380 aiNGLE CENTERS OP GRBATION. 



bution of fresh watei' productions; and thirdly, the 

 occniTence of the same terrestrial species on islands and 

 on the nearest mainland, 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 explained on the view of each species 

 having migrated from a single birthplace, then, consider- 

 ing our ignorance with respect to former climatical and geo- 

 graphical 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 spe- 

 cies 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 f orm er period, our general view will be much 

 strengthened ; for the explanation is obvious on the principle 

 of descent with modification. A volcanic island, for in- 

 stance, upheaved and formed at the distance of a few 

 hundreds of miles fj-om 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 centimes of creation 

 differs from another though allied question, namely, 

 whether all individuals of the same species are descended 

 from a single pair, or single hermaphrodite, or whether, as 

 some authors suppose, from many individuals simultane- 



