GEOGRAPHICAL DISTRIBUTION 329 



classes of facts, namely, the existence of the same species on the 

 summits of distant mountain ranges, and at distant points in the 

 Arctic and Antarctic regions; and secondly (in the following 

 chapter), the wide distribution of fresh-water productions; and 

 thirdly, the occurrence 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, considering our ignorance with respect to 

 former climatical 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 de- 

 scended 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 oc- 

 curred 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, up- 

 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 descendants, though modified, would 

 still be related by inheritance to the inhabitants of that con- 

 tinent. 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. Wal- 

 lace, 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 coinci- 

 dence to descent with modification. 



The question of single or multiple centres of creation differs 

 from another though allied question; namely, whether all indi- 

 viduals 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 be de- 

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



