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Show G EOG RA.Plll CAL DISTRIBUTION. [CHAP. XI. 306 migration; as does time for the slo~ process ~f modifi.ca· tion through natural selectio~. Widely-ranging. species, abounding in indi vi~uals, .which .have alre~dy tnumphed over 1 nany competitors In thmr O'!Il; Widely-extended homes, will have the best chance ?f seizing O"-?- new places, when they spread into new countr1.e~. In thmr. new homes they will be exposed to ~ew c?nd1twns1 3Jld w1ll frequent- 1 undergo further modi~catlon and ~mpr?vement ; a~d thus they will become still further victonous, and will produce groups of modifi~d de~cendants. On this principle of inheritance With modrfication, we can understand how it is that sections of genera, whole gen.era, and even families are confined to the same areas, as 1s so commonly and notoriously the case. .· . I believe as was remarked In the last chapter, 1n no law of neces~ary develop1nent. .As the va:iability of each species is an independent property, and Will be taken advantage of by natural _selection, only so fa: as it profits the individual in its complex struggle for hfe, so the degree of modification in dilferent species will be ~o unifo:rn quantity. If, for instance, a number of species, whiCh stand in direct competition with each other, migrate in a body into a new and afterwards isolated country, they will be little liable to modification; for neither migration nor isolation in themselves can do anything. These principles come into play only by bringing organisms i~to new relations with each other, and in a lesser degree With the surrounding physical conditions. .As w~ have seen in the last chapter that some forms have reta1ned nearly the same character from an enormously remote geological period, so certain species have migrated over vast spaces, and have not becon1e greatly modified. . On these views, it is obvious, that the several spec1es of the same genus, though inhabiting the most distant quarters of the world, must originally have proceeded from the same source, as they have desce"?-ded f:om the same progenit~r. In the case o~ those ~pecies, w~ICh have undergone dunng whole geolo~cal peno~s but. ht~le mod· ification there is not much difficulty 1n behev1ng that they m;y have migrated from the same region ; for dur· 0BA.P. XI.] SINGLE CENTRES OF OR"'ll:A.TION . . 307 1ng the vast geographical a d 1. will have supervened f:\ince n ~ Imatal changes which amount of migration i; poss'abnlmentBtim~s, a]most any cases, 1. n w hI'C h we have reasonl t e. b ut In rna th 1. ny o er of a g~nus have been produced ~it~ .I eve that th~ species cent times, there is great d 'ffi If In COI~paratively realso obvious that the indivld c~ Y ~n ~his head. It is though now inhabitina- distant ua ~ .0 1 t e sa~e species, have proceeded from ~ne s ot an ISO ate~ regions, must first produced: for as ex 1 ~ ' ":here thmr parents were ,incredible that individuJs ai~~~tn 1fe last chapter, it is ever have been produced throu ca Y the sam~ should parents specifically distinct. gh natural selection from We are thus brought to th . . largely discussed by natural' t quest\on whiCh has been have been created at one o;s::; name r' whether species surface. Undoubtedly th ore pOints of the earth's t d reme I'f fi culty in undersetr e da'r e vehr y many cases of ex-could possibly have miO'rate~n f Ing ow the san;e species several distant and is~lated ro~n some one point to the Nevertheless the simplicit of l~Int~, where now found. was first produced within y . ! VIew. that each species mind. He who rejects it : .si~g eh region captivates the nary generation with sub'se e~ec s t ~ verr;t causa of ordithe agency of a miracle r{·. ent !lligration, and calls in in ~ost cases the area i~habi~~du~versally .ad~itted, !hat o~s' and when a plant or . ~ a sp.emes lS contJnudistant from each other o ani~hl Inh~bits two points so n~ture, that the s ace co~l r WI an n;terval of such a migration, the faft is giv d not be e~tsily passed over by exceptional. Theca aci~n as s~met~Ing remarkable and more distinctly limit~d . ~of mig:rating across the sea is haps in anv other or()'a~~c ~r~stnal mammals, than perfi~ d no inexplicable c~ses f t~ngs; and, accordingly, we d~stant points of the w fa Nsame mammal inhabiting difficulty in suoh cases or G. t ~ ~e,~logist will feel any merly united to Euro easa rea nta:ln having been for~ e quadrupeds. ntt ifnth consequentl:y: possessing the uced at two separate o. t e same species can be promammal common t IE rn s, why do we not find a single o urope and Australia or South |