OCR Text |
Show 144 DARWINISM CHAP. would be ample beneficial varia~ions; if . a darker colour or 1 b 1 Or wincr were reqmred, there are always a con a onger ea ( l? . . d 1' h . · 1 . sidcrable number of md1v1duals d~rker an 1g ter m c~ on1 than the average, with longer or w1th shorter beaks and wmgs, and thus the beneficial variation must.always be.present. And so with every other part, organ, functwn, or hab1t ; l:ecau. e, fts varm· t"1 0 n , so far as we know, is . and alw. ays mu. st be m the two directions of excess and defect m rela~10n to tho mean ~monn t , whichever kind of va.riation is wanted 1s alwayfis ~r1e~,ent lT~ s~nn e degree, and thus the difficulty ~s to " bene cut vanat10 ns occurrin<Y, as if they were a spec1al and rar~ cbss, falls to Uw ground. 0 No doubt some organs may vary 1~ three or perhaps more directions, as in the length, breadth, th1ckness, or c1u~a ture of the bill. But these may be taken as separate varm tions each of which a<Yain occurs as "more" or "less "; and th ns the "'r ight" or "benebf icial" or "usef u 1" van.a tw. n mu_ t a l ways be present so long as any variation at all occ_urs; and ~t has ll~Jt yet been proved that in any large or .dommant. spec10s,. ot~ 111 any part, organ, or faculty of such spee1es, there 1s no vahr!at10n. And even were such a case found it would prove not mg, so lon<Y as in numerous other species variation wa. shown to exist; bec~use we know that great numbers of species and groups throuO'hout all O'Oolocrical time have died out, leaving 110 desce~dants; anl'the gbvious and sufficient explanation of this fact is, that they did not vary enough at the time when varin tion was required to brincr them into harmony with changed conditions. The objectioon as to the "right" or "benef i cw. I " variation occurring when required, seems therefore to have no weight in view of the actual facts of variation. Isolation to prevent Intercrossing. Most writers on the subject consider the isolation of a portion of a species a very important factor in the form<1tion of new species, while others maintain it to be absolutely essential. This latter view has arisen from an cxaggornkd opinion as to the power of intorcrossing to keep clown any variety or incipient species, and merge it in the parent sto~k But it is evident that this can only occur with varieties ' vh l ('h are not useful, or which, if useful, occur in very smaH numbers; and from this kind of variations it is clear tlu1t VI DIFFICULTIES AND OBJECTIONS 145 n· e1w s1p eci·e1s do not arise. Com1)lctc isolation ' a«'' 1"n ,, · ,.n oceanic IS a~H , WI 1 no doubt enable natural selection to act more rap1dly, for several reasons. In the first place the al) f cornpct1·t 1·0 n WI· 11 f or some time allow the new ' im ·, s. ontc c o · · 11 · m1gra.n s to mcrease. rapH y t1ll they reach the limi'ts of sn 1). 'I·S t encc They will th?n st~uggle a.moucr themselves, and by snrvival of the fittest w1ll qmcldy become ad:t])tcd to the new co l't· f h · . nc I IOns o t mr etwn·onmcnt. O~gans which they formerly needed, to . defend thems. elve.s agamst' or to esc•a ·p e fr·om , enemi·e s, b_cmg no l_ongcr reqmred, would be encumbrances to be got nd of, '.~h1le the power ~f al?P~"?priating aiHl digesting new and va11ed food would nsc m Importance Th 1 · h · · · us we may ~xp am ~ ~ ongm of so many flightless and ra.thcr bulky birds m oceamc 1sland.·, as the dodo, the cassowary and the t' t moas. A g;u.n , w h"1] c t h"I S process was goin<Y o, n the ex m1 te · 1 · o , comp e e Iso at:on would. prevent its beincor checkccl by the 1· mm1· gra t"1 011 of no\~ competi_tors or enemies, _which would be very likely to o~cur m a c?~tmuous area; wh1lc, of course, any intercrossincr With the ongmal unmodified stock would be absolutely pre~ ven_ted. If, nm;, befor? this change has gone very far, the varJCty spre~ds mto adJacent but rather distant islands th somewhat chffcrc?t. conditions in each may lead to' th: development of d1stmct forms constitutin<Y what are tci' 1 t · . o me( :epresen ative species ; and these we find in the separate Islands of .the Galapagos, the West Indies, and other ancient groups of Islands. But such. cases as. these will only lead to the production of a f_ew pecul1ar spec10s, descended from the original . cttlcrs whiC~ happ.encd to reach the islands ; whereas, in wide areas and m contmcnts, we have variation and a-daptation on a much larger scale; a1:d, whenever important physical chancres demand t~cm, With e~en greater rapidity. The far ~rcatcr co~pl~xity .of the e~VJr?nment, together with the occurrence of varm~wn~ m . const1tutwn and habits, will often allow of effec~1vc I.solati_on, even here, producing all the results of actual physicalisolatwn. As we have already explained one of the ~ost _fr~quent .mo.d~s in which natural selection' acts is by aptmo .some mchv1duals of a species to a somewhat different ml·o do of. hfe' whereby the.y a1, e a ble t o sei·z e upon unappropna. t~d P a.ces m nature, and m so doing they become practically L |