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Show GEOGRAPHIOAL DISTRIBUTION. [OIIAP. XI 308 America~ The conditions of life are ne:nly the same, so that a multitude of European animals an~ plants have become naturalised in America and Austraha; and some of the aboriginal plants are identically the same. at these distant points of the northern and southern hem1spheres ! The answer as I believe, is, that man1mals have not been able to migrate whereas some plants fro1n their varied n1eans of dispe;sal, have migrated acro~s .the ;ast and broken interspace. The great and stnlnng Influence which barriers of every kind have had on distribution, is intelligible only on the view that the great majority of species have been produced on one side alone, and have not been able to Inigrate to the other side. Some few families, many sub-families, very n1any genera, and a still greater nu1nber of sections of genera are confined to a single region ; and it has been observed by several naturalists, that the most natural genera, or those genera in which the species are most closely related to each other, are generally local, or confined to one area. What a strange anomaly it would be, if, when coming one step lower in the series, to the individuals of the same species, a directly opposite rule prevailed ; and species were not local, but had been produced in two or more distinct areaHs!e nce it see1ns to me, as it has to many other natu-ralists, that the view of each species having been produced in one area alone, and having subsequently migrated from that area as far as its powers of migration and subsistence under past and present conditions permitted, is the most probable. Undoubtedly many cases occur, in which we cannot explain how the same species could hav~ passed fron; one point to th~ other. But the geographical and chmatal changes, ,vhich have certainly occurred within recent geological thnes, must have interrupted or rendered discontinuous the for1nerly con· tinuous range of many species. So that w~ are reduced to consider ,vhether the exceptions to continuity of range are so numerous and of so grave a nature, that we ought to give up the belief, rendered probable by general .co~siderations, that each species has been produced w1th1n OIUP. XI.] SINGLE OENTRES OF OREATION . 309 one area, and has migrated thenc . would be hopelessly tedious to disc: as far It could: It cases of the same species now li . ss ~11 d ~he exceptional arated points ; nor do I 'for a Vlng a Istant and sepexplanation could be offered ofoment pre~end that any after some preliminary remarks J:aX:ll s~? cases. But the most striking classes of fact~ . ;;1 1 lscuss a few of of the same species on the summ. t afJ· y, the existence ranges, and at distant pojnts in ~~ 0 1.stant mountainregions; and secondly (in the foil e. arc~c and antarctic distribution of fresh-water produ~~~~~? apter~.the wide occurrence of the same terrestrial . ' and ~ udly, the on the mainland, thoug-h separatedstecY~B on lslands ~nd of open sea. If the existence of th y undred~ of miles ~ant and isolated points of the earth~ safe speme~ at dismstances be explained on the v· s sur ace, can 1n many !fiigrated from a single birth la~e'; of each spe?ies .having Ignorance with respect to for~e e ll the~,l considering our cal changes and vari . ~r c rna a and geographi- )>elief that this has ~!~ 0~h::1~~~l ~einl of transport, the mcomp~rably the safest. Iversa aw, seems to me In d1scussing this s b · t h same time to consideru a~e~inwe s all b~ enabled at the na~ely, whether the sevm~l d~s:.qu:lly l~portant for us, winch ?n my theory have all d me d~c~~es of a genus, pro~enitor, can have mi rated escen e . rom a common dunng some part of the· g . (undergoing modification ited by their pro enitor Ir migration) from the area inhabinvariably the ca~e that If It. can be s~own to be almost habitants are clos~ly ret {~gi~n, of which most of its ingenera with the s e . a e o, or belong to the same received at some f~r~: of ~ ~e~on~ region, has probably region, my theory will b per;o Ul~migrants from this other !Y understand on the p~. 8 ~e~gt fened ; for we can clearInhabitants .of a region Iili~~l~ o modification, why the ~nother region whe . t h be related to those of Island, for inst~nce :c~e~ ~s been stocked. A volcanic of a few hundreds ~f p .1 vf and formed at the distance abl" receive from it .m~hs rom a co~tinent, would pro b-an their descend~;~ tho~guhse ofd~filmde a few colonists, ' mo 1 e , would still be |