Selection.—The first and most important of the directive factors is natural selection, and the most distinctive contribution which Darwin and Wallace made to ætiology was to show how selection works and what it can effect. The process admits of brief statement.
Variability is a fact of life, the members of a family or species are not born alike; some may have qualities which give an advantage both as to hunger and love; others are relatively handicapped. But a struggle for existence, as Malthus called it, is also a fact of life, necessitated especially by two facts—first, that two parent organisms usually produce many more than two children organisms, and that population thus tends to outrun the means of subsistence; and, secondly, that organisms are at the best only relatively well-adapted to the complex and changeful conditions of their life. This struggle expresses itself not merely as an elbowing and jostling around the platter of subsistence, but at every point where the effectiveness of the response which the living creature makes to the stimuli playing upon it, is of critical moment. As Darwin said, though many seem to have forgotten, the phrase "struggle for existence" must be used "in a wide and metaphorical sense." It includes much more than an internecine scramble for the necessities of life; it includes all endeavours for and all changes that make towards preservation and welfare, not only of the individual, but of the offspring as well. In many cases, indeed, the struggle for existence both among men and beasts is fairly described as an endeavour after well-being, and what may have been primarily self-regarding impulses become replaced by others which are distinctively species-maintaining, the self failing to find full realisation apart from its kin and society.
Now, in this struggle for existence, which has so many expressions, the relatively less fit to the present conditions tend to be eliminated. Though the process may work out progress, as measured by degree of differentiation and integration, by increasing freedom and fullness of life, and has doubtless done so, yet until we come to its highest forms in subjective and finally rational selection, it works not towards an ideal but towards a relative fitness to present conditions. And this may spell degeneration, as in parasites, when an extrinsic standard is used. Tapeworms may be just as fit to survive as golden eagles. Again, the process of elimination does not necessarily mean that the handicapped variants come at once to a violent end, as when rat devours rat, or the cold decimates a flock of birds in a single night; it often simply means that the less fit die before the average time, and are less successful than their neighbours as regards pairing and having offspring. Moreover, although the selective process is primarily eliminative or destructive, like thinning turnips or pruning fruit-trees, we cannot separate its positive and negative aspects. That nothing succeeds like success is continually verifiable in nature, the fit variant gets a start just as surely as the unfit variant is handicapped; there is favouring and fostering just because there is sifting and singling.
Given variations and given some mode of selection in the manifold struggle for existence, the argument continues, then the result will be in Spencer's phrase "the survival of the fittest." And since many variations are transmitted from generation to generation, and may, through the pairing of similar or suitable mates, be gradually increased in amount and stability, the eliminative or selective process works towards the establishment of new adaptations and the origin of new species.
Darwin thought chiefly of the struggle between individuals—either between fellows of the same kin or between fellow-kin and foreign foes—and of the struggle between organisms and the inanimate environment. He also emphasised the sexual selection which occurs (a) when rival males fight or otherwise compete for the possession of a desired mate or mates, and in so doing reduce the leet, and (b) when the females appear to choose their mates from amid a crowd of suitors. While many now doubt if the range and effectiveness of preferential mating is so great as Darwin believed, there seems no reason to doubt that this mode of selection has been a factor in evolution. There are facts which warrant us in saying that das ewig weibliche plays a part in the upward march of life, that Cupid's darts as well as Death's arrows have evolutionary significance.
Even more important, however, are other extensions of the selection-idea. There may be struggle between groups as well as between individuals, as when one ant-colony goes to war with another, and there may be struggle of the parts within the organism just as there is struggle between organisms. There is struggle when one ovum survives in an ovary by devouring all its sister-cells, as in the case of Hydra and Tubularia, and, after allowing a wide margin for chance, there may be some form of selection among the crowd of spermatozoa encompassing the egg which only one will fertilise, just as there is some form of selection among the many drones which pursue the queen-bee in her nuptial flight. Weismann has carried the selection-idea to a logical finesse in his theory that there may be a struggle between the different sets of hereditary qualities in the germ-cell, or that there is a process of "germinal selection" at the very beginning of the individual life. There are, we admit, great differences between the struggle of hereditary items and the struggle of large parts within the organism; between intra-organismal and inter-organismal struggle; between the competition of individuals and the struggle against physical nature; between personal selection and the conflict of races; between objective and subjective selection; but, as it seems to us, they may be all expressed in the same formula if it is useful so to do.
Isolation.—In organic evolution variation supplies the materials which some form of selection sifts. But besides selection another directive factor has been indicated in what is called the theory of isolation. A formidable objection to the Darwinian doctrine, first clearly stated by Professor Fleeming Jenkin, is that variations of small amount and sparse occurrence would tend to be swamped out by inter-crossing before they had time to accumulate and gain stability. In artificial selection, the breeder takes measures to prevent this swamping-out by deliberately pairing similar or suitable forms together, or by deliberately removing unsuitable mateable forms; but what in Nature corresponds to the breeder?
It may be that similar variations occur in many individuals at once and many times over; it may be that many variations are not at first small in amount, but express big steps in organisation, as in Bateson's instances of Discontinuous Variation or in De Vries's instances of Mutation; it may be that many variations are not from the first unstable, but express changes of organic equilibrium which have come to stay if they get a chance at all; and it may be that the supposed swamping effects of inter-crossing are in part illusory, as is strongly suggested by some of the facts summed up in Mendel's Law; but there seems to be still room and need for the theory of Isolation worked out by Romanes, Gulick, and others.
They point out the great variety of ways in which, in the course of nature, the range of inter-crossing is restricted—e.g. by geographical barriers, by differences in habit, by psychical likes and dislikes, by reproductive variation causing mutual sterility between two sections of a species living on a common area, and so on. According to Romanes, "without isolation, or the prevention of free inter-crossing, organic evolution is in no case possible." The supporting body of illustrative facts is still unsatisfactorily small, but there seems sound sense in the idea.
An interesting corollary has been recently indicated by Professor Cossar Ewart. Breeding within a narrow range often occurs in nature, and often in human kind, being necessitated by geographical and other barriers. In artificial conditions, this in-breeding often results in the development of what is called prepotency. This means that certain forms have an unusual power of transmitting their peculiarities, even when mated with dissimilar forms, or, in other words, that some variations have a strong power of persistence. Therefore, wherever through in-breeding (which implies some form of isolation) prepotency has developed, there is no difficulty in understanding how even a small idiosyncrasy may come to stay, even although the bridegroom does not meet a bride endowed with a peculiarity like his own. Similarly, Dr A. Reibmayr has argued that the establishment of a successful human tribe or race involves periods of in-breeding (i.e., marriage within a limited range of relationship), with the effect of "fixing" constitutional characteristics, and periods of cross-breeding (i.e. marriage between members of distinct stocks), with the effect of promoting a new crop of variations or initiatives.