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Here it remains to add that such of them as are subject to like sets of incidentforces, are segregated. For by the process of "natural selection,"there is a continual purification of each species from those individualswhich depart from the common type in ways that unfit them for the conditionsof their existence. Consequently, there is a continual leaving behind ofthose individuals which are in all respects fit for the conditions of theirexistence, and are therefore nearly alike. The circumstances to which anyspecies is exposed, being an involved combination of incident forces; andthe members of the species having among them some that differ more than isusual from the average structure required for meeting these forces; it resultsthat these forces are constantly separating such divergent individuals fromthe rest, and so preserving the uniformity of the rest -- keeping up itsintegrity as a species or variety. Just as the changing autumn leaves arepicked 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 sievewhile the larger are kept back; so, the uniform incidence of external forcesaffects the members of a group of organisms similarly in proportion as theyare similar, and differently in proportion as they are different; and thusis ever segregating the like by parting the unlike from them. Whether theseseparated members are killed off, as mostly happens, or whether, as otherwisehappens, they survive and multiply into a distinct variety, in consequenceof their fitness to certain partially-unlike conditions, matters not to theargument. The one case conforms to the law that the unlike units of an aggregateare sorted into their kinds and parted, when uniformly subject to the sameincident forces, and the other to the converse law that the like units ofan aggregate are parted and separately grouped when subject to differentincident forces. And on consulting Mr. Darwin's remarks on divergence ofcharacter, it will be seen that the segregations thus caused tend ever tobecome more definite. §167. Mental evolution under one of its leading aspects, we foundto consist in the formation in the mind of groups of like objects and likerelations -- a differentiation of the various things originally confoundedtogether in one assemblage, and an integration of each separate order ofthings into a separate group (§153). Here it remains to point out thatwhile unlikeness in the incident forces is the cause of such differentiations,likeness in the incident forces is the cause of such integrations. For whatis the process through which classifications are established? How do plantsbecome grouped in the mind of the botanist into orders, genera, and species?

Each plant he examines yields him a certain complex impression. Now and thenhe picks up a plant like one before seen; and the recognition of it is theproduction in him of a like connected group of sensations, by a like connectedgroup of attributes. That is to say there is produced throughout the nerve-centresconcerned, a combined set of changes, similar to a combined set of changesbefore produced. Considered analytically, each such combined set of changesis a combined set of molecular modifications wrought in the affected partof the organism. On every repetition of the impression, a like combined setof molecular modifications is superposed on the previous ones, and makesthem greater: thus generating an internal plexus of.modifications, with itsanswering idea, corresponding to these similar external objects. Meanwhile,another kind of plant produces in the brain of the botanist another set ofmolecular modifications -- a set which does not agree with the one we havebeen considering, but disagrees with it; and by repetition of such thereis generated a different idea answering to a different species. What, now,is the nature of this process expressed in general terms? On the one handthere are the like and unlike things from which severity emanate the groupsof forces by which we perceive them. On the other hand, there are the organsof sense and percipient centres, through which, in the course of observation,these groups of forces pass. In passing through them the like groups of forcesare segregated, or separated from the unlike groups of forces; and each suchseparate 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 aseparation of mixed matters by the same force, there is a separation of mixedforces by the same matter; and here we may further see that the unlike forcesso separated, work unlike structural changes in the aggregate that separatesthem -- structural changes each of which thus represents the integrated seriesof motions that has produced it.