University of Virginia Library

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.