12 Geographical Diatribution of Animals. 



specfes of Leuciscus have been created in one of the basins, — 

 in the Danube for instance, — and have migrated in such a way, 

 that a certain number of the species should remain solely in 

 the Danube, while some others left the Danube altogether 

 to settle finally only in the Rhone, and others to settle only 

 in the Rhine ; that one accompanying those species peculiar 

 to the Rhone, remained in the Danube with those species 

 peculiar to it, and settled also in the Rhone, with those 

 species peculiar to that river, and also in the Rhine with the 

 species peculiar to the Rhine ? And whether we assume the 

 Rhone as the primitive centre, instead of the Danube or the 

 Rhine, the argument holds equally good. We have one 

 species common to the three rivers, and several species pecu- 

 liar to each, which could never have migrated (if migration 

 took place) in such a manner as to assume their present 

 combinations. But if, on the contrary, we suppose that all 

 the species originated in the rivers where they occur, then 

 we have again a multiple origin of that species which is 

 common to the three, for it were wonderful if that one alone 

 had migrated, when they are all so closely allied. Here, 

 again, we arrive at the conclusion, that the same species can 

 have a multiple origin, in the same manner as, from the con- 

 siderations alluded to before, we have decided that species 

 do not originate from single pairs, but in their natural pro- 

 portion with the other species with which they live simul- 

 taneously over the whole ground which they cover. And 

 this is the view which we take of the natural distribution of 

 animals, that they originated primitively over the whole ex- 

 tent of their natural distribution ; that they originated there, 

 not in pairs, but in large numbers, in such proportions as 

 suits their natural mode of living, and the preservation of 

 species ; and that the same species may have originated in 

 different unconnected parts of the more extensive circle of 

 their distribution. We are well aware that there are very 

 many species which are known to have spread beyond what 

 we w^ould call their natural limits ; species which did not 

 occur in North America before the settlement of the whites, 

 that are now abundant here over very extensive tracts of 

 country ; other species which have been introduced from 



