322 Single Centres of Creation. Chap, xil 



from a single birthplace ; then, considering our ignorance with re- 

 spect to former climatal and geographical changes and to the various 

 occasional means of transport, the belief that a single birthplace is 

 the law, seems to me incomparably the safest. 



In discussing this subject, we shall be enabled at the same time 

 to consider a point equally important for us, namely, whether the 

 several species of a genus, which must on our theory all be descended 

 from a common progenitor, can have migrated, undergoing modi- 

 fication during their migration, from some one area. If, when most 

 of the species inhabiting one region are different from those of 

 another region, though closely allied to them, it can be shown that 

 migration from the one region to the other has probably occurred 

 at some former period, our general view will be much strengthened ; 

 for the explanation is obvious on the principle of descent with modi- 

 fication. 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, inexplicable 

 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 modi- 

 fication. 



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 he de- 

 scended from a succession of modified varieties, that have sup- 

 planted 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 de- 

 scended 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 inter- 

 crossing; so that many individuals will go on simultaneously 

 changing, and the whole amount of modification at each stage will 



