Chap. XU.] SINGLE CENTRES OP CREATION. 139 



descent with modification. A volcanic island, for in- 

 stance, upheaved and formed at the distance of a few 

 hundreds of miles from a continent, would probably re- 

 ceive from it in the course of time a few colonists, and 

 their descendants, though modified, would still be re- 

 lated by inheritance to the inhabitants of that continent. 

 Cases of this nature are common, and are, as we shall 

 hereafter see, inexplicable on the theory of independ- 

 ent 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 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 crea- 

 tion 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 individ- 

 uals 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, name- 

 ly, 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 individ- 

 uals will go on simultaneously changing, and the whole 



