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Show ISLAND LIFE. (PAnT I, from the m. flux of new or more highly organise.d animals, w,e may there expect to fi n d the rema·m s of groups whiCh have elsewhere become extm. ct . I n I.S 1a n ds which have been long separal ted from the.u paren t con t'm en t~:s these condition. s •a re exactly fulfi led, and I. t I.S m. sue h tha t we find the most stnlnnOo ' exam. ples of tho preservatw· n of f ragme nts of primeval. oo -rou ps of. amma. ls, often wi·d ely separat ed f ro m each other, owm(o)' to theu hav. mg been preserve d a t remo t e Portions of the area of the. once . widespr.e ad parenta1 group. There are many other ways m whiCh portwns 0 f d ym· g ou t group s may be saved · Nocturna. l or subterra.n ean modes of life may save a. species from enemies or competit~rs, and many of the ancient types still existing have such habits. The dense gloom of equatorial forests also. affords m.eans of concea1 m ent and Protection' and we sometimes find m such localities a few remnants of low types in the midst of .a general assem bla ge of hi. o() 'her forms • Some of the most anment types now II. vm· g 1' n habi't caves ' like the Proteus,. or bury thems. elves in mud like the Lepidosiren, or in sand hke the A.mp~wxu s, the last being the most ancient of all v~rtebrates; wh1le the Galeopithecus and Tarsius of the Mal~y Islands and .the potto of west Africa, survive amid the higher . mammaha of t~e Asiatic and African continents owing to the1r nocturnal habits and concealment in the densest forests. . The Rise and Decay of Species and Genera.-The proco~mg sketch of the mode in which species and genera have ansen, have come to maturity, and then decay, leads us to som~ very important conclusions as to the mode of distribution of a~1mal~. When a species or a genus is increasin~ and spreadmg, It necessarily occupies a continuous area whiCh g:ts larger and larger till it reaches a maximum; and we. accordmgly find that almost all extensive groups are thus contmuous. When. dec~y commences, and the group, ceasing to be in harmony WI~h :ts environment, is encroached upon by other .forms, th~ contmu:ty may frequently be broken. Sometimes the outlymg s~ecies may be the first to become e~tinct, and the group may SI.mply diminish in area while keepmg a compact central mass' but more often the process of extinction will be very irr~gular, and may even divide the group into two or more dtsconnoctecl CHAP, Iv.] EVOLUTION THE KEY TO DISTRIDUTION. 63 portions. This is the more likely to be the case because the 1~ost recently formed species, probably adapted to local conditiOns and therefore most removed from the general type of the group, will have the best chance of surviving, and these may exist at several isolated points of the area once occupied by the whole group. We may thus understand how the phenomena of discontinuous areas has come about, and we may be sure that when allied species or varieties of the same species arc found widely separated from each other, they were once connected by intervening forms or by each extending till it overlapped the other's area. Discontinuo~ts Specific Areas, why rare.-But although discontinuous generic areas, or the separation from each other of species whose ancestors must once have occupied conterminous or overlapping areas, is of frequent occurrence, yet undoubted cases of discontinuous specific areas are very rare, except, as already stated, when one portion of a species inhabits an island. A. few examples among mammalia have been referred to in our first chapter, but it may be said that these are examples of the very common phenomenon of a species being only found in the station for which its organisation adapts it; so that forest or marsh or mountain animals are of course only found where there are forests, marshes, or mountains. This may be true, and when the separate forests or mountains inhabited by the same species are not far apart there is little that needs explanation; but in one of the cases referred to there was a gap of a thousand miles between two of the areas occupied by the species, and this being too far for the animal to traverse through an uncongenial territory, we are forced to the conclusion that it must at some former period and under different conditions have occupied a considerable portion of the intervening area. Among birds such cases of specific discontinuity are very rare and hardly ever quite satisfactory. This may be owing to birds being more rapidly influenced by changed conditions, so that when a species is divided the two poz:tions almost always become modified into varieties or distinct species ; w bile another reason may be that their powers of flight cause them to occupy on the average wider and less precisely defined areas than do_ the species |