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Show 374 DARWINISM CllAl'. Xll in almost all the tropical oceanic islands ; but . the particula1: mode in which they are able to traverse a w1de expan e of ocean, which is a perfect barrier to batr~chia and almost. o to snakes, has not yet been discovered. ~I.zard~ are found m all the larger Pacific Islands as far as Tah1t1, wh1le snakes do not extend beyond the Fiji Islands; and t~e latter are also ltb~ent from Mauritius and Bourbon, where hzards of seven or mgh t species abound. Naturalists resident in the Pacific Islaucls would make a valuable contribution to our science by study in()' the life-history of the native lizards, and endeavouring to as~ertain the special facilities they possess for crossing over wide spaces of ocean. CHAPTER XIII THE GEOLOGICAL EVIDENCES 01<' EVOLUTION What we may cxpect-·The number of known species of extinct animal - Causes of the imperfection of the geological record-Geological evidences of evolution- Shells-Crocodiles-The rhinoceros tribeThe pedigree of the horse tribe-Development of deer.' homs-Brain development- Local relations of fossil and living auimals -Cause of extinction of large animals-Indications of general progr ss in plauts and animals-The progressive development of plants-Possible cause of sudden late appearan ce of cxogens-Geological distribution of insects-Geological succession of vertebrata-Conclncling remarks. THE theory of evolution in the organic world necessarily implies that the forms of animals and plants have, broadly speaking, progressed from a more generalised to a more specialised structure, and from simpler to more complex forms. We know, however, that this progression has been by no means regular, but has been accompanied by repeated degradation and degeneration ; while extinction on an enormous scale has again and again stopped all progress in certain directions, and has often compelled a fresh start in development from some comparatively low and imperfect type. The enormous exton. ion of geological research in recent times has made us acqnainted with n, vast number of extinct organisms, so vast that in some important groups-such as the mollusca-the fo sil arc more numerous than the living species; while in the mctmmalia they are not much less numerous, the preponderance of living species being chiefly in the smaller anu in the arboreal forms which have not been so well preserved as the member of the larger groups. \Vith such a wealth of material to illustra.te the successive stages |