By a parallel process, the relations of co-existence and sequence amongimpressions, become sorted into kinds and grouped. When two phenomena thathave been experienced in a given order, are repeated in the same order, thosenerve-centres which before were affected by the transition are again affected;and such molecular modification as they received from the first motion propagatedthrough them is increased by this second motion. Each such motion works astructural alteration which, in conformity with the law set forth in ChapterIX, involves a diminished resistance to all such motions that afterwardsoccur. The segregation of these successive motions (or more strictly, thepermanently effective portions of expanded them in overcoming resistance)thus becomes the cause of, and the measure of, the mental connexions betweenthe impressions which the phenomena produced. Meanwhile, phenomena differentfrom these, being phenomena that affect different nervous elements, willhave their connexions severally represented by motions along other routes;and along each of these other routes, the nervous discharges will severallytake place with a readiness proportionate to the frequency with which experiencerepeats the connexions of phenomena. The classification of relations musthence go on pari passu with the classification of the related things. Incommon with the mixed sensations received from the external world, the mixedrelations it presents cannot be impressed on the organism.without more orless segregation of them resulting. And through this continuous sorting andgrouping of changes or motions, which constitutes nervous function, thereis gradually wrought that sorting and grouping of matter, which constitutesnervous structure. §168. In social evolution, the collecting together of the like andthe separation of the unlike by incident forces, is primarily displayed inthe same manner as we saw it to be among groups of inferior creatures. Thehuman races tend to differentiate and integrate, as do races of other livingforms.
Of the forces which effect and maintain the segregations of mankind, mayfirst be named those external ones classed as physical conditions. The climateand food which are favourable to an indigenous people, are more or less detrimentalto an alien people of different bodily constitution. In tropical regionsthe northern races cannot permanently exist: if not killed off in the firstgeneration, they are so in the second, and, as in India, can maintain theirfooting only by the artificial process of continuous immigration and emigration.
That is to say, the external forces acting equally on the inhabitants ofa given locality, tend to expel all who are not of a certain type, and thusto keep up the integration of those who are of that type. Even among theIndian peoples themselves the like happens: some of the hill-tribes beingsegregated by surviving the malarious influences which kill off Hindus whoenter their habitat. The other foxes conspiring to produce these nationalsegregations, are those mental ones shown in the affinities of men for otherslike themselves. Units of one society who are obliged to reside in another,generally form colonies in the midst of that other -- small societies oftheir own. Races which have been artificially severed, show tendencies tore-unite. Now though these segregations caused by the mutual likings of kindredmen, do not seem due to the general principle enunciated, they really arethus interpretable. When treating of the direction of motion (§80),it was shown that the actions performed by men for the satisfaction of theirwants, are always motions along lines of least resistance. The feelings characterizinga member of a given race, are feelings which get complete satisfaction onlyamong other members of that race a satisfaction partly derived from sympathywith those having like feelings, but mainly derived from the adapted socialconditions which grow up where such feelings prevail. When, therefore, acitizen 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 inthe direction of least resistance. Human motions, like all other motions,being determined by the distribution of forces, it follows that such segregationsof races as are not produced by incident external forces, are produced byforces which the units of the races exercise on one another.
During the development of each society we see analogous segregations causedin analogous ways. A few of them result from minor natural affinities; butthose 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 certainlikeness -- a likeness which, in respect of their powers of action, obscuresand subordinates their natural differences. Those trained to brain-work haveacquired a certain other community of character which makes them, as socialunits, 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 assimilatedmembers of any class who are brought up to the same calling. Even where thenecessities of their work forbid concentration in one locality, as amongartizans happens with masons and bricklayers, and among traders happens withthe retail distributers, and among professionals happens with the medicalmen, there are not wanting Operative Builders' Unions, and Grocers' Societies,and Medical Associations, implying a process of sifting out and grouping.
