Chapter 18   The Interpretation of Evolution

§146. Is this law ultimate or derivative? Must we rest satisfied with the conclusion that throughout all classes of concrete phenomena such is the course of transformation? Or is it possible for us to ascertain why such is the course of transformation? May we seek for some all-pervading principle which underlies this all-pervading process? Can the inductions set forth in the preceding four chapters be reduced to deductions?

Manifestly this community of result implies community of cause. It may be that of the cause no account can be given, further than that the Unknowable is manifested to us after this mode. Or, it may be that this mode of manifestation is implied by a simpler mode, from which these many complex effects follow. Analogy suggests the latter inference. Just as it was possible to interpret the empirical generalizations called Kepler's laws, as necessary consequences of the law of gravitation; so it may be possible to interpret the foregoing empirical generalizations as necessary consequences of some deeper law.

Unless we succeed in finding a rationale of this universal metamorphosis, we obviously fall short of that completely unified knowledge constituting Philosophy. As they at present stand, the several conclusions we have lately reached appear to be independent. There is no demonstrated connexion between increasing definiteness and increasing heterogeneity, or between both and increasing integration. Still less proof is there that these laws of the re-distribution of matter and motion, are necessarily correlated with those laws of the direction of motion and the rhythm of motion, previously set forth. But until we see these now separate truths to be implications of one truth, our knowledge remains imperfectly coherent.

§147. The task before us, then, is that of exhibiting the phenomena of Evolution in synthetic order. Setting out from an established ultimate principle, it has to be shown that the course of transformation among all kinds of existences, cannot but be that which we have seen it to be. It has to be shown that the re-distribution of matter and motion, must everywhere take place in those ways, and produce those traits, which celestial bodies, organisms, societies, alike display. And it has to be shown that in this universality of process, is traceable the same necessity which we find in each simplest movement around us, down to the accelerated fall of a stone or the recurrent beat of a harp-string.

In other words, the phenomena of Evolution have to be deduced from the Persistence of Force. As before said -- "to this an ultimate analysis brings us down, and on this a rational synthesis must build up." This being the ultimate truth which transcends experience by underlying it, furnishes a common basis on which the widest generalizations stand; and hence these widest generalizations are to be unified by referring them to this common basis. Already the truths that there is equivalence among transformed forces, that motion follows the line of least resistance or greatest traction and that it is universally rhythmic, we have found to be severally deducible from the persistence of force; and this affiliation of them on the persistence of force has reduced them to a coherent whole. Here we have similarly to affiliate the universal traits of Evolution, by showing that, given the persistence of force, the re-distribution of Matter and Motion necessarily proceeds in such ways as to produce these traits. By doing this we shall unite them as correlative manifestations of one law, at the same time that we unite this law with the foregoing simpler laws.

§148. Before proceeding it will be well to set down some principles that must be borne in mind. In interpreting Evolution we shall have to consider, under their special forms, the various resolutions of force or energy which accompany the re-distributions of matter and motion. Let us glance at such resolutions under their most general forms.

Any incident force is primarily divisible into its effective and non-effective portions. In mechanical impact the entire momentum of a striking body is never communicated to the body struck: even under those most favourable conditions in which the striking body loses all its sensible motion, there still remains with it some of the original momentum under the shape of that insensible motion produced among its particles by the collision. Again, of the light or heat falling on any mass, a part, more or less considerable, is reflected; and only the remaining part works molecular changes in the mass. Next it is to be noted that the effective force is itself divisible into the temporarily effective and the permanently effective. The units of an aggregate acted on may undergo only those rhythmical changes of relative position which constitute increased vibration; or they may also undergo changes of relative position which are not from instant to instant neutralized by opposite ones. Of these the first, disappearing in the shape of radiating undulations, leave the molecular arrangement as it originally was; while the second conduce to one form of that re-arrangement characterizing compound Evolution. Yet a further distinction has to be made. The permanently effective force works out changes of relative position of two kinds -- the insensible and the sensible. The insensible transpositions among the units are those constituting molecular changes, including what we call chemical composition and decomposition; and it is these which largely constitute the qualitative differences that arise in an aggregate. The sensible transpositions are such as result when certain of the units -- molar units as well as molecular units -- instead of being put into different relations with their immediate neighbours, are carried away from them and deposited elsewhere.

Concerning these divisions and subdivisions of any force affecting an aggregate, the fact which it chiefly concerts us to observe is, that they are complementary to one another. Of the whole incident force, the effective must be that which remains after deducting the non-effective. The two parts of the effective force must vary inversely as each other: where much of it is temporarily effective, little of it can be permanently effective; and vice versa. Lastly, the permanently effective force, being expended in working both the insensible re-arrangements which constitute molecular modification, and the sensible re-arrangements which result in structure, must generate of either kind an amount that is great or small in proportion as it has generated a small or great amount of the other.

Chapter 19   The Instability of the Homogenous Exemplifying Instability at Large(*)

<* The idea developed in this chapter originally formed part of an article on "Transcendental Physiology," published in 1857. See Essays, Vol. I.>

§149. The difficulty of dealing with transformations so many-sided as those which all existences have undergone, or are undergoing, is such as to make a definite or complete deductive interpretation seem almost hopeless. So to grasp the total process of re-distribution, as to see simultaneously its several necessary results in their actual interdependence, is scarcely possible. There is, however, a mode of rendering the process as a whole tolerably comprehensible. Though the genesis of the re-arrangement undergone by every evolving aggregate is in itself one, it presents to our intelligence several factors; and after interpreting the effects of each separately, we may, by synthesis of the interpretations, form an adequate conception.

The proposition which comes first in logical order, is, that some re-arrangement must result; and this proposition may be best dealt with under the more specific shape, that the condition of homogeneity is a condition of unstable equilibrium.

First, as to the meanings of the terms, respecting which some readers may need explanation. The state of "unstable equilibrium," so named in mechanics, is well illustrated by a stick standing on its lower end, in contrast with the state of stable equilibrium of a stick suspended by its upper end: the one instantly losing its equilibrium and the other regaining it if disturbed. But the reader must be warned against confusing the instability thus exemplified with the instability here to be treated of. The one shown by a stick on end may be called an external instability, while that which we have now to consider is an internal instability. It is not alleged that a homogeneous aggregate is liable because of its homogeneity to be overthrown or deranged by an external force. The allegation is that its component parts cannot maintain their arrangements unaltered: they must forthwith begin to change their relations to one another. Let us take a few illustrations.

Of mechanical ones the most familiar is that of the scales. If they be accurately made and not clogged by dirt or rust, it is impossible to keep a pair of scales perfectly balanced: eventually one scale will descend and the other ascend -- they will assume a heterogeneous relation. Could a mass of water be brought into a state of perfect homogeneity -- a state of complete quiescence, and exactly equal density throughout -- yet the radiation of heat from neighbouring bodies, by affecting differently its different parts, would inevitably produce inequalities of density and consequent currents; and would so render it to that extent heterogeneous. Take a piece of red-hot matter, and however evenly heated it may at first be, it will quickly cease to be so: the exterior, cooling faster than the interior will become different from it in temperature. And the lapse into heterogeneity of temperature, so obvious. in this extreme case, takes place more or less in the cases of all surrounding objects, which are ever being warmed or cooled. The action of chemical forces supplies other illustrations. Expose a fragment of metal to air or water, and in course of time it will be coated with a film of oxide, carbonate, or other compound: its outer parts will become unlike its inner parts. Often the heterogeneity produced by the actions of chemical forces on the surfaces of masses, is not striking, because the changed portions are soon washed away, or otherwise removed. But if this be prevented comparatively complex structures result. In some quarries of trap-rock there are striking examples. Not unfrequently a piece of trap may be found reduced, by the action of the weather, to a number of loosely-adherent coats, like those of an onion. Where the block has been undisturbed, we may trace the whole series of these, from the angular, irregular outer one, through successively included ones in which the shape becomes gradually rounded, ending at length in a spherical nucleus. On comparing the original mass of stone with this group of concentric coats, each differing from the rest in form, and probably in the state of decomposition it has arrived at, we get a marked illustration of the multiformity to which, in lapse of time, a uniform body may be brought by external chemical action. The instability of the homogeneous is equally seen in the changes set up throughout the interior of a mass, when it consists of units that are not rigidly bound together. The molecules of a slowly-settling precipitate do not remain separate, and equably distributed through the fluid in which they make their appearance. They aggregate either into crystalline grains or into flocculi; and where the mass of fluid is great and the process prolonged, these flocculi do not continue equi-distant, but asSemble into groups. That is to say, there is a destruction of the balance at first subsisting among the diffused particles, and also of the balance at first subsisting among the groups into which these particles unite.

The instability thus variously illustrated is consequent on the fact that the several parts of any homogeneous aggregate are exposed to different forces -- forces which differ either in kind or amount; and are of necessity differently modified. The relations of outside and inside, and of comparative nearness of the parts to neighbouring sources of influence, imply the reception of influences that are unlike in quantity or quality, or both: unlike changes, now temporary now permanent, being caused.

For like reasons the process must repeat itself in each of the component masses of units that are differentiated by the modifying forces. Each of these minor groups, like the major group, must gradually, in obedience to the unlike influences acting on it, lose its balance of parts, and pass from a uniform into a multiform state. And so on continuously. Whence, indeed, it follows that not only must the homogeneous lapse into non-homogeneous, but the more homogeneous must tend ever to become less homogeneous. If any given whole, instead of being absolutely uniform throughout, consist of parts distinguishable from one another -- if each of these parts, while somewhat unlike other parts, is uniform within itself; then, each of them being in unstable equilibrium, it follows that while the changes set up within it must render it multiform, they must at the same time render the whole more multiform than before. The general principle, now to be followed out in its applications, is thus somewhat more comprehensive than the title of the chapter implies.

No demurrer to the conclusions drawn, can be based on the truth that perfect homogeneity nowhere exists; since, whether that state with which we commence be or be not one of perfect homogeneity, the process must equally be towards a relative heterogeneity.

§150. The stars are distributed with a threefold irregularity. There is first the marked contrast between the Milky Way and other parts of the heavens, in respect of the quantities of stars within given visual areas. There are secondary contrasts of like kind in the Milky Way itself, which has its thick and thin places; as well as throughout the celestial spaces in general, which are more closely strewn in some regions than in others. And there is a third order of contrasts produced by the aggregation of stars into small clusters. Besides this heterogeneity in the distribution of stars, considered without distinctions of kind, a further heterogeneity is disclosed when they are classified by their differences of colour, which answer to differences of physical constitution. While yellow stars are found in all parts of the heavens, red and blue stars are not so: there are wide regions in which both red and blue stars are rare; there are regions in which the blue occur in considerable numbers, and there are other regions in which the red are comparatively abundant. Yet one more irregularity of like significance is presented by the nebulae. These are not dispersed with anything like uniformity, but are far more numerous around the poles of the galactic circle than in the neighbourhood of its plane.

No one will expect that anything like a definite interpretation of this structure can be given on the hypothesis of Evolution, or any other hypothesis. Such an interpretation would imply some reasonable assumption respecting the pre-existing distribution of the stellar matter and of the matter forming nebulae, and we have no warrant for any assumption. If we allow imagination to range back through antecedent Possibilities and probabilities, we see it to be unlikely that homogeneous matter filled the space which our Sidereal System now fills, at a time immediately preceding its initiation. Rather the evidence which the heavens present implies that the distribution out of which the present distribution arose, was irregular in all respects. Though certain traits of our galaxy suggest that it has a vague individuality, and that, along with their special motions, its stars have some general motion; yet the evidence forces on us the conclusion that many varieties of changes have been simultaneously going on in its different parts. We find nebulae in all stages of concentration, star-clusters variously condensed, groups of larger stars approximating in different degrees, as well as regions like those which the nubeculae occupy, presenting complex structures and apparently active changes. The most which can be said respecting this total distribution is that, subject as all parts of our Sidereal System are to the law of gravitation, the heterogeneities it exhibits, everywhere implying a progressing concentration, that is, integration, point backward to a less heterogeneous state and point forward to a more heterogeneous state. But, leaving aside this too transcendent question, we may without undue rashness consider from the evolution point of view the changes to be anticipated in one of those collections of matter described as a diffused nebulosity, or one of those more distinct ones of which the outlying parts are compared to wisps of cloud blown about by the wind. The only evolutional process which can at first be displayed is the primary one of integration -- the gathering together through mutual attraction of the parts; for in this early stage in which indefiniteness and incoherence are so fully exemplified, there does not yet exist such an aggregate as is capable of exhibiting secondary re-distributions: we have only the dispersed components of such an aggregate. Contemplating, then, only the process of integration, we may, without asking anything about the previous history of an irregular nebula, safely assume that its parts have their respective proper motions; for the chances are infinity to one against a state of rest relatively to one another. Further, the chances are infinity to one against their proper motions being such that during concentration they will cancel one another: the motion of some part, or the resultant of the motions of several parts, will constitute a proper motion distinct from that which mutual gravitation generates -- a motion which, unless just counterbalanced by an opposite one (again an infinite improbability) will generate rotation. It may, indeed, be argued that, apart from any pre-existing proper motions of its parts, a nebulous mass, if irregular, will acquire rotation while integrating; since each outlying fragment, arriving after the rest have been gathered together, is infinitely unlikely to fall into the mass in such a manner that its motion will be entirely cancelled by resistance; but, falling into it so as to be deflected laterally, will have its motion of approach so changed in direction as to become in part a motion of revolution: a resultant of all such motions, largely conflicting, being an eventual rotation of the mass. It must not, however, be assumed that this will necessarily be the rotation of a solitary aggregate. The great nebula in Andromeda does not appear on the way to form a single body; and is an advanced spiral of which the that in Canes Venatici outer parts have a tangential motion too great to permit of their being drawn into the centre. Rather the apparent implication of the structure is that there will be formed a cluster of masses revolving round a common centre of gravity. Such cases, joined with those of the annular nebula, suggest that often the processes of integration result in compound structures, various in their kinds, while in other cases, and perhaps most frequently, single masses of rotating nebulous matter are formed.

Ignoring all such possibilities and probabilities, however, and limiting our attention to that form of the nebular hypothesis which regards the solar system as having resulted from a rotating spheroid of diffused substance; let us consider what consequence the instability of the homogeneous necessitates. Being oblate in figure, unlike in the densities of its centre and surface, unlike in their temperatures, and probably unlike in the angular velocities of its parts, such a mass cannot be called homogeneous; and any further changes exhibited by it can illustrate the general law, only as being changes from a more homogeneous to a less homogeneous state. Just noting that one of these changes is the increasing oblateness of form, let us go on to observe those which are to be found in the transformations of such of its parts as are at first homogeneous within themselves. If we accept the conclusion that the equatorial portion of this rotating and contracting spheroid will, at successive stages, have a centrifugal force great enough to prevent nearer approach to the centre of rotation, and will so be left behind; we shall find, in the fate of the detached ring, an exemplification of the principle we are following out. Consisting of gaseous matter such a ring, even if uniform at the time of its detachment, could not continue so. In the absence of equality among the forces, internal and external, acting on it, there must be a point or points at which the cohesion of its parts would be less than elsewhere -- a point or points at which rupture would therefore take place. The original assumption was that the ring would rupture at one place only, and would then collapse on itself. But this was a more than questionable assumption: such, at least, I know to have been the opinion of the late Sir John Herschel. So vast a ring, consisting of matter having such feeble cohesion, must break up into many parts. Nevertheless, appeal to another high authority -- the late Sir G. B. Airy -- yielded verification for the belief that the ultimate result which Laplace predicted would take place. And here is furnished a further illustration of the instability of the homogeneous. For even supposing the masses of nebulous matter into which such a ring separated, were so much alike in their sizes and distances as to attract one another with exactly equal forces (which is infinitely improbable); yet the unequal actions of external disturbing forces would inevitably destroy their equilibrium -- there would be one or more points at which adjacent masses would begin to part company. Separation, once commenced, would with accelerating speed lead to a grouping of the masses. A like result would eventually take place with the groups thus formed; until they at length aggregated into a single mass.

§151. Already so many references have been made to the formation of a crust over the originally incandescent Earth, that it may be thought superfluous again to name it. It has not, however, been thus far considered in connexion with the general principle under discussion. Here it must be noted as a necessary consequence of the instability of the homogeneous. In this cooling and soldification of the Earth's surface, we have one of the simplest, as well as one of the most important, instances of that change from a uniform to a multiform state which occurs in any mass through exposure of its component parts to unlike conditions. To the differentiation of the Earth's exterior from its interior, thus brought about, we must add one of the most conspicuous differentiations which the exterior itself afterwards undergoes, as being similarly brought about. Were the forces to which the surface of the Earth is subject, alike in all directions, there would be no reason why certain of its parts should become permanently unlike the rest. But being unequally exposed to the chief external centre of force -- the Sun -- its main divisions become unequally modified. While the crust thickens and cools, there arises that contrast, now so decided, between the polar and equatorial regions.

Along with these most marked physical differentiations of the Earth, there have been going on numerous chemical differentiations, admitting of similar interpretation. Leaving aside all speculations concerning the origin of the so-called simple substances, it will suffice to show how in place of that comparative homogeneity of the Earth's crust, chemically considered, which must have existed when its temperature was high, there has arisen, during its cooling, an increasing chemical heterogeneity. Let us contemplate this change somewhat in detail. At an extreme heat the bodies we call elements cannot combine. Even under such heat as can be generated artificially, some very strong affinities yield; and the great majority of chemical compounds are decomposed at much lower temperatures. Probably, therefore, when the Earth was in its first state of incandescence, there were no chemical combinations. But without drawing this inference, let us set out with the unquestionable fact that the compounds which can exist at the highest temperatures, and which must therefore have been the first formed as the Earth cooled, are those of the simplest constitutions. The protoxides (including under that head the alkalies, earths, etc.) are, as a class,the most stable compounds known -- the least changeable by heat. These, consisting severally of one atom of each component element, are but one degree less homogeneous than the elements themselves. More heterogeneous than these, more decomposable by heat, and therefore later in the Earth's history, are the deutoxides, tritoxides, peroxides, etc.; in which two, three, four, or more atoms of oxygen are united with one atom of metal or other base. Still less able to resist heat are the salts, which present us with compound atoms each made up of five, six, seven, eight, ten, twelve, or more atoms, of three or more kinds. Then there are the hydrated salts of a yet greater heterogeneity, which undergo partial decomposition at much lower temperatures. After them come the further-complicated supersalts and double salts, having a stability again decreased; and so throughout. After making a few unimportant qualifications demanded by peculiar affinities, it may be asserted as a general law of these inorganic combinations that, other things equal, the stability decreases as the complexity increases. When we pass to the compounds which make up organic bodes, we find this general law further exemplified; we find much greater complexity and much less stability. A molecule of albumen, for instance, consists of more than two hundred ultimate units of five different kinds. According to the latest analyses it contains in each molecule, 72 of carbon, 18 of nitrogen, 1 of sulphur, 112 of hydrogen, and 22 of oxygen -- in all, 225 atoms; or, more strictly speaking, equivalents. And this substance is so unstable as to decompose at quite moderate temperatures; as that to which the outside of a joint of roasting meat is exposed. Possibly it will be objected that some inorganic compounds, as phosphuretted hydrogen, chloride of nitrogen, and the nitrogen-explosives in general, are more decomposable than most organic compounds. This is true. But the admission may be made without damage to the argument. The proposition is not that all simple combinations are more stable than all complex ones. To establish our inference it is necessary only to show that, as an average fact, the simple combinations can exist at a higher temperature than the complex ones. And this is beyond question. Thus it is manifest that the present chemical heterogeneity of the Earth's surface, and of the bodies upon it, has arisen by degrees as the decrease of heat has permitted; and that it has shown itself in three forms: -- first, in the multiplication of chemical compounds; second, in the greater number of different elements contained in the more modern of these compounds; and third, in the higher and more varied multiples in which these more numerous elements combine.

Without specifying them, it will suffice just to name the meteorologic processes eventually set up in the Earth's atmosphere, as further illustrating the alleged law. They equally display that destruction of a homogeneous state which results from unequal exposure to incident forces.

§152. Take a mass of unorganized but organizable matter -- either the body of one of the lowest living forms, or the germ of one of the higher: both comparatively homogeneous. Consider its circumstances. Either it is immersed in water or air or is contained within a parent organism. Wherever placed, however, its outer and inner parts stand differently related to surrounding agencies -- nutriment, oxygen, and the various stimuli. But this is not all. Whether it lies quiescent at the bottom of a pool or on the leaf of a plant; whether it moves through the water preserving some definite attitude; or whether it is in the inside of an adult; it equally happens that certain parts of its surface are more exposed to surrounding agencies than other parts -- in some cases more exposed to light, heat, or oxygen, and in other cases to the maternal tissues and their contents. Hence must follow the loss of its original equilibrium. This may take place in one of two ways. Either the disturbing forces may be such as to over-balance the affinities of the organic elements, and there results decomposition; or, as ordinarily occurs, such changes are induced as do not destroy the organic compounds but only modify them: the parts most exposed to the modifying forces being most modified. To elucidate this a few cases are required.

Observe first what appear to be exceptions. Certain minute animal forms present either no appreciable differentiations or differentiations so obscure as to be made out with great difficulty. Concerning these forms, however, note the fact that in all cases (some say in nearly all) the presence of a nucleus shows conformity to the general law, since it implies a contrast between the innermost protoplasm and the protoplasm surrounding it. But let us pass on to the seemingly exceptional fact that the surrounding protoplasm does not exhibit the kind of differentiation between inner and outer above alleged. To this objection, there immediately presents itself the answer that this homogeneous body-substance does not become heterogeneous because its parts are not subject to any permanent heterogeneity of conditions: it has no fixed surface. In all members of the lowest group, Proteomyxa, the protoplasm continually protrudes itself, now in thicker now in thinner processes -- pseudopodia; proved to have no limiting membranes by often coalescing. These, when they touch fragments of nutriment, contract and draw them into the mass of the body; so that what was just before external now becomes internal. Thus there are no fixed relations of parts and therefore no differentiations. And it is noteworthy that in certain of the Amoebae, less excursive than others of the type in the movements of their substance, we see an incipient differentiation: sometimes there is an investing film, "delicate and evanescent," implying that an outer part which is for a short time stationary, begins to be differentiated. Perceiving, then, that this apparent exception is in fact a verification, we go on to observe that permanent relations of inner and outer are followed by permanent differentiations. Elsewhere (Essays, i, 439) I have quoted from Sachs various proofs that a portion of protoplasm, whether normally detached, as in a spore, or abnormally detached, as by a rupture, forthwith becoming globular, at once acquires a surface denser than the interior; and Kerner similarly describes the protoplasm of a zoospore as "fixing itself and putting on a delicate cell-wall." These cases, joined with those of various Protozoa which, ceasing their active changes of form, pass into a resting stage and become enclosed in a cyst, and joined with the cases of Protophyta, like Sphaerella nivalis or "Red Snow," which, in its young stage ovoid, flagellate, locomotive, and secreting a skin, presently passes into a resting stage and becomes spherical and covered by a substantial cell-membrane, yield clear evidence that in these lowest types there is a lapse from a more homogeneous state into a less homogeneous state. And throughout the higher Protozoa and Protophyta, the primary contrast is between cell-membrane and cell-contents -- between the part exposed to environing forces and the part sheltered from them.

The transition -- the most important transition which the organic world presents -- between the simple forms above exemplified and those compound forms in which a number of such are united into a colony, is well seen in certain minute algae, Pandorina and Eudorina: each being a spherically-arranged colony of sixteen or thirty-two members. In this first advance from unicellular types to multicellular types we find conformity to the general law in so far that the hollow sphere conspicuously displays the primary contrast between outer and inner; a primitive amorphous cluster has undergone a marked differentiation of parts corresponding to the difference of conditions. Still more instructive is the evidence furnished by types slightly in advance of these -- Pleodorina and Volvox; the first consisting of some 128 cells and the second of 10,000 or more. Hollow spheres like the foregoing, they present in common the significant trait that, revolving, as they do, on a constant axis and moving forward approximately in the line of that axis, their two ends are exposed to slightly different conditions, and the primitive homogeneity of the members of the colony has, in consequence, lapsed into appropriate heterogeneity. These ciliated alga-cells, whether living singly or joined into groups, severally have a minute red speck which is proved to be sensitive to light, and causes motion towards it. Now in these compound forms just named, the eye-spots are more developed in those cells forming the anterior part of the spherical colony-cells which also carry on more actively the nutritive function; while those cells which form the posterior part of the sphere, and carry on the reproductive function, have smaller eye-spots. On passing to the animal kingdom (which at its root is so little differentiated from the vegetal kingdom that there are unsettled disputes respecting the inclusion of the lowest forms in the one or the other) we meet with parallel illustrations. The nucleated cell, which is the common starting point for all organisms, animal and vegetal, presents us as before with the primary contrast between inner and outer. And as in the multicellular plants so in the multicellular animals, a like primary contrast is forthwith repeated in the initial clusters of cells. Produced by the repeated fissions of the primitive germ-cell, each such cluster presently forms itself into a hollow sphere: the "cleavage cavity" being manifestly homologous with the cavity of the Vilvox-sphere.*<* I may remark in passing that in the one case (and possibly by inheritance in the other) this formation of a hollow sphere is the result of the more rapid growth of the outer parts than the inner parts of a solid group. Being dependent for nutrition on light and carbon-dioxide in the water, the outside components of a Volvox (either the cells or the chlorophyll in each cell) have a great advantage over the cells or portions of cells which are more centrally placed; and it needs but to consider what happens if the periphery of a sphere increases at a proportionately greater rate than its contents to see that it must either leave the contents behind or draw them after it and become hollow. An analogous effect of excessive peripheral growth may occasionally be seen exempted when, after a dry fit during which potatoes have not grown much, there comes rain and a rapid increase of bulk; this being the explanation of the fact that in very large potatoes there is not uncommonly a split in the interior, caused by the strain which the disproportionate growth of the periphery necessarily causes.> In simple types of Metazoa, as the hydroid polyps, the blastula, being thus established in conformity with the primary contrast of conditions, there presently begins a secondary differentiation which, like that we have seen in the Volvox but in a more pronounced manner, answers to the secondary contrast of conditions; for this spherical assemblage of cells becomes ovoid, and by the aid of its cilia moves through the water broad end foremost: the lapse from homogeneity of form being in some cases made more pronounced by the assumption of a sausage-shape. Simultaneously the component cells of the two ends become unlike in character. A far more marked differentiation, or lapse into greater heterogeneity, is seen when this single-layered spheroid of ciliated cells is changed into a double-layered spheroid by introversion of one side: a sack with the mouth sewn up and the bottom thrust in as far as it will go, serving to illustrate the relations of parts. Hence results the gastrula with its ectoderm and endoderm; severally playing contrasted parts in subsequent development. So that at successive stages there is repeated this rise of a contrast of structures answering to a contrast of conditions -- that which occurs in the simple cell, that which occurs in the hollow sphere of such cells, and that which occurs in the double-walled sphere.

Illustrations presenting the law under another aspect -- one from each organic kingdom -- are instructive. The ciliated germ or planula of a Zoophyte which, during its locomotive stage, is distinguishable only into outer and inner tissues, no sooner becomes fixed than its upper end begins to assume a different structure from its lower. The disc-shaped gemmae of the Marchantia, originally alike on both surfaces, and falling at random with either side uppermost, immediately begin to develop rootlets on their under sides and stomata on their upper sides: a fact proving beyond question, that this primary differentiation is determined by this fundamental contrast of conditions.

Of course in the germs of higher organisms, the metamorphoses immediately due to the instability of the homogeneous, are soon masked by those due to the assumption of the hereditary type. Even in the early stages above described there are to be traced modifications thus originating. Even before the primary cell-multiplication begins, there is said to be an observable distinction between the two poles of the egg-cell, foreshadowing the different germ-layers. Of course as development progresses assumption of the transmitted type of structure quickly obscures these primary lapses from homogeneity; though for some time the fundamental relations of inner and outer are recognizable in the differentiations. But what has been said suffices to establish the alleged general truth. It is enough that incipient organisms, setting out from relatively homogeneous arrangements, forthwith begin to fall into relatively heterogeneous ones. It is enough that the most conspicuous differentiations which they display, correspond to the most marked differences of conditions to which their parts are subject. It is enough that the habitual contrast between outside and inside, which we know is produced in inorganic masses by unlikeness of exposure to incident forces, is paralleled by the first contrast which makes its appearance in all organic masses.

It remains to point out that in the assemblage of organisms constituting a species, the principle enunciated is no less traceable. We have abundant materials for the induction that each species will not remain uniform -- is ever becoming to some extent multiform; and there is ground for the deduction that this lapse from homogeneity to heterogeneity is caused by the subjection of its members to unlike circumstances. Tending ever to spread from its original habitat into adjacent habitats, each species must have its peripheral parts subject to sets of forces unlike those to which its central parts are subject, and so must tend to have its peripheral members made different from its central members.

§153. Among mental phenomena full establishment of the alleged law would involve an analysis too extensive for the occasion. To show satisfactorily how states of consciousness, relatively homogeneous, become heterogeneous through differences in the changes wrought by different external forces, would require us to trace out the organization of early experiences. Without here attempting this it must suffice to set down the conclusions to be drawn.

The development of intelligence is, under one of its chief aspects, a classifying of the unlike things previously confounded together -- a formation of sub-classes and sub-sub-classes, until the once confused aggregate of objects known, is resolved into an aggregate which unites great heterogeneity among its multiplied groups, with complete homogeneity among the members of each group. On following through ascending grades of creatures, the genesis of that vast structure of knowledge acquired by sight, we see that in the first stage, where eye-specks suffice only for discriminating light from darkness, there can be no classifications of objects seen, save those based on the manner in which light is obstructed, and the degree in which it is obstructed. By such undeveloped visual organs, the shadows perceived would be merely distinguished into those of the stationary objects which the creature passed during its own movements, and those of the moving objects which came near while it was at rest; so that the extremely general classification of visible things into stationary and moving, would be the earliest formed. A kindred step follows. While the simplest eyes cannot distinguish between an obstruction of light caused by a small object close to, and an obstruction caused by a large object at some distance, eyes a little more developed can distinguish them; whence must result a vague differentiation of the class of moving objects into the nearer and the more remote. Further developments which make possible a better estimation of distances by adjustment of the optic axes, and those which, through enlargement and subdivision of the retina, make possible the discrimination of shapes, must give greater definiteness to the classes already formed, and subdivide these into smaller classes, consisting of objects less unlike. In every infant may be traced the analogous transformation of a confused aggregate of impressions of surrounding things, not recognized as differing in their distances, sizes, and shapes, into separate classes of things unlike one another in these and various other respects. And in both cases the change from this first indefinite, incoherent, and comparatively homogeneous consciousness, to a definite, coherent, and heterogeneous one, is due to differences in the actions of incident forces on the organism. These brief indications must suffice. Probably they will give adequate clue to an argument by which each reader may satisfy himself that the course of mental evolution offers no exception to the general law. In further aid of such an argument, I will here add an illustration which is comprehensible apart from the process of mental evolution as a whole.

It has been remarked (I am told by Coleridge) that with the advance of language, words which were originally alike in their meanings acquire unlike meanings -- a change he expressed by the formidable word "de-synonymization." Among indigenous words this loss of equivalence cannot be clearly shown; because in them the divergences of meaning began before the dawn of literature. But among words that have been coined, or adopted from other languages, since the writing of books commenced, it is demonstrable. By the old divines, miscreant was used in its etymological sense of unbeliever; but in modern speech it has entirely lost this sense. Similarly with evil-doer and malefactor. Exactly synonymous as these are by derivation, they are no longer synonymous by usage. By a malefactor we now under stand a convicted criminal, which is far from being the acceptation of evil-doer. The verb produce bears in Euclid its primary meaning -- to prolong or draw out; but the now largely-developed meanings of produce, have little in common with the meanings of prolong, or draw out. In the Church of England liturgy an odd effect now results from the occurrence of prevent in its original sense -- to come before, instead of its modern specialized sense -- to come before with the effect of arresting. But the most conclusive cases are those in which the contrasted words consist of the same parts differently combined, as in go under and undergo. We go under a tree, and we undergo a pain. But though, if analytically considered, the meanings would be the same were the words transposed, habit has so far modified their meanings that we could not without absurdity speak of undergoing a tree and going under a pain. Many such instances show that between two words which are originally of like force, an equilibrium cannot be maintained. Unless they are daily used in exactly equal degrees, in exactly similar relations (which is infinitely improbable), there necessarily aries a habit of associating one rather than the other with particular acts, or objects. Such a habit once commenced, becomes confirmed; and gradually their homogeneity of meaning disappears.

Should any difficulty be felt in understanding how these mental changes exemplify a law of physical transformations that are wrought by physical forces, it will disappear on contemplating acts of mind as nervous functions. It will be seen that each loss of equilibrium above instanced, is a loss of functional equality between some two elements of the nervous system. And it will be seen that, as in other cases, this loss of functional equality is due to differences in the incidence of forces.

§154. Masses of men, in common with all other masses, show a like proclivity similarly caused. Small combinations and large societies equally manifest it; and in the one, as in the other, both governmental and industrial differentiations are initiated by it. Let us glance at the facts under these heads.

A business-partnership, balanced as the authorities of its members may theoretically be, presently becomes a union in which the authority of one partner is tacitly recognized as greater than that of the other or others. Though the shareholders have given equal powers to the directors of their company, inequalities of power soon arise among them; and often the supremacy of some one director grows so marked, that his decisions determine the course which the board takes. Nor in associations for political, charitable, literary, or other purposes, do we fail to find a like process of division into dominant and subordinate parties; each having its leader, its members of less influence, and its mass of uninfluential members. These minor instances in which unorganized groups of men, standing in homogeneous relations, may be watched gradually passing into organized groups of men standing in heterogeneous relations, give us key to social inequalities. Barbarous and civilized communities are alike characterized by separation into classes, as well as by separation of each class into more important and less important units; and this structure is the gradually-consolidated result of a process like that daily exemplified in trading and other combinations. So long as men are constituted to act on one another, either by physical force or by force of character the struggles for supremacy must finally be decided in favour of some class or some one; and the difference once commenced must tend to become ever more marked. Its unstable equilibrium being destroyed, the uniform must gravitate with increasing rapidity into the multiform. And so supremacy and subordination must establish themselves, as we see they do, throughout the whole structure of a society, from the great class-divisions pervading its entire body, down to village cliques, and even down to every posse of schoolboys. Probably it will be objected that such changes result, not from the homogeneity of the original aggregations, but from their non-homogeneity -- from certain slight differences existing among their units at the outset. This is doubtless the proximate cause. In strictness, such changes must be regarded as transformations of the relatively homogeneous into the relatively heterogeneous. But an aggregation of men absolutely alike in their endowments, would eventually undergo a similar transformation. For in the absence of uniformity in the lives severally led by them -- in their occupations, physical conditions, domestic relations, and trains of thought and feeling -- there must arise differences among them; and these must eventually initiate social differentiations. Even inequalities of health caused by accidents will, by entailing inequalities of physical and mental power, disturb the exact balance of mutual influences among the units; and the balance once disturbed, will inevitably be lost.

Turning to the industrial organization, and noting that its division into regulative and operative is primarily determined, like the preceeding, by differences of power (women and slaves being the first working classes); admitting, too, that even among savages some small specializations arise from individual aptitudes; we go on to observe that the large industrial divisions into which societies gravitate, are due to unlikenesses of external circumstances. Such divisions are absent until such unlikenesses are established. Nomadic tribes do not permanently expose any groups of their members to special local conditions; nor does a stationary tribe, when occupying only a small area, maintain from generation to generation marked contrasts in the local conditions of its members; and in such tribes there are no decided economic differentiations. But a community which, by conquest, or otherwise, has overspread a large tract, and has become so far settled that its members live and die in their respective districts, keeps its several sections in different circumstances; and then they no longer remain alike in their occupations. Those who live dispersed continue to hunt or cultivate the earth; those who spread to the sea-shore fall into maritime occupations; while the inhabitants of some spot chosen, perhaps for its centrality, as one of periodic assemblage, become traders, and a town springs up. In the adaptations of these social units to their respective functions, we see a progress from uniformity to multiformity caused by unlike incidence of forces. Later in the process of social evolution these local adaptations are greatly multiplied. Differences in soil and climate, cause the rural inhabitants in different parts of the kingdom to have their occupations partially specialized, and to be come known as chiefly producing cattle, or sheep, or wheat, or oats, or hops, or fruit. People living where coal-fields are discovered are transformed into colliers; Cornishmen take to mining because Cornwall is metalliferous; and iron-manufacture is the dominant industry where iron-stone is plentiful. Liverpool has taken to importing cotton, because of its proximity to the district where cotton-goods are made; and for analogous reasons Hull has become the chief port at which foreign wools are brought in. Thus in general and in detail, industrial heterogeneities of the social organism primary depend on local influences. Those divisions of labour which, under another aspect, were interpreted as due to the setting up of motion in the directions of least resistance (§80), are here interpreted as due to differences in the incident forces; and the two interpretations are quite consistent with each other. For that which in each determines the direction of least resistance, is the distribution of the forces to be overcome; and hence unlikenesses of distribution in separate localities, entails unlikenesses in the lines of human actions in those localities -- entails industrial differentiations.

§155. It has still to be shown that this general truth is demonstrable a priori -- that the instability of the homogeneous is a corollary from the persistence of force. Already this has been tacitly implied, but here it will be proper to expand the tacit implication into definite proof.

On striking a mass of matter with such force as either to indent it or make it fly to pieces, we see both that the blow affects differently its different parts, and that the differences are consequent on the unlike relations of its parts to the force impressed. The part struck is driven in towards the centre of the mass. It thus compresses, and tends to displace, the more centrally situated portions. These, however, cannot be compressed or thrust out of their places without pressing on surrounding portions. And when the blow is violent enough to fracture the mass, we see, in the radial dispersion of the fragments, that the original momentum has been divided into numerous minor momenta, unlike in their directions. We see that the parts are differently affected by the disruptive force, because they are differently related to it in their directions and attachments -- that the effects being the joint products of the force and the conditions cannot be alike in parts which are differently conditioned. A body on which radiant heat is falling, exemplifies this truth still more clearly. Take the simplest case -- that of a sphere. While the part nearest to the radiating centre receives the rays at right angles, the rays strike the other parts of the exposed side at all angles from 90° down to 0°. The molecular vibrations propagated through the mass from the surface which receives the heat, proceed inwards at angles differing for each point. Further, the interior parts reached by the vibrations proceeding from all points of the heated side, must be dissimilarly affected in proportion as their positions are dissimilar. So that whether they be on the recipient area, in the middle, or at the remote side, the constituent molecules are thrown into states of vibration more or less unlike one another.

But now, what is the ultimate meaning of the conclusion that a force produces different changes throughout a uniform mass, because the parts of the mass stand in different relations to the force? Fully to understand this, we must contemplate each part as simultaneously subject to other forces -- those of gravitation, of cohesion, molecular motion, etc. The effect wrought by an additional force, must be a resultant of it and the forces already in action. If the forces already in action on two parts of any aggregate, are different in their resultant directions, the effects produced on these two parts by equal additional forces must be different in their directions. Why must they be different? Because such unlikeness as exists between the two sets of factors, is made by the presence in the one of some specially-directed force that is not present in the other; and that this force will produce an effect, rendering the total result in the one case unlike that in the other, is a necessary corollary from the persistence of force. Still more manifest does it become that the dissimilarly-placed parts of any aggregate must be dissimilarly modified by an incident force, when we remember that the quantities of the incident force to which they are severally subject, are not equal, as above supposed, but are nearly always unequal. Look again at the above examples. The amounts of any external radiant force which the different parts of an aggregate receive, are widely contrasted: we have the contrast between the quantity falling on the side next the radiating centre, and the quantity, or rather no quantity, falling on the opposite side; we have contrasts in the quantities received by differently-placed areas on the exposed side; and we have endless contrasts between the quantities received by the various parts of the interior. Similarly when mechanical force is expended on any aggregate, either by collision, continued pressure, or tension, the amounts of strain distributed throughout the mass are manifestly unlike for unlike positions. And it is obvious that ordinary chemical action affects surface more than centre, and often one part of the surface more than another. But to say the different parts of an aggregate receive different quantities of any force capable of changing them, is to say that if they were before homogeneous they must be rendered to a proportionate extent heterogeneous; since, force being persistent, the different quantities of it falling on the different parts, must work in them different quantities of effect-different changes. Yet one more kindred deduction is required to complete the argument. Even apart from the action of any external force, the equilibrium of a homogeneous aggregate must be destroyed by the unequal actions of its parts on one another. That mutual influence which produces aggregation (not to mention other mutual influences) must work different effects on the different parts; since they are severally exposed to it in unlike amounts and directions. This will be clearly seen on remembering that the portions of which the whole is made up, may be severally regarded as minor wholes; that on each of these minor wholes, the action of the entire aggregate then becomes an external incident force; that such external incident force must, as above shown, work unlike changes in the parts of any such minor whole; and that if the minor wholes are severally thus rendered heterogeneous, the entire aggregate is rendered heterogeneous.

The instability of the homogeneous is thus deducible from that primordial truth which underlies our intelligence. One stable homogeneity only, is hypothetically possible. If centres of force, absolutely uniform in their powers, were diffused with absolute uniformity through unlimited space, they would remain in equilibrium. This however, though a verbally intelligible supposition, is one that cannot be represented in thought; since unlimited space is inconceivable. But all finite forms of the homogeneous -- all forms of it which we can know or conceive, must inevitably lapse into heterogeneity; and the less heterogeneous must lapse into the more heterogeneous. In three several ways does the persistence of force necessitate this. Setting external agencies aside, each unit of a homogeneous whole must be differently affected from any of the rest by the aggregate action of the rest upon it. The resultant force exercised by the aggregate on each unit, being in no two cases alike in both amount and direction, and usually not in either, any incident force, even if uniform in amount and direction, cannot produce like effects on the units. And as the various positions of the parts in relation to any incident force, prevents them from receiving it in uniform amounts and directions, a further difference in the effects wrought on them inevitably arises.

One further remark is needed. The conclusion that the changes with which Evolution commences, are thus necessitated, has to be supplemented by the conclusion that these changes must continue. The absolutely homogeneous (supposing it to exist) must lose its equilibrium; and the relatively homogeneous must lapse into the relatively less homogeneous. That which is true of any total mass, is true of the parts into which it segregates. The uniformity of each such part must as inevitably be lost in multiformity, as was that of the original whole; and for like reasons. And thus the continued changes characterizing Evolution, in so far as they are constituted by the lapse of the homogeneous into the heterogeneous, and of the less heterogeneous into the more heterogeneous, are necessary consequences of the persistence of force.

[A small change in the definition of Evolution indicated in a note at the end of Chapter XVII of this part, must be recalled as involving a correlative change in this chapter. Here, as before, the required change, though already implied (page 367), has not been sufficiently emphasized, and lack of the emphasis invites misinterpretation. For reasons like those before given, the requisite explanations cannot be made in this place. The reader will find them in Appendix A.

Replies to certain criticisms on the general doctrine set forth in this chapter will be found in Appendix C.]