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Show 310 GEOGRAPHICAL DISTRIBUTION. [OIIAP. XI. plainly related by inheritance to the inhabitants of the continent. Cases of this nature arc com1non, and are, as we shall hereafter more fully see, inexplicable on the theory of independent creation. Thi~ view of the relatio~ of species in one region to those 1n an?ther, docs n_ot d1ffer much (by substituting the word vanety for speCies) from that lately advanced in an ingenious .paper ~y Mr. vVallace in which he concludes, that " every speCies has con1e into' existence coincident both in s~ace and time with a pre-existing closely allied species.' And I now know from correspondence, that this coincidence he attributes to generation with modification. The previous remarks on " single and multiple centres of creation " do not directly bear on another allied question,- namely whether all the individuals of the same species have descended from a single pair, or single hermaphrodite, or whether, as some authors suppose, from many individuals simultaneously created. With those organic beings which never intercross (if such exist), the species, on my theory, must have descended fr01n a succession of improved varieties, which will never have blended with other individuals or varieties, but will have supplanted each other; so that, at each successive stage of modification and improvement, all the individuals of each variety will have descended from a single parent. But in the majority of cases, namely, with all organisms which habitually unite for each birth, or which often intercross, I believe that during the slow process of modification the individuals of the species will have been kept nearly uniform by intercrossing ; so that many jndividuals will have gone on sbnultaneously changing, and the whole amount of modification will not have been due, at each stage, to descent from a single parent. To illustrate what I mean: our English race-horses differ slightly from the horses of every other breed ; but they do not owe their difference and superiority to descent from any single pair, but to continued care in selecting and training many in-dividuals during many generations. Before discussing the three classes of facts, which I have selected as presenting the greatest amount of diffi- OJIAP. XI.] MEANS OF DISPERSAL. 311 culty on the theory of " s · must say a few words on th~ng1 e cenftre~ of creation" I means o dispersal. ' .Means of .Dispersal.-Sir C L have ably treated this subject · I yell a.nd other authors briefest abstract of the mo ·. can give here only the of climate must have had a re Imp?rt~nt facts. Change tion: a region when its cliJ~;"erfulu~fiuence on migrabeen a high road for mi ratione was dilfcrei~t may have I shall, ~owever, presentlg have' but. now be.Impassable; the subJect in some detail Ch to discuss tlns branch of must also have been highly infiuang~sl ~f level in tf!e land now separates two marine fau en~Ia . a narro;v Isthmus formerly have been submer dnas' submerge It, or let it now blend or may fonnerly ~e ' a~f the two faunas will now extends, land rna at a £ ave en~ed : where the sea islands or possibly e~en o~~er period have connected have allowed terrestrial ~~~ In~nts together, and thus the other. No geologist $·ll ;.ctlons to pass frorn one to of level have occurred wfth. Isfhte th~t great mutations ganisms. Edward Forbes . IJ?- t de 1_eriod of e~isting orthe Atlantic must recentl~n~s e ~ at all the Islands in Europe or Africa, and Euro ave. ee~ co~nected with Other authors have thus h pe .hkewis~ With America. ocean, and have united alypo~hetlcally bridged over every land. If indeed the ar mos every Island to some maintrusted, it must be ad!"?-ments used by Forbes are to be exists which has not re~tt~f t~at scar?ely a single island nent. This view cuts th!nGy d~en united to some contiof the same species to the mos~r d .Ian knot. of the dispersal many a difficulty : but to th ~tant foints,, and removes ~re not authorized in admittin e est o my JUdgment we Ical changes within the . g such e~or;mous geographseems to me that h period of existing species. It oscillations of levei~ ave abundant evidence of great vast changes in thei~n p ou~. continents ; but not of such united them within the OSI IOn an_d extension, as to have the several intervenin recent :per.Iod to each other and to the former existence ofg ocea~Ic Islands. I freely admit the sea, which may h many Islands, now buried beneath ave served as halting places , for |
