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Show 150 • DARWINISM CILAP the isolated portion of the species. The struggle for exi:toncc will differ in its severity and irl its incidence from that which affects the bulk of the species. The absence of some one insect or other creature inimical to the young animal or plant may cause a vast difference in its conditions of existence, and may n~cessitate a modification of its external or internal characters in quite a different direction from that which happened to be present in the average of the individuals which were first isolated. On the whole, then, we conclude that, while isobtion is n.n important factor in effecting some modification of specie., it iH so, not on account of any effect produced, or influence ex r te<l by isolation per se, but because it is always and necessarily accompanied by a cha.nge of environment, both physica,l and biological. Natural selection will then begin to act in adapting the isolated portion to its new conditions, and wilt do this the more quickly and the more effectually because of the isolation. We have, however, seen reason to beli eve tlt:tt geographical or loca1 isolation is by no means essential to the differentiation of species, because the same result is brono·ht about by the incipient species acquiring difl'erent habits" or frequenting a different station; and also by the fact thaL different varieties of the same species are known to prefer to pair with their like, and thus to bring a.bout a physioloc•i cal isolation of the most effective kind. This pa,rt of the subject will be again referred to when the very difficult problems presented by hybridity are discussed.1 Cases in which Isolation is Ineffective. One objection to the views of those who, like Mr. Guli ek, believe isolation itself to be a ca.use of modific<Ltion of RJH'('ies deserves attention, namely, the entire absence of cha.ngc where, 1 In M~. Gulick's last fa per (.Tmwnctl q( L 1'nn. Soc. Zool., vol. xx. pp. 189- 274) he d1sctvses the varwns forms of isolation above refernHl Lo u nd<'r 110 less ~han thirty-eight difl'erent divisions and subdivisions, with a;1 elahnntLe termm_ology,_ and he nrgues that the e will frequently bring ahout clivergcnt evolu~wn w1thou~ any_ change in the environment or any action ol' 11:tlural Aelectwn. The d1scusswn of the problem here given will, I beli eve, suffi eienLly expose the fallacy of his contention ; bnt his illu stration of the v:ui c1l and often recondite modes by which practical isolation may be broucrht ahont, may help to remove one of the popular difli culties in the way of the action of natural selection in the origination of s1)ecies, Vl DIFFICULTIES AND OBJECTIONS 151 if this were a v It - - - Ireland h er causa, we should expect to find it In h . l we ave an excellent test ca,sc for we know th. . t . as )COn separated f B . . . ' a 1t epoch, certainly' man;o; nt~n smcc the end of the glacial its mammals reptiles oro~sa~ y~~rs. Yet hardly one of sl.ightcst cha~gc, even' alth~~ h ~~ ~sc~ has ~ndcrgon~ _the d1fferellce in the · g ere Is ccrtamly a d1stmct CllVIronment botl · . That changes have not occurred th .~ morgamc and organic. perhaps due to the less sever.e strucuIOrlucg hfo natu· rta l selecti.o n, is t. he smaller numb er of competm. cro os pc · r e. x1bs enc.cf o. wmg. to Itself were an efficient ca f o c~cs) ut, 1 IsolatiOn tivcly, it is incredible th~:~ ~~cii~~dco~tm~ously and cumulabeen produced in thousands of . c anoe should not have occurred in this, and man o~cai s. That ~o su~h change has prove that it is not in itsel~ . er casfes of .Isola~wn, seems to Th . a cause o modification ere yet rcmam a number of d'ffi 1 . . relating to the que tion of h b ··d·t I cu. ties and ?bjections as to require a separate chap£:/~o~ rh ';~Icdh are so l~portant en a equate discussion. |