Chap. XII.] SINGLE CENTRES OF CREATION. 139 



modification. A volcanic island, for instance, upheaved 

 and formed at the distance of a few hundreds of miles 

 from a continent, would probably receive from it in the 

 course of time a few colonists, and their descendants, 

 though modified, would still be related by inheritance to 

 the inhabitants of that continent. Cases of this nature 

 are common, and are, as we shall hereafter see, inexplic- 

 able on the theory of independent creation. This view 

 of the relation of the species of one region to those of 

 another, does not differ much from that advanced by Mr. 

 Wallace, who concludes that " every species has come 

 into existence coincident both in space and time with a 

 pre-existing closely allied species." And it is now well 

 known that he attributes this coincidence to descent with 

 modification. 



The question of single or multiple centres of creation 

 differs from another though allied question, — namely, 

 whether all the individuals of the same species are 

 descended from a single pair, or single hermaphrodite, or 

 whether, as some authors suppose, from many individuals 

 simultaneously created. With organic beings which 

 never intercross, if such exist, each species must be de- 

 scended from a succession of modified varieties, that 

 have supplanted each other, but have never blended with 

 other individuals or varieties of the same species ; so that, 

 at each successive stage of modification, all the individuals 

 of the same form will be descended from a single parent. 

 But in the great majority of cases, namely, with all 

 organisms which habitually unite for each birth, or which 

 occasionally intercross, the individuals of the same 

 species inhabiting the same area will be kept nearly 

 uniform by intercrossing ; so that many individuals will 

 go on simultaneously changing, and the whole amount 

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