GEOGRAPHICAL DISTRIBUTION 147 



heaved 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 de- 

 scendants, 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 here- 

 after see, inexplicable on the theory of independent cre- 

 ation. 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 spe- 

 cies." 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 in- 

 dividuals simultaneously created. With organic beings 

 which never intercross, if such exist, each species must 

 be descended 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 individ- 

 uals 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 



