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Show 394 DARWINISM CHAP. f t d t eth as I·n the American fossil horses; or in the in-ee an e ' · h · th t, creased development of the bmnchmg orns, as m ~ rue I h f these cases specialisation and aclaptatwn to deer. n eac o h l h . the con d1.t .w ns of the environment appear to have ·r eac e'cf ·t m1 r 1. 't and any chanO'e of these conditions, especmlly 1 1t )e altiD aIl ls ,r a•p id or accompba nied by t h e compe t1't 'w n of 1e ss d. eve.l opecl 1J ut more a d ap t..--.ble forms ' is liable to cause the extmctwn ho f 1 t h. O'hly developed O'rouns. Such we know was t e t ,1 e mo'st h tlhbe horse tribe m. bA menr.c ::t, whl·C h tota1 1 y cl 1' sappe::t1·e (1 Cci.Se Wl b t 1 in that continent at an epoch so re?ent t at we canno )e snre that the disappearance was not vntnesse~, perha~s c~use<l, 1 . while even in the Eastern hem1sphere 1t 1s the sJmY al mle arn s' pec· ws-the asses and the zebras-that have persisht ed, while the larger and more highlY: developed true horses, ave almost, if not quite, disappeared m a stat? of natm~e. So :ve find, both in Australia :wd South Amenca, tha~ 1~ a, qmte recent period many of the largest and most speemhsed forms have become extinct, while only the ~maHer types h~ve snr vived to our day; and a similar fact 1s to be obs~rved m mn.JJY of the ea.rlier geological epochs, a gro~p progressmg an.d I:ca('h• 0' a ma,ximum of size or complexity and then dymg out, mb . . or lea,ving at most but few and p1gmy reprcsenta,tlVes. Cause of Extinction of La1·ge Animals. Now there are several reasons for the repeated extinction of large rather than of sma,ll animals: In the fir. t p~ acl', animals of O'reat bulk require a proportwna,te supply of foot1, and any a<l~erse chn.nge of conditions woul<l a,{fect them more seriously than it would sm~Lllcr anima,ls. In the next l~l n.ce, the extreme speciali ation of many of thes.e lar~e amnmb wonld render it less easy for them to be m?<.hfied u: any new direction uited to changed conditions. St11l more Im~)o r t:lllt, perhaps, is the fact t?at very large animal. alw;:tys .111 ' r~:~s~ slowly as compared with small OHCs-th~ clephan~ p1 oclnc .111.~ a sinO'le young one every three years, wh1le a rabl?1t may h.1' L a litt~r of seven or eight young t·wo or three times. a ~ ·ar. Now the probability of favourable varia~ions will be m d~rc~~ proportion to the population of the specws, ~mel n,s the small.l l animals are not only many hundred times more numer~us tll ,1ll the la,rgest, but also increase perhaps a hnnclred tlm s a. XIII THE GEOLOGICAL EVIDENCES OF EVOLUTION 395 rapidly, they are able to become quickly modified by variation an~ natural selection in harmony with changed conditions, while the large a~d bulky species, being unable to vary quickly enough, arc obhged to succumb in the struggle for existen~ e ... As Profess~r Marsh well observes: "In every vigorous pnmitlve type which was de tined to survive many geological changes, there seems to have been a tendency to throw off lateral branches, which became biO'hly specialised and soon died out, because they were unable fo adapt themselves to new conditions." And he goes on to show bow the whole narrow pa~h off the persistent Suilline type, throughout tho entire senes o the American tertiaries, is strewed with the remains of such. ambitious offs.hoots, many of them attaining the size of a rhmoceros; "'_Vhile. the typical pig, with an obstinacy never l?st, has held on m spite of catastrophes and evolution, and still hves in America to-day." l1uiications of Geneml P1·ogression in Plants and Anirnals. ?ne of tb~ most powerful arguments formerly auducecl agamst evolutwn was, that geology afforded no evidence of th.e gradual development of organic forms, but that whole tnbes .and classes . appeared suddenly at definite epochs, and o.ften m great variety and exhibiting a very perfect organisation. The mammalia, for example, were Ion()' thono·ht to have first appeared in .Tertiary .times, where they ~re rep~'esented in some of the earher deposits by all the great divisions of the clas~ fully developed-:-carnivora, rodents, insectivora, marsupmls, anu even the penssodactyle aod artiodactylc divisions of t~e ungula~a-as clearly ~efined as at the pre. ent day. The discovery m 181 ~ of a smgle lower jaw in the Stonesfielcl ~late of. Oxfm~dshire hardly threw doubt on the generalisation, smce .either :t.s mammalian character ·wa.s denied, or the geological positiOn of the strata, in which it wa.s founcl, was ~eld to have been erroneously determine<.!. But since then at J~tervals of .many years, other remains of mammalia have b~en d1sc_overed m the Secondary strata, ranging hom the Upper Oohte to the Upper 'I;'rias both in Europe and the United States, and one. even (Tritylodon) in the Trias of Sontb Africa. All thes~ are e1ther ma,rsupials, or of some still lmver type of mammaha ; but they consist of many distinct forms classeu in |