ever-increasing irregularity; and that thus the present aspect of the heavens is not, so far as we can judge, incongruous with the hypothesis of a general evolution consequent on the instability of the homogeneous.
Descending to that more limited form of the nebular hypothesis which regards the solar system as having resulted by gradual concentration; and assuming this concentration to have advanced so far as to produce a rotating spheroid of nebulous matter; let us consider what further consequence the instability of the homogeneous necessitates. Having become oblate in figure, unlike in the densities of its centre and surface, unlike in their temperatures, and unlike in the velocities with which its parts move round their common axis, such a mass can no longer be called homogeneous; and therefore any further changes exhibited by it as a whole, can illustrate the general law, only as being changes from a more homogeneous to a less homogeneous state. Changes of this kind are to be found in the transformations of such of its parts as are still homogeneous within themselves. If we accept the conclusion of Laplace, that the equatorial portion of this rotating and contracting spheroid will at successive stages acquire a centrifugal force great enough to prevent any nearer approach to the centre round which it rotates, and will so be left behind by the inner parts of the spheroid in its still-continued contraction; we shall find, in the fate of the detached ring, a fresh exemplification of the principle we are following out. Consisting of gaseous matter, such a ring, even if absolutely uniform at the time of its detachment, cannot continue so. To maintain its equilibrium there must be an almost perfect uniformity in the action of all external forces upon it (almost, we must say, because the cohesion, even of extremely attenuated matter, might suffice to neutralize very minute disturbances); and against this the probabilities are immense. In the absence of equality among the forces, internal and external, acting on such a ring, there must be a point or points at which the cohesion of its parts is less than elsewhere—a point or points at which rupture will therefore take place. Laplace assumed that the ring would rupture at one place only; and would then collapse on itself. But this is a more than questionable assumption—such at least I know to be the opinion of an authority second to none among those now living. So vast a ring, consisting of matter having such feeble cohesion, must break up into many parts. Nevertheless, it is still inferrable from the instability of the homogeneous, that the ultimate result which Laplace predicted would take place. For even supposing the masses of nebulous matter into which such a ring separated, were so equal in their sizes and distances as to attract each other with exactly equal forces (which is infinitely improbable); yet the unequal action 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 ever-accelerating speed lead to a grouping of the masses. And obviously a like result would eventually take place with the groups thus formed; until they at length aggregated into a single mass.
Leaving the region of speculative astronomy, let us consider the Solar System as it at present exists. And here it will be well, in the first place, to note a fact which may be thought at variance with the foregoing argument—namely, the still-continued existence of Saturn’s rings; and especially of the internal nebulous ring lately discovered. To the objection that the outer rings maintain their equilibrium, the reply is that the comparatively great cohesion of liquid or solid substance would suffice to prevent any slight tendency to rupture from taking effect. And that a nebulous ring here still preserves its continuity, does not really negative the foregoing conclusion; since it happens under the quite exceptional influence of those symmetrically disposed forces which the external rings exercise on it. Here indeed it deserves to be noted, that though at first sight the Saturnian system appears at variance with the doctrine that a state of homogeneity is one of unstable equilibrium, it does in reality furnish a curious confirmation of this doctrine. For Saturn is not quite concentric with his rings; and it has been proved mathematically that were he and his rings concentrically situated, they could not remain so: the homogeneous relation being unstable, would gravitate into a heterogeneous one. And this fact serves to remind us of the allied one presented throughout the whole Solar System. All orbits, whether of planets or satellites, are more or less excentric—none of them are perfect circles; and were they perfect circles they would soon become ellipses. Mutual perturbations would inevitably generate excentricities. That is to say, the homogeneous relations would lapse into heterogeneous ones.
* * * * * § 111. Already so many references have been made to the gradual formation of a crust over the originally incandescent Earth, that it may be thought superfluous again to name it. It has not, however, been before considered in connexion with the general principle under discussion. Here then it must be noted as a necessary consequence of the instability of the homogeneous. In this cooling down and solidification 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 different parts to different 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 conditions to which the surface of the Earth is exposed, alike in all directions, there would be no obvious 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: as 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, which are manifestly consequent on the instability of the homogeneous, there have been going on numerous chemical differentiations, admitting of similar interpretation. Without raising the question whether, as some think, the so-called simple substances are themselves compounded of unknown elements (elements which we cannot separate by artificial heat, but which existed separately when the heat of the Earth was greater than any which we can produce),—without raising this question, it will suffice the present purpose 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: each element or compound, being unable to maintain its homogeneity in presence of various surrounding affinities, having fallen into heterogeneous combinations. Let us contemplate this change somewhat in detail. There is every reason to believe that 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. Whence it seems not improbable that, when the Earth was in its first state of incandescence, there were no chemical combinations at all. 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, &c.—are, as a class, the most fixed compounds known: the majority of them resisting decomposition by any heat we can generate. These, consisting severally of one atom of each component element, are combinations of the simplest order—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, &c.; 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, if not 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, I believe no chemist will deny it to be a general law of these inorganic combinations that, other things equal, the stability decreases as the complexity increases. And then when we pass to the compounds that make up organic bodies, we find this general law still further exemplified: we find much greater complexity and much less stability. An atom of albumen, for instance, consists of 482 ultimate atoms of five different kinds. Fibrine, still more intricate in constitution, contains in each atom, 298 atoms of carbon, 49 of nitrogen, 2 of sulphur, 228 of hydrogen, and 92 of oxygen—in all, 660 atoms; or, more strictly speaking—equivalents. And these two substances are so unstable as to decompose at quite moderate temperatures; as that to which the outside of a joint of roast meat is exposed. Possibly it will be objected that some inorganic compounds, as phosphuretted hydrogen and chloride of nitrogen, 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 fixed 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 wholly beyond question. Thus it is manifest that the present chemical heterogeneity of the Earth’s surface 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.
* * * * * * * * * * § 112. 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. Consider its circumstances. Either it is immersed in water or air, or it 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 the water 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 results 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 others to the maternal tissues and their contents. Hence must follow the destruction of its original equilibrium. This may take place in one of two ways. Either the disturbing forces may be such as to overbalance the affinities of the organic elements, in which case there result those changes which are known as decomposition; or, as is ordinarily the case, 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, suppose we take a few cases.
Note first what appear to be exceptions. Certain minute animal forms present us either with no appreciable differentiations or with differentiations so obscure as to be made out with great difficulty. In the Rhizopods, the substance of the jelly-like body remains throughout life unorganized, even to the extent of having no limiting membrane; as is proved by the fact that the thread-like processes protruded by the mass, coalesce on touching each other. Whether or not the nearly allied _Amœba_, of which the less numerous and more bulky processes do not coalesce, has, as lately alleged, something like a cell-wall and a nucleus, it is clear that the distinction of parts is very slight; since particles of food pass bodily into the inside through any part of the periphery, and since when the creature is crushed to pieces, each piece behaves as the whole did. Now these cases, in which there is either no contrast of structure between exterior and interior or very little, though seemingly opposed to the above inference, are really very significant evidences of its truth. For what is the peculiarity of this division of the _Protozoa_? Its members undergo perpetual and irregular changes of form—they show no persistent relation of parts. What lately formed a portion of the interior is now protruded, and, as a temporary limb, is attached to some object it happens to touch. What is now a part of the surface will presently be drawn, along with the atom of nutriment sticking to it, into the centre of the mass. Either the relations of inner and outer have no permanent existence, or they are very slightly marked. But by the hypothesis, it is only because of their unlike positions with respect to modifying forces, that the originally like units of a living mass become unlike. We must therefore expect no established differentiation of parts in creatures which exhibit no established differences of position in their parts; and we must expect extremely little differentiation of parts where the differences of position are but little determined—which is just what we find. This negative evidence is borne out by positive evidence. When we turn from these proteiform specks of living jelly to organisms having an unchanging distribution of substance, we find differences of tissue corresponding to differences of relative position. In all the higher _Protozoa_, as also in the _Protophyta_, we meet with a fundamental differentiation into cell-membrane and cell-contents; answering to that fundamental contrast of conditions implied by the terms outside and inside. On passing from what are roughly classed as unicellular organisms, to the lowest of those which consist of aggregated cells, we equally observe the connection between structural differences and differences of circumstance. Negatively, we see that in the sponge, permeated throughout by currents of sea-water, the indefiniteness of organization corresponds with the absence of definite unlikeness of conditions: the peripheral and central portions are as little contrasted in structure as in exposure to surrounding agencies. While positively, we see that in a form like the _Thalassicolla_, which, though equally humble, maintains its outer and inner parts in permanently unlike circumstances, there is displayed a rude structure obviously subordinated to the primary relations of centre and surface: in all its many and important varieties, the parts exhibit a more or less concentric arrangement.
After this primary modification, by which the outer tissues are differentiated from the inner, the next in order of constancy and importance is that by which some part of the outer tissues is differentiated from the rest; and this corresponds with the almost universal fact that some part of the outer tissues is more exposed to certain environing influences than the rest. Here, as before, the apparent exceptions are extremely significant. Some of the lowest vegetal organisms, as the _Hematococci_ and _Protococci_, evenly imbedded in a mass of mucus, or dispersed through the Arctic snow, display no differentiations of surface; the several parts of their surfaces being subjected to no definite contrasts of conditions. Ciliated spheres such as the _Volvox_ have no parts of their periphery unlike other parts; and it is not to be expected that they should have; since, as they revolve in all directions, they do not, in traversing the water, permanently expose any part to special conditions. But when we come to organisms that are either fixed, or while moving preserve definite attitudes, we no longer find uniformity of surface. The most general fact which can be asserted with respect to the structures of plants and animals, is, that however much alike in shape and texture the various parts of the exterior may at first be, they acquire unlikenesses corresponding to the unlikenesses of their relations to surrounding agencies. The ciliated germ 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 _gemmæ_ of the _Marchantia_, originally alike on both surfaces, and falling at random with either side uppermost, immediately begin to develop rootlets on the under side, and _stomata_ on the upper side: 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. Such early changes, however, as are common to all classes of organisms, and so cannot be ascribed to heredity, entirely conform to the hypothesis. A germ which has undergone no developmental modifications, consists of a spheroidal group of homogeneous cells. Universally, the first step in its evolution is the establishment of a difference between some of the peripheral cells and the cells which form the interior—some of the peripheral cells, after repeated spontaneous fissions, coalesce into a membrane; and by continuance of the process this membrane spreads until it speedily invests the entire mass, as in mammals, or, as in birds, stops short of that for some time. Here we have two significant facts. The first is, that the primary unlikeness arises between the exterior and the interior. The second is, that the change which thus initiates development, does not take place simultaneously over the whole exterior; but commences at one place, and gradually involves the rest. Now these facts are just those which might be inferred from the instability of the homogeneous. The surface must, more than any other part, become unlike the centre, because it is most dissimilarly conditioned; and all parts of the surface cannot simultaneously exhibit this differentiation, because they cannot be exposed to the incident forces with absolute uniformity. One other general fact of like implication remains. Whatever be the extent of this peripheral layer of cells, or blastoderm as it is called, it presently divides into two layers—the serous and mucous; or, as they have been otherwise called, the ectoderm and the endoderm. The first of these is formed from that portion of the layer which lies in contact with surrounding agents; and the second of them is formed from that portion of the layer which lies in contact with the contained mass of yelk. That is to say, after the primary differentiation, more or less extensive, of surface from centre, the resulting superficial portion undergoes a secondary differentiation into inner and outer parts—a differentiation which is clearly of the same order with the preceding, and answers to the next most marked contrast of conditions.
But, as already hinted, this principle, understood in the simple form here presented, supplies no key to the detailed phenomena of organic development. It fails entirely to explain generic and specific peculiarities; and indeed leaves us equally in the dark respecting those more important distinctions by which families and orders are marked out. Why two ova, similarly exposed in the same pool, should become the one a fish, and the other a reptile, it cannot tell us. That from two different eggs placed under the same hen, should respectively come forth a duckling and a chicken, is a fact not to be accounted for on the hypothesis above developed. We have here no alternative but to fall back upon the unexplained principle of hereditary transmission. The capacity possessed by an unorganized germ of unfolding into a complex adult, which repeats ancestral traits in the minutest details, and that even when it has been placed in conditions unlike those of its ancestors, is a capacity we cannot at present understand. That a microscopic portion of seemingly structureless matter should embody an influence of such kind, that the resulting man will in fifty years after become gouty or insane, is a truth which would be incredible were it not daily illustrated. Should it however turn out, as we shall hereafter find reason for suspecting, that these complex differentiations which adults exhibit, are themselves the slowly accumulated and transmitted results of a process like that seen in the first changes of the germ; it will follow that even those embryonic changes due to hereditary influence, are remote consequences of the alleged law. Should it be shown that the slight modifications wrought during life on each adult, and bequeathed to offspring along with all like preceding modifications, are themselves unlikenesses of parts that are produced by unlikenesses of conditions; then it will follow that the modifications displayed in the course of embryonic development, are partly direct consequences of the instability of the homogeneous, and partly indirect consequences of it. To give reasons for entertaining this hypothesis, however, is not needful for the justification of the position here taken. It is enough that the most conspicuous differentiations which incipient organisms universally 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 strictly paralleled by the first contrast that 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 equally traceable. We have abundant materials for the induction that each species will not remain uniform, but 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 sets of circumstances. The fact that in every species, animal and vegetal, the individuals are never quite alike; joined with the fact that there is in every species a tendency to the production of differences marked enough to constitute varieties; form a sufficiently wide basis for the induction. While the deduction is confirmed by the familiar experience that varieties are most numerous and decided where, as among cultivated plants and domestic animals, the conditions of life depart from the original ones, most widely and in the most numerous ways. Whether we regard “natural selection” as wholly, or only in part, the agency through which varieties are established, matters not to the general conclusion. For as the survival of any variety proves its constitution to be in harmony with a certain aggregate of surrounding forces—as the multiplication of a variety and the usurpation by it of an area previously occupied by some other part of the species, implies different effects produced by such aggregate of forces on the two, it is clear that this aggregate of forces is the real cause of the differentiation—it is clear that if the variety supplants the original species in some localities but not in others, it does so because the aggregate of forces in the one locality is unlike that in the other—it is clear that the lapse of the species from a state of homogeneity to a state of heterogeneity arises from the exposure of its different parts to different aggregates of forces.
* * * * * § 113. Among mental phenomena it is difficult to establish the alleged law without an analysis too extensive for the occasion. To show satisfactorily how states of consciousness, originally homogeneous, become heterogeneous through differences in the changes wrought by different forces, would require us carefully to trace out the organization of early experiences. Were this done it would become manifest that the development of intelligence, is, under one of its chief aspects, a dividing into separate classes, the unlike things previously confounded together in one class—a formation of sub-classes and sub-sub-classes, until the once confused aggregate of objects known, is resolved into an aggregate which unites extreme heterogeneity among its multiplied groups, with complete homogeneity among the members of each group. If, for example, we followed, through ascending grades of creatures, the genesis of that vast structure of knowledge acquired by sight, we should find that in the first stage, where eyes suffice for nothing beyond the discrimination of light from darkness, the only possible classifications of objects seen, must be those based on the manner in which light is obstructed, and the degree in which it is obstructed. We should find that by such undeveloped visual organs, the shadows traversing the rudimentary retina 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 the creature while it was at rest; and that so the extremely general classification of visible things into stationary and moving, would be the earliest formed. We should find that whereas the simplest eyes are not fitted to 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 must be competent to such a distinction; whence must result a vague differentiation of the class of moving objects, into the nearer and the more remote. We should find that such further improvements in vision as those 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 have the effects of giving greater definiteness to the classes already formed, and of sub-dividing these into smaller classes, consisting of objects less unlike. And we should find that each additional refinement of the perceptive organs, must similarly lead to a multiplication of divisions and a sharpening of the limits of each division. In every infant might be traced the analogous transformation of a confused aggregate of impressions of surrounding objects, not recognized as differing in their distances, sizes, and shapes, into separate classes of objects unlike each other in these and various other respects. And in the one case as in the other, it might be shown that 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 of what might be shown, did space permit, must here 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 that is comprehensible apart from the process of mental evolution as a whole.
It has been remarked (I am told by Coleridge, though I have been unable to find the passage) that with the advance of language, words which were originally alike in their meanings acquire unlike meanings—a change which he expresses by the formidable word “desynonymization.” Among indigenous words this loss of equivalence cannot be clearly shown; because in them the divergencies 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. In the old divines, _miscreant_ is 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 understand 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 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 of these expressions 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. Countless such instances might be brought to 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 (against which there are infinite probabilities), there necessarily arises 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. In each individual we may see the tendency which inevitably leads to this result. A certain vocabulary and a certain set of phrases, distinguish the speech of each person: each person habitually uses certain words in places where other words are habitually used by other persons; and there is a continual recurrence of favourite