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Show GEOGRAPHICAL DISTRIBUTION. [CHAP. XII. 352 . . f the world of other spec1es existence at remote P?1nts 0th and more general way. allied to it, is shown Ill anf er ago that in those genera Mr. Gould remarked to me ;ng orld many of the species of birds whi~h range ovel ~a~ hardly doubt that this ru.le have very w1de ranges. 't ould be difficult to prove 1t. is generally true, tfough 1 -; it strikingly displayed in A1nongst mammas, we se in the Felidre and Oanidre. Bats, an.d i? a lesser ~:!~:distribution of butter~ies a~d We see It, If Vfe ~om:pth most fresh-water productions, In beetles. So It IS Wl . e over the world, and many which so man~ genera 1 U:~ous ranges. It is not n1eant individual species ~ave en r all the species have a wid.e that in world-ra~g~~~ege~:~e on an average a w~de range; range, or event a f lhe s ecies range very widely; for but onl;r. that ~o-{:e ~ich wiaely-ranging species v~ry and t~e fa~Ibty Wlt £ w will largely determine thmr av~rgive nse to new . or;ns two varieties of the same species age range. F?r Ins ance, and the species thus has an inhabit America and Europh e, · t' had been a little . but if t e vana 10n . . immense range ' . t. would have been ranked as dis-greater, t~e two varle Ies mmon ran e would have be.en tinct species, and th~ 1 cf s is it rrfeant that a species gre~tly reducedi ~~! th~ capacity of ~rossing barriers whiCh a~pare~~ ~ in the case of certain powerfullyand rang~ng WI. e y, as .· ran e widely; for we should winged birds, will necessanly . deYy implies not only the never forget ~hat to ~ange :; the more important power power of c.rossi~g b~rrd~r:, bt lands in the struggle for life of being VIctoriou~ In IS an n the view of all the spewith foreign assoCia~es. d But oded from a single parent, cies of a gend~sth.bvi~~ toe~~e: most remote points of the though now IS ri u. e d I believe as a general rule world, we ought to findt a? t of the species range very we do find, t~at .some a eas that the unmodified pare?t widely ; for lt ~s necessary . modification during Its s~oul~ range Wide{;{' ~~~:~f~~Ugunder diverse condit~ons diffusion, and shou P . f its offspring, firstly Into favourable for the co~ version. oto new s ecies. newI nva riet~es ~nd ultim~dtelyd·l~ 'bution~f certain genera, considering the WI e IS ri CHAP. XII.l OCEANIC ISLANDS. 353 we should bear in mind that some are extremely ancient, and must have branched off from a common parent at a remote epoch ; so that in such cases there will have been ample time for great climatal and geographical changes and for accidents of transport; and consequently for the migration of some of the species into all quarters of the world, where they may have become slightly modified in relation to their new conditions. There is, also, some reason to believe from geological evidence that organisms low in the scale within each great class, generally change at a slower rate than the higher forms ; and consequently the lower forms will have had a better chance of ranging widely and of still retaining the same specific character. This fact, together with the seeds and eggs of many low forms being very minute and better fitted for distant transportation, probably accounts for a law which has long been observed, and which has lately been admirably discussed by Alph. de Oandolle in regard to plants, namely, that the lower any group of organisms is, the more widely it is apt to range. The relations just discussed,-namely, low and slowlychanging organisms ranging n1ore widely than the high, -some of the species of widely-ranging genera themselves ranging widely,-such facts, as alpine, lacustrine, and marsh productions being related (with the exceptions before specified) to those on the surrounding low lands and dry lands, though these stations are so different-the very close relation of the distinct species \vhich inhabit the islets of the Saine archipelago,-and especially the striking relation of the inhabitants of each whole archipelago or island to those of the nearest mainland,-are, I thi.nl{, utterly inexplicable on the ordinary view of the independent creation of each species, but are explicable on the view of colonisation fr01n the nearest and readiest source, together with the subsequent n1odification and better adaptation of the colonists to their new homes. Summary of last and present Ohapters.-ln these chapters I have endeavoured to show, that if we make due allowance for our ignorance of the full effects of all |
