OCR Text |
Show I 146 DARWINISM. OHAP, isola,ted from their parent form. Let us suppose, for example, that one portion of a species usually living in forests nwgcs into the open plains, and finding abundance of food remains there permanently. So long as the strun·glc for existence is not exceptionally severe, these two portions of the species may remain almost unchanged ; but suppose some fresh enemies arc attracted to the plains by the presence of these new immigrants, then variation and natural selection would lead to the preservation of those individuals best able to cope with the difficulty, and thus the open country form would become modified into a marked variety or into a distinct species ; and there would evidently be little chance of this modifi ca,tion being checked by intercrossing with the parent form which remained in the forest. Another mode of isola.tion is brought about by the variety -either owing to habits, climate, or constitutional changebreeding at a slightly different time from the parent species. This is known to produce complete isolation in the case of many varieties of plants. Yet another mode of isob.tion is brought about by changes of colour, and by the fact that in a wild state animals .of similar colours prefer to keep toaether and refuse to pair with individuals of another colour. b Tho probable reason and utility of this habit will be cxphincd in another chapter, but the fact is well illustrated hy the cattle which have run wild in the Falkland Islands. These are of several different colours, but each colour keeps in a separate herd, often restricted to one part of the isla.ml ; and one of these varieties-the mouse-coloured-is said to breed a month earlier than the others· so that if tl1iH variety inhabited a larger area it might ;cry soon be esta h- 1ished as a distinct race or spccies.1 Of course where the chan~e of . habits or of station is still greater, as when a terrestnal an~mal. becomes sub-aquatic, or when aqua.tic animals come. to hve m tree-tops, as with the frogs and crustacea desc:1?ed at p. 118, the danger of intcrcrossing is reduced to a mm1mum. Several writers, however, not content with the indirect effects of isolation here indicated, maintain that it is in itself a cause of modification, and ultimately of the origination of 1 See Va1-iation of Animals and Plants, vol. i. p. 86. VI DIFFICULTIES AND OBJECTIONS ===~:::---~--~===----~147 new species. This was the keynote of M essay on "Variation of Species, 1 I" h rd ~ ernon Wolbston's adopted hy the Hcv. J. G G 1'. 1P~1 ) h 1 s. 0 m 18!56, ancl it is f E l . . u l c ( ll1 1 ' p:tJ)Cr " D. o vo ntwn under one Set of E t ·1 ' ?1 .1 1 versit.y 1 . S '7 x crna Conchti " (J Ann. oc. ~ool., vol. xi. p. 496). The . ons oum. that there IS an inherent t d Idea seems to be d . en cncy to vari t" · Ivcrgent lines and that wh . .' a. JOn m certain I·S O 1a t cc1, even th' ouO'h under ideen t" one p01twn of · 1 . . a speCJes is sets up a divcrO'cnbce which c n ~ca clon<htwns, tba.t tendency farther away fro~n the orio·inalarncs. t 1:tt portion farther and b o species This · · h c supportccl by the case of the I l. b 1. vJcw IS c]cl to Islands, which certainly p . tanc s c Is of the Sandwich h rosen some v . p cnomcna. In this comparat· 1 · 01Y remarkable about 300 species of land shells IV:l y small area .there are to one family (or sub-£ .1. ' most all ~f wlnch LelonO' nowhere else in the world~nn ~~he tl~c Ac~~ttnlClli~w, found extreme restriction of th . mtcrc. tJJJg pomt is the average rancrc of each s c . spc~JCs and varieties. The h '] b < pCCICS IS only fiv . . w l e some arc restricted to but on , e or f'JX miles, and only a very few ranO'e over he 1 or. two squn.re miles, ~egion that extends ove~ one of athw o c lS1a~cl. The forest Island of Oahu, is about f ·t 'l e. mountam -ranges of the 'I . Ol y ml es ll1 lcnuth ] fi m1 es m breadth· and tl · ll . b · anc vc or six 17 5 ' ns sma tern tory f . b species, represented by 7 00 urm. cs about Gulick states that the t . or 800 varieties. Mr h ' vcgc atwn of the l' ff' . on t e same side of th' . c 1 erent valleys h Ismn~~mu~ ili as a molluscn.n fauna differi~a . c same, yet each of any other "We f ol m some degree from that . · requcnt y find a m ~everal successive valleys b ]]' 1 gcn~s represented feedmg on the same t' y a lCC spccJCs, sometimes , some 1mes on d 'it 1 every sueh case the 11 l ercnt p ant . In furnish the most nearr; ~rs dthtt arc nearest to each other varieties of each species a r~!cn orms ~ and a full set of the between the more divcrO'~nt t ts a mmutc. gradation of forms separated localities" H ypes found m the more widely cannot be attribut~d t e l~rgei that t~csc constn.nt differences in different valleys o othna ura s.clcctwn, hccausc they occur f d 1 . n e same side of th . oo ' c Imate and en . c mountam where th ere I.s no gr'e ater diffecmrel es a.r e the . sa, me; unc] also, h' ccau o dry side of the mountainnc:hm l~assJng.from the rainy to the . s an Ill passmg from one valley to |