402 ORIGIN OF SPECIES 



same terrestrial species on islands and on the nearest main- 

 land, though separated by hundreds of miles of open sea. 

 If the existence of the same species at distant and isolated 

 points of the earth's surface, can in many instances be ex- 

 plained on the view of each species having migrated 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 modification 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 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, inexplicable on the theory of inde- 

 pendent 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 dif- 

 fers from another though allied question, — namely, whether 

 all the individuals of the same species are descended from a 

 single pair, or single hermaphrodite, or wtiether, as some 

 authors suppose, from many individuals simultaneously ere- 



