Segregation §163. The general interpretation of Evolution is far from being completedin the preceding chapters. We must contemplate its changes under yet anotheraspect, before we can form a definite conception of the process constitutedby them. Though the laws already set forth furnish a key to the re-arrangementof parts which Evolution exhibits, in so far as it is an advance from theuniform to the multiform, they furnish no key to this rearrangement in sofar as it is an advance from the indefinite to the definite. On studyingthe actions and reactions everywhere going on, we have found it to followfrom a certain primordial truth, that the homogeneous must lapse into theheterogeneous, and that the heterogeneous must become more heterogeneous;but we have not discovered why the differently-affected parts of any simplewhole, become clearly marked off from one another, at the same time thatthey become unlike. Thus far no reason has been given why there should notordinarily arise a vague chaotic heterogeneity, in place of that orderlyheterogeneity displayed in Evolution. It still remains to find out the causeof that local integration which accompanied local differentiation -- thatgradually-completed segregation of like units into a group, distinctly separatedfrom neighbouring groups which are severally made up of other kinds of units.
The rationale will be conveniently introduced by a few instances in whichwe may watch this segregative process taking place.
When, late in September, the trees are gaining their autumn colours, andwe 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. Outof the mixed mass of foliage on each branch, the strong current of air carriesaway the decaying and brightly-tinted leaves, but fails to detach those whichare still green. And while these last, frayed and seared by long-continuedbeatings against one another, give a sombre colour to the woods, the redand yellow and orange leaves are collected together in ditches and behindwalls 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 leavesare picked out from among their still-living companions and gathered in placesby themselves. Again, the separation of particles of different sizes, asdust and sand from pebbles, may be similarly effected, as we see on everyroad in March. And from the days of Homer downwards, the power of currentsof air, natural and artificial, to part from one another units of unlikecharacters, has been habitually utilized in the winnowing of chaff from wheat.
In every brook we see how the mixed materials carried down are separatelydeposited -- how in rapids the bottom gives rest to nothing but bouldersand pebbles; how where the current is not so strong, sand is let fall; andhow, in still places, there is a sediment of mud. This selective actionof moving water is commonly applied in the arts to obtain masses of particlesof different degrees of fineness. Emery, for example, after being ground,is carried by a slow current through successive compartments; in the firstof which the largest grains subside; in the second of which the grains thatsettle before the water has escaped, are somewhat smaller; in the third smallerstill; until in the last there are deposited those finest particles whichhave not previously been able to reach the bottom. And in a way that is differentthough equally significant, this segregative effect of water in motion, isexemplified in the carrying away of soluble from insoluble matters -- anapplication of it hourly made in every laboratory. The effects of the uniformforces which aerial and aqueous currents exercise, are paralleled by thoseof uniform forces of other orders. Electric attraction will separate smallbodies from large, or light bodies from heavy. By magnetism, grains of ironmay be selected from other grains; as by the Sheffield grinder, whose magnetizedgauze-mask filters out the steel-dust his wheel gives off, from the stone-dustwhich accompanies it. And how the affinity of any agent acting differentlyon the mixed components of a body, enables us to take away some componentand leave the rest behind, is perpetually shown in chemical experiments.
What, now, is the general truth here variously presented? How are thesefacts, and countless similar ones, to be expressed in terms that embracethem all? In each case we see in action a force which may be regarded assimple or uniform-fluid motion in a certain direction at a certain velocity;electric or magnetic attraction of a given amount; chemical affinity of aparticular kind; or rather, in strictness, the acting force is compoundedof one of these with some other uniform force, as gravitation, etc. In eachcase we have an aggregate made up of unlike units -- either atoms of differentsubstances combined or intimately mingled, or fragments of the same substanceof different sizes, or other constituent parts that are unlike in their specificgravities, shapes, or other attributes. And in each case these unlike units,or groups of units, of which the aggregate consists, are, under the influenceof some resultant force acting indiscriminately on them all, separated fromone another -- segregated into minor aggregates, each consisting of unitsthat 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 commoninterpretation of them.
