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Show 120 DARWINISM CHAP. and recoO'nition by members of the same variety or incipient species. bIt has also bee~ observed that ea: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 be 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. The horse tribe commencing with an early four-toed ancestor in the Eocen~ age, has increased in size and in perfect adaptation of feet and, teet~ to. a life on open plains, and has reached its highest pcr- 1 fectwn m the horse, the ass, and the zebra. In birds, also, we se~ an a.dvance from the imperfect tooth-billed and reptiletailed birds of the secondary epoch, to tho wonderfully developed falcons, crows, and swallows of our time. So, tho ferns, lycop?ds, conifers, and monocotyledons of the pahBozoic and mesozOic rocks, have developed into the marvellous wealth of forms of the higher dicotyledons that now adorn the earth. But this remarkable advance in the hiO'ber and larO'cr groups d t . 1 . b b b oes no unph y any umversal law of proO'ress in orO'anisation b b , ecause we av~ at the same time numerous examples (as has been already pomted out) of the persistence of lowly organised v NATURAL SELECTION 121 forms, and also of absolute degra-dation or degeneration. Rerpents, for example, h:wc been dcvclopc<l from some lizard-like type which has lost its limb ; and thongh this lo · has enabled them to occupy fresh places in nature and to increase and flourish to a marvellous extent, yet it mnst be considered to b a retrogrc sion rather than an ad vance in organisation. The same remark will apply to the whaJc tribe among mammals; to the blind amphibia and insects of the great caverns; and amonO' plants to the numerous ca. cs in which flowers, once spcci~ly adn.ptccl to he fertilised by insects, have lost their gay corollas and their special adaptations, and have bec~mc dearacled into wind-fertilised form . . Snch arc our plantam., 0 . . . our meadow burnet, and even, as some hotamsts mmntam, our rushes, sedges, and grasses. The <.;a.w:;es \vhich have le<l to this degeneration will be di cussed in <t futuro chapter; hut the facts are nndispntod, and they show us that although variation and the struggle for existence m<Ly lead, on the whole ' to a continued advance of orga11isation; yet they a.l so lead in many cases to a retrooTo .. ion, when such retrogr?~s1on mn.y aiel in the preservation of any form nndcr new cond1t10ns. They also lead to the persi. tcncc, wiLh .. light mo<lific<ttions, of numerous lowly organised form· which <trc . uited to l~l~tcos which hio·hcr forms could not fully occupy, or to concbt10n under which they could not exist. • uch <Lrc tho ocean depths, the soi.l of the earth, the mud of river., deep caverns, subterranean wa.tcrs, etc.; and it is in su<.;h places as those, as well as in some oceanic islands which competing higher forms have not been able to reach, that ·we :find many 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 extormina. tcd by higher types. Surnmm·y of the first Five Chapters. We have now passed in review, in more or Jess d ta~l, the main facts on which the theory of "the orjgin of specJC by means of natura.l selection" is founded. In future ch:tpters we shall have to deal ma.inly with the application of the theory to explain the varied and complex phenomena pres.ented _by the orga.nic world; and, al o, to discuss some of the theones put forth by modern writers, either as being more fuudamcntal than |