wide areas of cloud, but which here unveils vast tracts of sea, there of level land, there of mountains, there of snow and ice, they cause in it countless different movements. Currents of air of all sizes, directions, velocities, and temperatures, are set up; as are also marine currents similarly contrasted in their characters. In this region the surface is giving off vapour; in that, dew is being precipitated; and in another, rain is descending -- unlikenesses which arise from the changing ratio between the absorption and radiation of heat in each place. At one hour a rapid fall in temperature leads to the formation of ice, with an accompanying expansion throughout the moist bodies frozen; while at another a thaw unlocks the dislocated fragments of these bodies. And then, passing to a second stage of complication, we see that the many kinds of motion directly or indirectly caused by the Sun's rays, severally produce results which vary with the conditions. Oxidation, drought, wind, frost, rain, glaciers, rivers, waves, and other denuding agents effect disintegrations that are determined in their amounts and qualities by local circumstances. Acting on a tract of granite, such agents here work scarcely an appreciable effect; there cause exfoliations of the surface and a resulting heap of debris and boulders; and elsewhere, after decomposing the feldspar into a white clay, carry away this with the accompanying quartz and mica, and deposit them in separate beds, fluviatile or marine. When the exposed land consists of several unlike formations, sedimentary and igneous, changes proportionately more heterogeneous are wrought. The formations being disintegrable in different degrees, there follows an increased irregularity of surface. The areas drained by adjacent rivers being differently constituted, these rivers carry down to the sea unlike combinations of ingredients; and so sundry new strata of distinct compositions arise. And here, indeed, we may see very clearly how the heterogeneity of the effects increases in a geometrical progression with the heterogeneity of the object acted upon. Let us, for the fuller elucidation of this truth in relation to the inorganic world, consider what would follow from an extensive cosmical catastrophe -- say a great subsidence throughout Central America. The immediate results would themselves be sufficiently complex. Besides the numberless dislocations of strata, the ejections of igneous matter, the propagation of earthquake vibrations many thousands of miles around, the loud explosions, and the escape of gases, there would be an inrush of the Atlantic and Pacific Oceans, a subsequent recoil of enormous waves, which would traverse both these oceans and produce myriads of changes along their shores, and corresponding atmospheric waves complicated by the currents surrounding each volcanic vent, as well as electrical discharges with which eruptions are accompanied. But these temporary effects would be insignificant compared with the permanent ones. The complex currents of the Atlantic and Pacific would be altered in their directions and amounts. The distribution of heat achieved by these currents would be different from what it is. The arrangement of the isothermal lines, not only on the neighbouring continents but even throughout Europe, would be changed. The tides would flow differently from what they do now. There would be more or less modification of the winds in their periods, strengths, directions, qualities; and rain would fall scarcely anywhere at the same times and in the same quantities as at present. In these many changes, each including countless minor ones, may be seen the immense heterogeneity of the results wrought out by one force, when that force expends itself on a previously complicated area: the implication being that from the beginning the complication has advanced at an increasing rate.

§159. We have next to trace throughout organic evolution, this same all-pervading principle. And here, where the transformation of the homogeneous into the heterogeneous was first observed, the production of many changes by one cause is least easy to demonstrate in a direct way. Heredity complicates everything. Nevertheless, by indirect evidence we may establish our proposition.

By way of preparation observe how numerous are the changes which any marked stimulus works on an adult organism -- a human being for instance. An alarming sound or sight, besides impressions on the organs of sense and the nerves, may produce a start, a scream, a distortion of the face, a trembling consequent on general muscular relaxation, a burst of perspiration, and perhaps an arrest of the heart followed by syncope; and if the system be feeble, an illness with its long train of complicated symptom may set in. Similarly in cases of disease. A minute portion of the small-pox virus taken into the system will, in a severe case, cause, during the first stage, rigors, heat of skin, accelerated pulse, furred tongue, loss of appetite, thirst, epigastric uneasiness, vomiting, headache, pats in the back and limbs, muscular weakness, convulsions, delirium, etc.; in the second stage, cutaneous eruption, itching, tingling, sore throat, swelled fauces, salivation, cough, hoarseness, dyspnoea, etc.; and in the third stage, oedematous inflammations, pneumonia, pleurisy, diarrhoea, inflammation of the brain, ophthalmia, erysipelas, etc.: each of which enumerated symptoms is itself more or less complex. Now it needs only to consider that this working of many changes by one force on an adult organism, must be partially paralleled in an embryo-organism, to understand that in it too there must be a multiplication of effects, ever tending to produce increasing heterogeneity. Each organ as it is developed, serves, by its actions and reactions on the rest, to initiate new complexities. The first pulsations of the foetal heart must simultaneously aid the unfolding of every part. The growth of each tissue, by taking from the blood special proportions of elements, must modify the constitution of the blood; and so must modify the nutrition of all the other tissues. The distributive actions, implying as they do a certain waste, necessitate an addition to the blood of effete matters, which must influence the rest of the system, and perhaps, as some think, initiate the formation of excretory organs. The nervous connexions established among the viscera must further multiply their mutual influences. And so is it with every modification of structure -- every additional part and every alteration in the ratios of parts. Proof of a more direct kind is furnished by the fact, that the same germ may be evolved into different forms according to circumstances. Thus, during its earliest stages, every germ is sexless -- originates either male or female as the balance of forces acting on it determines. Again, there is the familiar truth that the larva of a working-bee will develop into a queen-bee if, before a certain period, it is fed after a manner like that in which the larvae of queen-bees are fed. Then there is the still more striking evidence furnished by ants and termites. Riley, Grassi, Haviland, and Hart, have shown that differences of nutrition not only originate the differences between males and females but also the different traits of solders, workers, and nurses.*<* See Principles of Biology, Vol. I, pp. 680-8.> Varying degree of nutrition, after initiating the unlikeness of sex, then determines the unlikenesses of external organs possessed by the various classes of sexless individuals. Next comes the evidence, still more directly relevant, supplied by the effects of castration. If the removal of certain organs prevents the development of certain other organs in remote parts of the system -- in man the vocal structures, the beard, some traits of general form, some instincts and other mental characters -- then it is clear that where these organs have not been removed, the presence of them determines the occurrence of these various changes of development, and doubtless many minor ones which are unobtrusive. Here the fact that one cause produces many effects in the course of organic evolution is indisputable. Doubtless we are, and must ever continue, unable to conceive those mysterious properties which make the germ when subject to fit influences undergo the special changes initiating, and mainly constituting, the transformations of an unfolding organism; though we may consistently suppose that they represent an infinite series of inherited modifications consequent on the instability of the homogeneous, the multiplication of effects, and one further factor still to be set forth. All here contended is that, given a germ possessing these mysterious properties, the evolution of an organism from it depends, in part, on that multiplication of effects which we have seen to be one cause of evolution in general, so far as we have yet traced it.

When, leaving the development of single plants and animals, we pass to that of the Earth's Flora and Fauna, the course of the argument again becomes clear and simple. Though, as before admitted, the fragmentary facts Palaeontology has accumulated, do not clearly warrant us in saying that, in the lapse of geologic time, there have been evolved more heterogeneous organisms, and more heterogeneous assemblages of organisms; yet we shall now see that there must ever have been a tendency towards these results. We shall find that the production of many effects by one cause, which, as already shown, has been all along increasing the physical heterogeneity of the Earth, has further necessitated an increasing heterogeneity of its inhabiting organisms, individually and collectively. An illustration will make this clear.

Suppose that by upheavals, occurring, as they are known to do, at long intervals, the East Indian Archipelago were raised into a continent, and a chain of mountains formed along the axis of elevation. By the first of these upheavals, the plants and animals of Borneo, Sumatra, New Guinea, and the rest, would be subjected to slightly-modified sets of conditions. The climate of each would be altered in temperature, in humidity, and in its periodical variations, while the local differences would be multiplied. The modifications would effect, perhaps inappreciably, the entire Flora and Fauna of the region. The change of level would entail additional modifications, varying in different species, and also in different members of the same species, according to their distance from the axis of elevation. Plants growing only on the sea-shore in special localities, might become extinct. Others, living only in swamps of a certain humidity, would, if they survived at all, probably undergo visible changes of appearance. While more marked alterations would occur in some of the plants that spread over the lands newly raised out of the water. The animals and insects living on these modified plants, would themselves be in some degree modified by changes of food, as well as by changes of climate; and the modifications would be more marked where, from the dwindling or disappearance of one kind of plant, an allied kind was eaten. In the lapse of the many generations arising before the next upheaval, the sensible or insensible alterations thus produced in each species, would become organized -- in all the races which survived there would be more or less adaptation to the new conditions. The next upheaval would superinduce further organic changes, implying wider divergences from the primary forms; and so repeatedly. Now, however, observe that this revolution would not be a substitution of a thousand modified species for the thousand original species; but in place of the thousand original species there would arise several thousand species, or varieties, or changed forms. Each species being distributed over an area of some extent, and tending continually to colonize the new area exposed, its different members would be subject to different sets of changes. Plants and animals migrating towards the equator would not be affected in the same way with others migrating from it. Those which spread towards the new shores, would undergo changes unlike the changes undergone by those which spread into the mountains. Thus, each original race of organisms would become the root from which diverged several races, differing more or less from it and from one another; and while some of these might subsequently disappear, probably more than one would survive into the next geologic period. Not only would certain modifications be thus caused by changes of physical conditions and food, but also, in some cases, other modifications caused by changes of habit. The fauna of each island, peopling, step by step, the newly-raised tracts, would eventually come in contact with the faunas of other islands; and some members of these other faunas would be unlike any creatures before seen. Herbivores meeting with new beasts of prey would, in some cases, be led into modes of defence or escape differing from those previously used; and simultaneously the beasts of prey would modify their modes of pursuit and attack. We know that when circumstances demand it, such changes of habit do take place in animals; and we know that if the new habits become the dominant ones, they must eventually in some degree alter the organization. Note, now, a further consequence. There must arise not simply a tendency towards the differentiation of each race of organisms into several races; but also a tendency to the occasional production of a somewhat higher organism. Taken in the mass, these divergent varieties, which have been caused by fresh physical conditions and habits of life, will exhibit alterations quite indefinite in kind and degree, and alterations that do not necessarily constitute an advance. Probably in most cases the modified type will be not appreciably more heterogeneous than the original one. But it must now and then occur that some division of a species, falling into circumstances which give it rather more complex experiences, and demand actions somewhat more involved, will have certain of its organs further differentiated in proportionately small degrees -- will become slightly more heterogeneous. Hence, there will from time to time arise an increased heterogeneity both of the Earth's flora and fauna, and of individual races included in them. Omitting detailed explanations, and allowing for qualifications which cannot here be specified, it is sufficiently clear that geological mutations have all along tended to complicate the forms of life, whether regarded separately or collectively That multiplication of effects which has been a part-cause of the transformation of the Earth's crust from the simple into the complex, has simultaneously led to a parallel transformation of the Life upon its surface.(*)<fn* Had this paragraph, first published in the Westminster Review in April, 1857, been written after the appearance of Mr. Darwin's work on The Origin of Species, instead of before, it would doubtless have been otherwise expressed. Reference would have been made to the process of "natural selection," as greatly facilitating the differentiations described. As it is, however, I prefer to let the passage stand in its original shape; partly because it seems to me that these successive changes of conditions would produce divergent varieties or species, apart from the influence of "natural selection" (though in less numerous ways as well as less rapidly); and partly because I conceive that in the absence of these successive changes of conditions, "natural selection" would effect comparatively little. Let me add that though these positions are not enunciated in The Origin of Species, yet a common friend gives me reason to think that Mr. Darwin would coincide in them.

The deduction here drawn from the established truths of geology and the general laws of life, gains immensely in weight on finding it to be in harmony with an induction drawn from direct experience. Just that divergence of many races from one race, above described as continually occurring during geologic time, we know to have occurred during the pre-historic and historic periods, in man and domestic animals. And just that multiplication of effects which we concluded must have been instrumental to the first, we see has in great measure wrought the last. Single causes, as famine, pressure of population, war, have periodically led to further dispersions of men and of dependent creatures: each such dispersion initiating new modifications, new varieties. Whether all the human races be or be not derived from one stock, philology shows that in many cases a group of races, now easily distinguishable from one another, was originally one race -- that the diffusion of one race into different regions and conditions of existence has produced many altered forms of it. Similarly with domestic animals. Though in some cases, as that of dogs, community of origin will perhaps be disputed, yet in other cases, as that of the sheep or the cattle of our own country, it will not be questioned that local differences of climate, food, and treatment, have transformed one original breed into many breeds, now become so far distinct as to produce unstable hybrids. Moreover, through the complication of effects flowing from single causes, we here find, what we before inferred, not only an increase of general heterogeneity, but also of special heterogeneity. While of the divergent divisions and subdivisions of the human race, many have undergone changes not constituting an advance; others have become more heterogeneous. The civilized European departs more widely from the mammalian archetype than does the Australian.

§160. A sense-impression does not expend itself in arousing some single state of consciousness; but the state of consciousness aroused is made up of various represented sensations connected by co-existence or sequence with the presented sensation. And that, in proportion as the grade of intelligence is high, the number of ideas suggested is great, may be readily inferred. Let us, however, look at the proof that here, too, each change is the parent of many changes and that the multiplication increases in proportion as the area affected is complex.

Were some hitherto unknown bird, driven by stress of weather from the remote north, to make its appearance on our shores, it would excite no speculation in the sheep or cattle amid which it alighted: a perception of it as a creature like those constantly flying about, would be the sole interruption of that dull current of consciousness which accompanies grazing and rumination. The cowherd, by whom we may suppose the exhausted bird to be presently caught, would probably gaze at it with some slight curiosity, as being unlike any he had before seen would note its most conspicuous markings, and vaguely ponder on the questions, where it came from, and how it came. By the sight of it, the village bird-stuffer would have suggested to him sundry forms to which it bore a little resemblance; would receive from it more numerous and more specific impressions respecting structure and plumage; would be reminded of other birds brought by storms from foreign parts; would tell who found them, who stuffed them, who bought them. Supposing the unknown bird taken to a naturalist of the old school, interested only in externals, (one of those described by Edward Forbes, as examining animals as though they were skins filled with straw,) it would excite in him a more involved series of mental changes. There would be an elaborate examination of the feathers, a noting of all their technical distinctions, with a reduction of these perceptions to certain equivalent written symbols; reasons for referring the new form to a particular family order, and genus would be sought out and written down; communications with the secretary of some society or editor of some journal, would follow; and probably there would be not a few thoughts about the addition of the ii to the describer's name, to form the name of the species. Lastly, in the comparative anatomist such a new species, should it have any marked internal peculiarity, might produce additional sets of changes -- might suggest modified views respecting the relationships of the division to which it belonged; or, perhaps, alter his conceptions of the homologies and developments of certain organs; and the conclusions drawn might possibly enter as elements into still wider inquiries concerning the origin of organic forms.

From ideas let us turn to emotions. In a young child, a father's anger produces little else than vague fear -- a sense of impending evil, taking various shapes of physical suffering or deprivation of pleasures. In elder children the same harsh words will arouse additional feelings: sometimes a sense of shame, of penitence, or of sorrow for having offended; at other times, a sense of injustice and a consequent anger. In the wife, yet a further range of feelings may come into existence -- perhaps wounded affection, perhaps self-pity for ill-usage, perhaps contempt for groundless irritability, perhaps sympathy for some suffering which the irritability indicates, perhaps anxiety about an unknown misfortune which she thinks has produced it. Nor are we without evidence that among adults, the like differences of development are accompanied by like differences in the number of emotions aroused, in combination or rapid succession: the lower natures being characterized by that impulsiveness which results from the uncontrolled action of a few feelings; and the higher natures being characterized by the simultaneous action of many secondary feelings, modifying those first awakened.

Perhaps it will be objected that the illustrations here given, are drawn from the functional changes of the nervous system, not from its structural changes; and that what is proved among the first does not necessarily hold among the last. This must be admitted. Those, however, who recognize the truth that the structural changes are the slowly accumulated results of the functional changes, will readily draw the corollary that a part-cause of the evolution of the nervous system, as of other evolution, is this multiplication of effects which becomes ever greater as the development becomes higher.

§161. If the advance of Man towards greater heterogeneity, in both body and mind, is in part traceable to the production of many effects by one cause, still more clearly may the advance of Society towards greater heterogeneity be so explained.

Consider the growth of industrial organization. When some individual of a tribe displays unusual aptitude for making weapons, which were before made by each man for himself, there arises a tendency towards the differentiation of that individual into a maker of weapons. His companions, warriors and hunters all of them, severally wishing to have the best weapons that can be made, are certain to offer strong inducements to this skilled individual to make weapons for them. He, on the other hand, having both an unusual faculty, and an unusual liking, for making weapons (capacity and desire being commonly associated), is predisposed to fulfil these commissions on the offer of adequate rewards: especially as his love of distinction is also gratified. This first specialization of function, once commenced, tends ever to become more decided. On the side of the weapon-maker, continued practice gives increased skill. On the side of his clients, cessation of practice entails decreased skill. Thus this social movement tends to become more decided in the direction in which it was first set up; and the incipient heterogeneity is, on the average of cases, likely to become permanent for that generation, if no longer.

Such a differentiation has a tendency to initiate other differentiations. The advance described implies the introduction of barter. The maker of weapons has to be paid in such other articles as he agrees to take. Now he will not habitually exchange for one kind of article. He does not want mats only, or skins, or fishing-gear. He wants all these, and on each occasion will bargain for the particular things he then most needs. What follows? If among the members of the tribe there exist any slight differences of skill in the manufacture of these various things the weapon-maker will take from each one the thing which that one excels in making. But he who has bartered away his mats or his fishing-gear, must make other mats or fishing-gear for himself; and in so doing must, in some degree, further develop his aptitude. If such transactions are repeated, these specializations may become appreciable. And whether or not there ensue distinct differentiations of other individuals into makers of particular articles, it is clear that the one original cause produces not only the first dual effect, but a number of secondary dual effects, like in kind but minor in degree. This process, of which traces may be seen among groups of schoolboys, cannot well produce a lasting distribution of functions in an unsettled tribe; but where there grows up a fixed and multiplying community, it will become permanent, and increase with each generation. An addition to the number of citizens, involving a greater demand for every commodity, intensifies the functional activity of each specialized person or class; and this renders the specialization more definite where it exists, and establishes it where it is nascent. By increasing the pressure on the means of subsistence, a larger population again augments these results; since every individual is forced more and more to confine himself to that which he can do best, and by which he can gain most. And this industrial progress opens the way for further growth of population, which reacts as before. Under the same stimuli new occupations arise. Among competing workers, some discover better processes or better materials. The substitution of bronze for stone entails on him who first makes it a great increase of demand -- so great an increase that presently all his time is occupied in making the bronze for the articles he sells, and he is obliged to depute the fashioning of these articles to others; so that eventually the making of bronze, thus differentiated from a pre-existing occupation, becomes an occupation by itself. But now mark the ramified changes which follow this change. Bronze soon replaces stone not only in the articles it was first used for, but in many others; and so affects the manufacture of them. Further, it affects the processes which such improved utensils subserve, and the resulting products -- modifies buildings, carvings, dress, personal decorations. And all these changes react on the people-increase their manipulative skill, their intelligence, their comfort-refine their habits and tastes.

This increasing social heterogeneity that results from the production of many effects by one cause, cannot of Course be followed out. But leaving the intermediate phases of social development, let us take an illustration from its passing phase. To trace the effects of steampower, in its manifold applications to mining, navigation, and manufactures, would carry us into unmanageable detail. Let us confine ourselves to the latest embodiment of steam-power -- the locomotive engine. This, as the proximate cause of our railway-system, has changed the face of the country, the course of trade, and the habits of the people. Consider, first, the complicated sets of changes that precede the making of every railway -- the provisional arrangements, the meetings, the registration, the trial-section, the parliamentary survey, the lithographed plans, the books of reference, the local deposits and notices, the application to Parliament, the passing Standing-Orders Committee, the first, second, and third readings: each of which brief heads indicates a multiplicity of transactions, and a further development of sundry occupations, (as those of engineers, surveyors, lithographers, parliamentary agents, share-brokers,) and the creation of sundry others (as those of traffic-takers, reference-makers). Consider, next, the yet more marked changes implied in railway construction -- the cuttings, embankings, tunnellings, diversions of roads; the building of bridges, viaducts, and stations; the laying down of ballast, sleepers, and rails; the making of engines, tenders, carriages, and wagons: which processes, acting upon numerous trades, increase the importation of timber, the quarrying of stone, the manufacture of iron, the mining of coal, the burning of bricks; institute a variety of special manufactures weekly advertised in the Railway Times; and call into being some new classes of workers-drivers, stokers, cleaners, plate-layers, signalmen. Then come the changes, more numerous and involved still, which railways in action produce on the community at large. The organization of every business is modified. Ease of communication makes it better to do directly what was before done by proxy; agencies are established where previously they would not have paid; goods are obtained from remote wholesale houses instead of near retail ones; and commodities are used which distance once rendered inaccessible. Rapidity and economy of carriage tend to specialize more than ever the industries of different districts -- to confine each manufacture to the parts in which, from local advantages, it can be best carried on. Cheap distribution equalizes prices, and also, on the average, lowers prices: thus bringing divers articles within the reach of those before unable to buy them. At the same time the practice of travelling is immensely extended. People who before could not afford it, take annual trips to the sea, visit their distant relations, make tours, and so are benefited in body, feelings, and intellect. The prompter transmission of letters and of news produces further changes -- makes the pulse of the nation faster. Yet more, there arises a wide dissemination of cheap literature through railway book-stalls, and of advertisements in railway carriages: both of them aiding ulterior progress. So that beyond imagination are the changes, thus briefly indicated, consequent on the invention of the locomotive engine.

It should be added that we here see more clearly than, ever, how in proportion as the area over which any influence extends becomes heterogeneous, the results are in a yet higher degree multiplied in number and kind. While among the uncivilized men to whom it was first known, caoutchouc caused but few changes, among ourselves the changes have been so many and varied that the history of them occupies a volume. Upon the small, homogeneous community inhabiting one of the Hebrides, the electric telegraph would produce, were it used, scarcely any results; but in England the results it produces are multitudinous.

Space permitting, the synthesis might here be pursued in relation to all the subtler products of social life. It might be shown how, in Science, an advance of one division presently advances other divisions -- how Astronomy has been immensely forwarded by discoveries in Optics, while other optical discoveries have initiated Microscopic Anatomy, and greatly aided the growth of Physiology -- how Chemistry has indirectly increased our knowledge of Electricity, Magnetism, Biology, Geology -- how Electricity has reacted on Chemistry and Magnetism, developed our views of Light and Heat, and disclosed sundry laws of nervous action. But it would needlessly tax the reader's patience to detail, in their many ramifications, these various changes; so involved and subtle as to be followed with difficulty.

§162. After the argument which closed the last chapter, a parallel one here seems scarcely required. For symmetry's sake, however, it will be proper briefly to point out how the multiplication of effects, like the instability of the homogeneous, is a corollary from the persistence of force.

Things which we call different are things which react in different ways; and we can know them as different only by the differences in their reactions. When we distinguish bodies as hard or soft, rough or smooth, we mean that certain like muscular forces expended on them are followed by unlike reactive forces, causing unlike sets of sensations. Objects classed as red, blue, yellow etc., are objects which decompose light in contrasted ways; that is, we know contrasts of colour as contrasts in the changes produced in a uniform incident force. The proposition that the different parts of any whole must react differently on a uniform incident force, and must thus reduce it to a group of multiform forces, is in essence a truism. Suppose we reduce this truism to its lowest terms.

When, from unlikeness between the effects they produce on consciousness, we predicate unlikeness between two objects, what is our warrant? and what do we mean by the unlikeness, objectively considered? Our warrant is the persistence of force. Some kind or amount of change has been wrought in us by the one which has not been wrought by the other. This change we ascribe to some force exercised by the one which the other has not exercised. And we have no alternative but to do this, or to assert that the change had no antecedent, which is to deny the persistence of force. Whence it is further manifest that what we regard as the objective unlikeness is the presence in the one of some force, or set of forces, not present in the other -- something in the kinds or amounts or directions of the constituent forces of the one, which those of the other do not parallel. But now if things or parts of things which we call different, are those of which the constituent forces differ in one or more respects, what must happen to any like forces, or any uniform force, falling on them? Such like forces, or parts of a uniform force, must be differently modified. The force which is present in the one and not in the other, must be an element in the conflict -- must produce its equivalent reaction; and must so affect the total reaction. To say otherwise is to say that this differential force will produce no effect, which is to say that force is not persistent.

I need not develop this corollary further. It manifestly follows that a uniform force falling on a uniform aggregate, must undergo dispersion; that falling on an aggregate made up of unlike parts, it must undergo dispersion from each part, as well as qualitative differentiations; that in proportion as the parts are unlike, these qualitative differentiations must be marked; that in proportion to the number of the parts, they must be numerous; that the secondary forces so produced must undergo further transformations while working equivalent transformations in the parts that change them; and similarly with the forces they generate. Thus the conclusions that a part-cause of Evolution is the multiplication of effects, and that this increases in geometrical progression as the heterogeneity becomes greater, are not only to be established inductively, but are deducible from the deepest of all truths.

Chapter 21   Segregation

§163. The general interpretation of Evolution is far from being completed in the preceding chapters. We must contemplate its changes under yet another aspect, before we can form a definite conception of the process constituted by them. Though the laws already set forth furnish a key to the re-arrangement of parts which Evolution exhibits, in so far as it is an advance from the uniform to the multiform, they furnish no key to this rearrangement in so far as it is an advance from the indefinite to the definite. On studying the actions and reactions everywhere going on, we have found it to follow from a certain primordial truth, that the homogeneous must lapse into the heterogeneous, and that the heterogeneous must become more heterogeneous; but we have not discovered why the differently-affected parts of any simple whole, become clearly marked off from one another, at the same time that they become unlike. Thus far no reason has been given why there should not ordinarily arise a vague chaotic heterogeneity, in place of that orderly heterogeneity displayed in Evolution. It still remains to find out the cause of that local integration which accompanied local differentiation -- that gradually-completed segregation of like units into a group, distinctly separated from neighbouring groups which are severally made up of other kinds of units. The rationale will be conveniently introduced by a few instances in which we may watch this segregative process taking place.

When, late in September, the trees are gaining their autumn colours, and we are hoping soon to see a further change increasing the beauty of the landscape, we are sometimes disappointed by the occurrence of an equinoctial gale. Out of the mixed mass of foliage on each branch , the strong current of air carries away the decaying and brightly-tinted leaves, but fails to detach those which are still green. And while these last, frayed and seared by long-continued beatings against one another, give a sombre colour to the woods, the red and yellow and orange leaves are collected together in ditches and behind walls and in corners where eddies allow them to settle. That is to say , by that uniform force which the wind exerts on both kinds, the dying leaves are picked out from among their still-living companions and gathered in places by themselves. Again, the separation of particles of different sizes, as dust and sand from pebbles, may be similarly effected, as we see on every road in March. And from the days of Homer downwards, the power of currents of air, natural and artificial, to part from one another units of unlike characters, has been habitually utilized in the winnowing of chaff from wheat. In every brook we see how the mixed materials carried down are separately deposited -- how in rapids the bottom gives rest to nothing but boulders and pebbles; how where the current is not so strong, sand is let fall; and how , in still places, there is a sediment of mud. This selective action of moving water is commonly applied in the arts to obtain masses of particles of different degrees of fineness. Emery, for example, after being ground, is carried by a slow current through successive compartments; in the first of which the largest grains subside; in the second of which the grains that settle before the water has escaped, are somewhat smaller; in the third smaller still; until in the last there are deposited those finest particles which have not previously been able to reach the bottom. And in a way that is different though equally significant, this segregative effect of water in motion, is exemplified in the carrying away of soluble from insoluble matters -- an application of it hourly made in every laboratory. The effects of the uniform forces which aerial and aqueous currents exercise, are paralleled by those of uniform forces of other orders. Electric attraction will separate small bodies from large, or light bodies from heavy. By magnetism, grains of iron may be selected from other grains; as by the Sheffield grinder, whose magnetized gauze-mask filters out the steel-dust his wheel gives off , from the stone-dust which accompanies it. And how the affinity of any agent acting differently on the mixed components of a body, enables us to take away some component and leave the rest behind , is perpetually shown in chemical experiments.

What, now, is the general truth here variously presented? How are these facts, and countless similar ones, to be expressed in terms that embrace them all? In each case we see in action a force which may be regarded as simple or uniform-fluid motion in a certain direction at a certain velocity; electric or magnetic attraction of a given amount; chemical affinity of a particular kind; or rather, in strictness, the acting force is compounded of one of these with some other uniform force, as gravitation, etc. In each case we have an aggregate made up of unlike units -- either atoms of different substances combined or intimately mingled, or fragments of the same substance of different sizes, or other constituent parts that are unlike in their specific gravities, shapes, or other attributes. And in each case these unlike units, or groups of units, of which the aggregate consists, are, under the influence of some resultant force acting indiscriminately on them all, separated from one another -- segregated into minor aggregates, each consisting of units that are severally like one another and unlike those of the other minor aggregates. Such being the common aspect of these changes, let us look for the common interpretation of them.

In the chapter on "The Instability of the Homogeneous," it was shown that a uniform force falling on any aggregate, produces unlike modifications in its different parts -- turns the uniform into the multiform and the multiform into the more multiform. The transformation thus wrought, consists of either insensible or sensible changes of relative position among the units, or of both. Such portion of the permanently effective force as reaches each different part, or differently-conditioned part, may be expended in modifying the mutual relations of its constituents; or it may be expended in moving the part to another place; or it may be expended partially in the first and partially in the second. And if little or none is absorbed in re-arranging the components of a compound unit, much or the whole must show itself in motion of such compound unit to some other place in the aggregate. and conversely. What must follow from this, in cases where none or only part of the force generates chemical re-distributions, what physical re-distributions must be generated? Parts that are similar to each other will be similarly acted on by the force, while parts that are dissimilar will be dissimilarly acted on. Hence the permanently effective incident force, when wholly or partially transformed into mechanical motion of the units, will produce like motions in units that are alike, and unlike motions in units that are unlike. If then, in an aggregate containing two or more orders of mixed units, those of the same order will be moved in the same way, and in a way that differs from that in which units of other orders are moved, the respective orders must segregate. A group of like things on which are impressed motions that are alike in amount and direction, must be transferred as a group to another place, and if they are mingled with some group of other things, on which the motions impressed are like one another, but unlike those of the first group in amount or direction or both, these other things must be transferred as a group to some other place -- the mixed units must undergo a simultaneous selection and separation.

Further to elucidate this process, let me set down a few instances in which we may see that the definiteness of the separation is in proportion to the definiteness of the differences among the units. Take a handful of pounded substance, containing fragments of all sizes, and let it fall gradually while a gentle breeze is blowing. The large fragments will be collected on the ground almost immediately under the hand; somewhat smaller fragments will be carried a little to the leeward; still smaller ones further away; and those minute particles we call dust, will be drifted far before they reach the earth: that is, the segregation is indefinite where the differences among the fragments are indefinite, though the divergences are greatest where the differences are greatest. If, again, the handful be made up of distinct orders of units -- as pebbles, coarse sand, and dust -- these will, under like conditions, be segregated with greater definiteness. The pebbles will drop almost vertically; the sand, falling obliquely, will deposit itself within a tolerably circumscribed space beyond the pebbles; while the dust will be blown almost horizontally to a great distance. A case in which another kind of force comes into play, will still better illustrate this truth. Through a mixed aggregate of soluble and insoluble substances, let water slowly percolate. There will in the first place be a distinct parting of the substances that are the most widely unlike: the soluble will be carried away; the insoluble will remain behind. Further, some separation, though a less definite one, will be effected among the soluble substances; since the first part of the current will remove the most soluble in the largest amounts, and after these have been dissolved, it will continue to bring out the remaining less soluble. Even the undissolved matters will have simultaneously undergone some segregation; for the percolating fluid will carry down the minute fragments from among the large ones, and will often deposit those of small specific gravity in one place, and those of great specific gravity in another. To complete the elucidation we must glance at the obverse fact; namely that mixed units which differ but slightly, are moved in but slightly different ways by incident forces, and can therefore be separated only by such adjustments of the incident forces as allow slight differences to become appreciable factors in the result. The parting of alcohol from water by distillation is a good example. Here we have molecules consisting of oxygen and hydrogen, mingled with molecules consisting of oxygen, hydrogen, and carbon. The two orders of molecules have a considerable likeness of nature: they similarly maintain a fluid form at ordinary temperatures; they similarly become gaseous more and more rapidly as the temperature is raised; and they boil at points not very far apart. Now this comparative likeness of the molecules is accompanied by difficulty in segregating them. If the mixed fluid is unduly heated, much water distils over with the alcohol: it is only within a narrow range of temperature that molecules of the one kind are driven off rather than the others; and even then not a few of the others accompany them. The most interesting and instructive example, however, is furnished by certain phenomena of crystallization. When several salts that have little analogy of constitution, are dissolved in the same body of water, they are separated without much trouble, by crystallization: subject as they are to uniform forces, they segregate. The crystals of each salt do, indeed, usually contain certain small amounts of the other salts present in the solution; but from these they are severally freed by repeated re-solutions and crystallizations. Mark now, however, that the reverse is the case when the salts contained in the same body of water are chemically homologous. The nitrates of baryta and lead, or the sulphates of zinc, soda, and magnesia, unite in the same crystals; nor will they crystallize separately if these crystals be dissolved afresh, and afresh crystallized. On seeking the cause of this anomaly, chemists found that such salts were isomorphous-that their molecules, though not chemically identical, are identical in the proportions of acid, base, and water, composing them, and in the crystalline forms they assume when uniting. Here, then, we see clearly that units of unlike kinds are selected out and separated with a readiness proportionate to the degree of their unlikeness.

There is a converse cause of segregation which it is needless here to treat of with equal fullness. If different units acted on by the same force, must be differently moved; so, conversely units of the same kind must be differently moved by different forces. Supposing some group of units forming part of a homogeneous aggregate, are unitedly exposed to a force which is unlike in amount or direction to the force acting on the rest of the aggregate, then this group of units will separate from the rest, provided that, of the force so acting on it, there remains any portion not dissipated in molecular vibrations or absorbed in producing molecular rearrangements. After all that has been said above, this proposition needs no defence.

Before ending our preliminary exposition, a complementary truth must be specified; namely that mixed forces are segregated by the reaction of uniform matters, just as mixed matters are segregated by the action of uniform forces. Of this truth a complete and sufficient illustration is furnished by the dispersion of refracted light. A beam of light, made up of ethereal undulations of different orders, is not uniformly deflected by a homogeneous refracting body; but the different orders of undulations it contains are deflected at different angles: the result being that these different orders of undulations are separated and integrated, and so produce the colours of the spectrum. A segregation of another kind occurs when rays of light traverse an obstructing medium. Those which consist of comparatively short undulations are absorbed before those which consist of comparatively long ones; and the red rays, which consist of the longest undulations, alone penetrate when the obstruction is very great. How, conversely, there is produced a separation of like forces by the reaction of unlike matters, is also made manifest by the phenomena of refraction; since adjacent and parallel beams of light, falling on, and passing through, unlike substances, are made to diverge.

§164. In vague ways the heavenly bodies exemplify that cause of material segregation last assigned -- the action of unlike forces on like units.

I say in vague ways because our Sidereal System displays more of aggregation than of segregation. That the irregular swarms of stars constituting the Milky Way with its branches and gaps and denser regions, have been gathered together from a more widely diffused state, may be reasonably inferred; though as we know nothing of the preceding distribution such a change cannot be proved: still less can there be proved a segregative process.

It is true that in clusters of stars, beginning with those having members considerably dispersed and ending with those having members closely concentrated-globular clusters -- we see strong evidence of aggregation; and it may be contended that since the mutual gravitations of the stars forming a cluster, differ in their degrees and directions from those of the stars from which they have separated, there is a kind of segregation. But it must be admitted that the conformity to the above-named principle is but an indefinite one.

There are, however, two classes of facts which exhibit segregation, though they leave us ignorant of its causes. The first is that star-clusters are abundant along the course of the Milky Way: by far the larger number of them lying in the neighbourhood of its plane and relatively few in regions on either side. The second is that, contrariwise, the nebulae are sparsely scattered in and about the galactic circle and are relatively numerous in the spaces remote from it. Though there are thus presented two cases of segregation there is no evidence that these different classes of bodies have been separated from a mixed assemblage, nor is there any indication of the forces by which this contrast in distribution has been produced. We can only say that the facts are congruous with the belief that segregation, probably indirect rather than direct in its cause, has been going on.

The formation and detachment of a nebulous ring, illustrates the same general principle. To conclude, as Laplace did, that the equatorial portion of a rotating nebulous spheroid will, during concentration, acquire a centrifugal force sufficient to prevent it from following the rest of the contracting mass, is to conclude that such portions will remain behind as are in common subject to a certain differential force. The line of division between the ring and the spheroid, must be a line inside of which the aggregative force is greater than the force resisting aggregation; and outside of which the force resisting aggregation is greater than the aggregative force. Hence the alleged process conforms to the law that among like units, exposed to unlike forces, the similarly conditioned separate from the dissimilarly conditioned.

§165. Those geologic changes usually classed as aqueous, display under numerous forms the segregation of unlike units by a uniform incident force. On seashores the waves are ever sorting-out and separating the mixed materials against which they break. From each mass of fallen cliff, the tide carries away all those particles which are so small as to remain long suspended in the water; and, at some distance from shore, deposits them in the shape of fine sediment. Large particles, sinking with comparative rapidity, are accumulated into beds of sand near low water-mark. The small pebbles collect together at the bottom of the incline up which the breakers rush; and on the top lie the larger stones and boulders. Still more specific segregations may occasionally be observed. Flat pebbles, produced by the breaking down of laminated rock, are sometimes separately collected in one part of a shingle bank. On this shore the deposit is wholly of mud; on that it is wholly of sand. Here we find a sheltered cove filled with small pebbles almost of one size; and there, in a curved bay one end of which is more exposed than the other we see a progressive increase in the massiveness of the stones as we walk from the less exposed to the more exposed end. Trace the history of each geologic deposit, and we are quickly led down to the fact that mixed fragments of matter, differing in their sizes or weights, are, when exposed to the momentum and friction of water, joined with the attraction of the Earth, selected from one another, and united into groups of comparatively like fragments. And we see that, other things equal, the separation is definite in proportion as the differences of the units are marked. After they have been formed, sedimentary strata exhibit segregations of another kind. The flints and the nodules of iron pyrites that are found in chalk, as well as the silicious concretions which sometimes occur in limestone, are interpreted as aggregations of molecules of silex or sulphuret of iron, originally diffused through the deposit, but gradually collected round centres, notwithstanding the solid or semi-solid state of the surrounding matter. Bog iron-ore supplies the conditions and the result in still more obvious correlation.

Among igneous changes we do not find so many examples of the process described. Nevertheless, geological phenomena of this order are not barren of illustrations. Where the mixed matters composing the Earth's crust have been raised to a very high temperature, segregation commonly takes place as the temperature falls. Sundry of the substances that escape in a gaseous form from volcanoes, sublime into crystals on coming against cool surfaces; and solidifying, as these substances do, at different temperatures, they are deposited at different parts of the crevices through which they are emitted together. The best illustration, however, is furnished by the changes that occur during the slow cooling of igneous rock. When, through one of the fractures from time to time made in the Earth's crust, a portion of the molten nucleus is extruded, and when this is cooled with comparative rapidity, there results trap or basalt -- a substance that is uniform in texture, though made up of various ingredients. But when, not escaping through the superficial strata, such a portion of the molten nucleus is slowly cooled, granite is the result: the mingled particles of quartz, feldspar, and mica, being kept for a long time in a fluid and semi-fluid state -- a state of comparative mobility-undergo those changes of position which the forces impressed on them by their fellow units necessitate. The differential forces arising from mutual polarity, segregate the quartz, feldspar, and mica, into crystals. How completely this is dependent on the long-continued agitation of the mixed particles, and consequent long-continued movableness by small differential forces, is proved by the fact that in a granite dyke the crystals in the centre, where the fluidity or semi-fluidity continued for a longer time, are much larger than those at the sides, where contact with the neighbouring rock caused more rapid cooling and solidification.

§166. The actions going on throughout an organism are so involved, that we cannot expect to identify the forces by which particular segregations are effected. Among the few instances admitting. of interpretation, the best are those in which mechanical pressures and tensions are the agencies at work.

The spine of a vertebrate animal is subjected to certain general strains -- the weight of the body, together with the reactions involved by all considerable muscular efforts; and under these conditions it has become segregated as a whole. At the same time being exposed to different forces during those lateral bendings which the movements necessitate, its parts retain a certain separateness. If we trace up the development of the vertebral column from its primitive form of a cartilaginous cord in the lowest fishes, we see that, throughout, it maintains an integration corresponding to the unity of the incident forces, joined with a division into segments corresponding to the variety of the incident forces. Each segment, considered apart, exemplifies the truth more simply. A vertebra is not a single bone, but consists of a central mass with sundry appendages or processes, and in unfinished types of vertebra these appendages are separate from the central mass, and, indeed, exist before it makes its appearance. But these several independent bones constituting a primitive spinal segment, are subjected to a certain aggregate of forces which agree more than they differ: as the fulcrum to a group of muscles habitually acting together, they perpetually undergo certain reactions in common. And accordingly, in the course of development, they gradually coalesce. Still clearer is the illustration furnished by spinal segments that become fused together where they are together exposed to some predominant strain. The sacrum consists of a group of vertebra firmly united. In the ostrich and its congeners there are from seventeen to twenty sacral vertebra; and, besides being confluent with one another , these are confluent with the iliac bones, which run on each side of them. If, now , we assume these vertebra to have been originally separate, as they still are in the embryo bird, and if we consider the forces to which they must in such case have been exposed, we shall see that their union results in the alleged way. For through these vertebra the entire weight of the body is transferred to the legs: the legs support the pelvic arch; the pelvic arch supports the sacrum; and to the sacrum is articulated the rest of the spine, with all the organs attached to it and upheld by it. Hence, if separate, the sacral vertebra must be held firmly together by strongly-contracted muscles, and must, by implication, be prevented from partaking in those lateral movements which the other vertebra undergo -- they must be subjected to a common strain, while they are preserved from strains which would affect them differently; and so they fulfil the conditions under which segregation occurs. But the cases in which cause and effect are brought into the most obvious relation, are supplied by the limbs. The metacarpal bones (those which in man support the palm of the hand) are separate from one another in most mammals: the separate actions of the toes entailing on them slight amounts of separate movements. This is not so however in the ox-tribe and the horse-tribe. In the ox-tribe, only the middle metacarpals (third and fourth) are developed; and these, attaining massive proportions, coalesce to form the cannon bone. In the horse-tribe, the segregation is what we may distinguish as indirect: the second and fourth metacarpals are present only as rudiments united to the sides of the third, while the third is immensely developed; thus forming a cannon bone which differs from that of the ox in being a single cylinder, instead of two cylinders fused together. The metatarsus in these quadrupeds exhibits parallel changes. Now each of these metamorphoses occurs where the different bones grouped together have no longer any different functions, but retain only a common function. The feet of oxen and horses are used solely for locomotion -- are not put, like those of unguiculate mammals, to purposes which involve some relative movements of the metacarpals. Thus there directly or indirectly results a single mass of bone where the incident force is single. And for the inference that these facts have a causal connexion, we find confirmation throughout the entire class of birds, in the wings and legs of which, like segregations are found under like conditions. While this sheet is passing through the press (1862), a fact illustrating this general truth in a yet more remarkable manner, has been mentioned to me by Prof. Huxley who kindly allows me to make use of it while still unpublished by him. The Glyptodon, an extinct mammal found fossilized in South America, has long been known as a large uncouth creature allied to the Armadillo, but having a massive dermal armour consisting of polygonal plates closely fitted together so as to make a vast box, inclosing the body in such way as effectually to prevent it from being bent, laterally or vertically, in the slightest degree. This box, which must have weighed several hundredweight, was supported on the spinous processes of the vertebrae, and on the adjacent bones of the pelvic and thoracic arches. And the significant fact is that here, where the trunk vertebrae were together exposed to the pressure of this heavy dermal armour, at the same time that, by its rigidity , they were preserved from all relative movements, they were united into one solid, continuous bone.

The formation and maintenance of a species, considered as an assemblage of similar organisms, is interpretable in an analogous way. Already we have seen that in so far as the members of a species are subject to different sets of incident forces, they are differentiated, or divided into varieties. Here it remains to add that such of them as are subject to like sets of incident forces, are segregated. For by the process of "natural selection," there is a continual purification of each species from those individuals which depart from the common type in ways that unfit them for the conditions of their existence. Consequently, there is a continual leaving behind of those individuals which are in all respects fit for the conditions of their existence, and are therefore nearly alike. The circumstances to which any species is exposed, being an involved combination of incident forces; and the members of the species having among them some that differ more than is usual from the average structure required for meeting these forces; it results that these forces are constantly separating such divergent individuals from the rest, and so preserving the uniformity of the rest -- keeping up its integrity as a species or variety. Just as the changing autumn leaves are picked out by the wind from among the green ones around them, or just as, to use Prof. Huxley's simile, the smaller fragments pass through a sieve while the larger are kept back; so, the uniform incidence of external forces affects the members of a group of organisms similarly in proportion as they are similar, and differently in proportion as they are different; and thus is ever segregating the like by parting the unlike from them. Whether these separated members are killed off, as mostly happens, or whether, as otherwise happens, they survive and multiply into a distinct variety, in consequence of their fitness to certain partially-unlike conditions, matters not to the argument. The one case conforms to the law that the unlike units of an aggregate are sorted into their kinds and parted, when uniformly subject to the same incident forces, and the other to the converse law that the like units of an aggregate are parted and separately grouped when subject to different incident forces. And on consulting Mr. Darwin's remarks on divergence of character, it will be seen that the segregations thus caused tend ever to become more definite.

§167. Mental evolution under one of its leading aspects, we found to consist in the formation in the mind of groups of like objects and like relations -- a differentiation of the various things originally confounded together in one assemblage, and an integration of each separate order of things into a separate group (§153). Here it remains to point out that while unlikeness in the incident forces is the cause of such differentiations, likeness in the incident forces is the cause of such integrations. For what is the process through which classifications are established? How do plants become grouped in the mind of the botanist into orders, genera, and species? Each plant he examines yields him a certain complex impression. Now and then he picks up a plant like one before seen; and the recognition of it is the production in him of a like connected group of sensations, by a like connected group of attributes. That is to say there is produced throughout the nerve-centres concerned, a combined set of changes, similar to a combined set of changes before produced. Considered analytically, each such combined set of changes is a combined set of molecular modifications wrought in the affected part of the organism. On every repetition of the impression, a like combined set of molecular modifications is superposed on the previous ones, and makes them greater: thus generating an internal plexus of.modifications, with its answering idea, corresponding to these similar external objects. Meanwhile, another kind of plant produces in the brain of the botanist another set of molecular modifications -- a set which does not agree with the one we have been considering, but disagrees with it; and by repetition of such there is generated a different idea answering to a different species. What, now, is the nature of this process expressed in general terms? On the one hand there are the like and unlike things from which severity emanate the groups of forces by which we perceive them. On the other hand, there are the organs of sense and percipient centres, through which, in the course of observation, these groups of forces pass. In passing through them the like groups of forces are segregated, or separated from the unlike groups of forces; and each such separate series of groups of forces, answering to an external genus or species, produces an idea of the genus or species. We before saw that as well as a separation of mixed matters by the same force, there is a separation of mixed forces by the same matter; and here we may further see that the unlike forces so separated, work unlike structural changes in the aggregate that separates them -- structural changes each of which thus represents the integrated series of motions that has produced it.

By a parallel process, the relations of co-existence and sequence among impressions, become sorted into kinds and grouped. When two phenomena that have been experienced in a given order, are repeated in the same order, those nerve-centres which before were affected by the transition are again affected; and such molecular modification as they received from the first motion propagated through them is increased by this second motion. Each such motion works a structural alteration which, in conformity with the law set forth in Chapter IX, involves a diminished resistance to all such motions that afterwards occur. The segregation of these successive motions (or more strictly, the permanently effective portions of expanded them in overcoming resistance) thus becomes the cause of, and the measure of, the mental connexions between the impressions which the phenomena produced. Meanwhile, phenomena different from these, being phenomena that affect different nervous elements, will have their connexions severally represented by motions along other routes; and along each of these other routes, the nervous discharges will severally take place with a readiness proportionate to the frequency with which experience repeats the connexions of phenomena. The classification of relations must hence go on pari passu with the classification of the related things. In common with the mixed sensations received from the external world, the mixed relations it presents cannot be impressed on the organism.without more or less segregation of them resulting. And through this continuous sorting and grouping of changes or motions, which constitutes nervous function, there is gradually wrought that sorting and grouping of matter, which constitutes nervous structure.

§168. In social evolution, the collecting together of the like and the separation of the unlike by incident forces, is primarily displayed in the same manner as we saw it to be among groups of inferior creatures. The human races tend to differentiate and integrate, as do races of other living forms.

Of the forces which effect and maintain the segregations of mankind, may first be named those external ones classed as physical conditions. The climate and food which are favourable to an indigenous people, are more or less detrimental to an alien people of different bodily constitution. In tropical regions the northern races cannot permanently exist: if not killed off in the first generation, they are so in the second, and, as in India, can maintain their footing only by the artificial process of continuous immigration and emigration. That is to say, the external forces acting equally on the inhabitants of a given locality, tend to expel all who are not of a certain type, and thus to keep up the integration of those who are of that type. Even among the Indian peoples themselves the like happens: some of the hill-tribes being segregated by surviving the malarious influences which kill off Hindus who enter their habitat. The other foxes conspiring to produce these national segregations, are those mental ones shown in the affinities of men for others like themselves. Units of one society who are obliged to reside in another, generally form colonies in the midst of that other -- small societies of their own. Races which have been artificially severed, show tendencies to re-unite. Now though these segregations caused by the mutual likings of kindred men, do not seem due to the general principle enunciated, they really are thus interpretable. When treating of the direction of motion (§80), it was shown that the actions performed by men for the satisfaction of their wants, are always motions along lines of least resistance. The feelings characterizing a member of a given race, are feelings which get complete satisfaction only among other members of that race a satisfaction partly derived from sympathy with those having like feelings, but mainly derived from the adapted social conditions which grow up where such feelings prevail. When, therefore, a citizen of any nation is, as we see, attracted towards others of his nation, the rationale is that certain agencies which we call desires, move him in the direction of least resistance. Human motions, like all other motions, being determined by the distribution of forces, it follows that such segregations of races as are not produced by incident external forces, are produced by forces which the units of the races exercise on one another.

During the development of each society we see analogous segregations caused in analogous ways. A few of them result from minor natural affinities; but those most important ones which constitute political and industrial organization, result from the union of men in whom similarities have been produced by training. Men brought up to bodily labour are men who have had wrought in them a certain likeness -- a likeness which, in respect of their powers of action, obscures and subordinates their natural differences. Those trained to brain-work have acquired a certain other community of character which makes them, as social units, more like one another than like those trained to manual occupations. And there arise class-segregations answering to these super-induced likenesses. More definite segregations take place among the more definitely assimilated members of any class who are brought up to the same calling. Even where the necessities of their work forbid concentration in one locality, as among artizans happens with masons and bricklayers, and among traders happens with the retail distributers, and among professionals happens with the medical men, there are not wanting Operative Builders' Unions, and Grocers' Societies, and Medical Associations, implying a process of sifting out and grouping. And where, as among the manufacturing classes, the functions discharged do not require the dispersion of citizens who are artificially assimilated, there is an aggregation of them in special localities, and a consequent increase in the definiteness of industrial divisions. If, now, we seek the causes of these segregations, considered as results of force and motion, we are brought to the same general principle as before. This likeness produced in the members of any class or sub-class by training, is an aptitude acquired by them for satisfying their wants in like ways. That is, the occupation has become to each a line of least resistance. Hence under that pressure which determines all men to activity these similarly -- modified social units are similarly affected, and tend to take similar courses. If, then, there be any locality which, either by its physical peculiarities or by peculiarities wrought on it during social evolution, is rendered a place where a certain kind of industrial action meets with less resistance than elsewhere, it follows from the law of direction of motion that those social units who have been moulded to this kind of industrial action, will be segregated by moving towards this place. If, for instance, the proximity of coal and iron mines to a navigable river, gives to Glasgow an advantage in the building of iron-ships-if the total labour required to produce a given vessel, and get its equivalent in food and clothing, is less there than elsewhere; there is caused a concentration of iron-ship builders at Glasgow, either by detention of the population born to iron-ship building, or by immigration of those elsewhere engaged in it, or by both. The principle equally holds where the occupation is mercantile instead of manufacturing. Stock-brokers cluster where the amount of effort to be severally gone through by them in discharging their functions, and obtaining their profits, is less than elsewhere. A local exchange having once been established, becomes a place where the resistance to be overcome by each is smaller than in any other place; and, being like units under stress of common desires, pursuit of the course of least resistance by each involves their aggregation around this place.

Of course, with units so complex as those which constitute a society , and with forces so involved as those which move them, the resulting selections and separations must be far more entangled, or far less definite, than those we have hitherto considered. For men's likenesses being of various kinds, lead to various orders of segregation. There are likenesses of disposition, likenesses of taste, likenesses produced by education, likenesses that result from class-habits, likenesses of political feeling; and it needs but to glance round at the caste-divisions, the associations for philanthropic, scientific, and artistic purposes, the religious parties and social cliques, to see that some species of likeness among the component members of each body determines their union. Now the different segregative processes, by traversing one another and often by their indirect antagonism, more or less obscure one another's effects, and prevent any one differentiated class from completely integrating. But if this cause of incompleteness be borne in mind, social segregations will be seen to conform to the same principle as all other segregations.

§169. Can the general truth thus variously illustrated be deduced from the persistence of forte, in common with foregoing truths? Probably the exposition at the beginning of the chapter will have led most readers to conclude that it can be so deduced.

The abstract propositions involved are these: -- First, that like units, subject to a uniform force capable of producing motions in them, will be moved to like degrees in the same direction. Second, that like units if exposed to unlike forces capable of producing motion in them, will be differently moved-moved either in different directions or to different degrees in the Same direction. Third, that unlike units if acted on by a uniform force capable of producing motion in them, will be differently moved -- moved either in different directions or to different degrees in the same direction. Fourth, that the incident forces themselves must be affected in analogous ways: like forces falling on like units must be similarly modified by the conflict; unlike forces falling on like units must be dissimilarly modified; and like forces falling on unlike units must be dissimilarly modified. These propositions may be reduced to a still more abstract form. They all imply that in the actions and reactions of force and matter, an unlikeness in either of the factors necessitates an unlikeness in the effects, and that in the absence of unlikeness in either of the factors the effects must be alike.

When they are thus generalized, the dependence of these propositions on the persistence of force is obvious. Any two forces that are not alike, are forces which differ either in their amounts or directions or both; and by what is called the resolution of forces, it may be proved that this difference is constituted by the presence in the one of some force not present in the other. Similarly, any two units or portions of matter which are unlike in size, form, weight, or other attribute, can be known as unlike only through some unlikeness in the forces they impress on us; and hence this unlikeness also, is constituted by the presence in the one of some force or forces not present in the other. Such being the common nature of these unlikenesses, what is the corollary? Any unlikeness in the incident forces, where the things acted on are alike, must generate a difference between the effects; since, otherwise, the differential force produces no effect, and force is not persistent. Any unlikeness in the things acted on, where the incident forces are alike, must generate a difference between the effects; since, otherwise, the differential force whereby these things are made unlike, produces no effect, and force is not persistent. While, conversely, if the forces acting and the things acted on are alike, the effects must be alike; since, otherwise, a differential effect can be produced without a differential cause, and force is not persistent.

Thus these general truths being necessary implications. of the persistence of force, all the re-distributions above traced out as characterizing Evolution in its various phases, are also implications of the persistence of force. If of the mixed units making up any aggregate, those of the same kind have like motions impressed on them by a uniform force, while units of another kind are moved by this uniform force in ways more or less unlike the ways in which those of the first kind are moved, the two kinds must separate and integrate. If the units are alike and the forces unlike, a division of the differently affected units is equally necessitated. Thus there inevitably arises the demarcated grouping which we everywhere see. By virtue of this segregation, growing ever more decided while there remains any possibility of increasing it, the change from uniformity to multiformity is accompanied by a change from indistinctness in the relations of parts to distinctness in the relations of parts. As we before saw that the transformation of the homogeneous into the heterogeneous is inferable from that ultimate truth which transcends proof; so we here see that from this same truth is inferable the transformation of an indefinite homogeneity into a definite heterogeneity.