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Show 120 DARWINISM CHAP. and recorrnition by members of the same variety or incipient species. 0 It has also bee~ observed that e~ch differ~ntly coloured variety of wild ammals, or of domesticated ammals which have run wild, keep together, and refuse to pair with individuals of the other colours; and this must of itself act to keep the races separate as completely as physical isolation. On the Advance of Organisation by Natural Selection. As natural selection acts solely by the preservation of useful variations, or those which are beneficial to the organism under the conditions to which it is exposed, the result must necessarily be that each species or group tends to become more and more improved in relation to its conditions. Hence we should expect that the larger groups in each class of animals and plants-those which have persisted and have been abundant throughout geological ages-would, almost necessarily, have arrived at a high degree of organisation, both physical and mental. Illustrations of this are to bo seen everywhere. Among mammalia we have the carnivora, which from Eocene times have been becoming more and more specialised, till they have culminated in the cat and dog tribes, which have reached a degree of perfection both in structure and intelligence fully equal to that of any other animals. In another line of development, the herbivora have been specialised for living solely on vegetable food till they have culminated in the sheep, the cattle, the deer, and the antelopes. Tho horse tribe, commencing with an early four-toed ancestor in the Eocene age, has increased in size and in perfect adaptation of feet and 1 teet? to. a life on open plains, and has reached its highest per- 1 fectwn m the horse, the ass, and the zebra. In birds, also, we see an advance from the imperfect tooth-billed and reptiletailed birds of the secondary epoch, to the wonderfully developed falcons, crows, and swallows of our time. So, the ferns, lycop?ds, conifers, and monocotyledons of the palreozoic and mesozoic rocks, have developed into the marvellous wealth of forms of the higher dicotyledons that now adorn the earth. But t~is remarkable. advance in the higher and larger groups does not Imply any umversal law of prorrress in orrranisation b ecause we ha v~ at the same time numer0o us exampbl es (as has' been already pomted out) of the persistence of lowly organised v NATURAL SELECTION 121 forms, and also of absolute degradation or degeneration. Serpents, for example, have been dcvclopccl from some lizard-like type which has lost its limb. ; and though this los· has enabled them to occupy fresh places in nature and to increase and flourish to a marvellous extent, yet it must be considered to be a retrogression rather than an ad vance in organisation. The same remark will apply to the whale tribe among mammals; to the blind amphibia and insects of the great caverns; ancl among plants to the numerous cases in which flowers, once specially achtptcd to be fertilised by insects, have lost their o·ay corollas and their special ad:tpta.tions, and have become dccq·aded into wind-fertilised forms. ~nch arc our plantain., ou~ meadow hurnot, and even, as some hotn.nists maintain, om rushes, sedges, and grasses. The can. cs which have led to this degeneration will be discussed in a future chapter; bnt the facts arc undisputed, and they show us that although variation and the struggle for cxi tcncc may lead, on the whole, to a continued advance of organisation; yet they also lead in many cases to a retrogression, when such retrogression mn.y aiel in the preservation of n.ny form under new co ndi tions. They also lead to the persisten ce, with slight modifications, of numerous lowly organised forms which arc suited to places which higher forms could not fully occupy, or to conditions under which they could not exist. Such arc the ocean depths, the soil of the earth, the mud of ri vcrs, deep caverns, subterranean waters, etc. ; and it is in such places as these, as well as in some oceanic islands which competing higher forms hn.ve not been able to reach, that we find ma.ny curious relics of an earlier world, which, in the free air and sunlight and in the great continents, have long since been driven out or exterminated by higher types. Summary of the first Five Chapters. We have now passed in review, in more or less detail, the main facts on which the theory of "the origin of species by means of natural selection" is founded. In future chapters we shall have to deal ma.inly with the application of the theory to explain the varied and complex phenomena pres.entcd by the organic world ; and, also, to discuss some of the theories put forth by modern writers, either as being more fundamental than |
