302 SINGLE CENTRES OF CREATION. [CHAP. XIL 



that any explanation could be offered of many instances. But, 

 after some preliminary remarks, I will discuss a few of the most 

 striking 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 freshwater 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 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, under- 

 going 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 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 attri- 

 butes this coincidence to descent with modification. 



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 

 be descended from a succession of modified varieties, that have 

 supplanted each other, but have never blended with other indi- 



