354 GEOGRAPHICAL DISTRIBUTION. Chap. XI. 



continuity of range are so numerous and of so grave 

 a nature, that we ought to give up the belief, rendered 

 probable by general considerations, that each species 

 has been produced within one area, and has migrated 

 thence as far as it could. It would be hopelessly tedious 

 to discuss all the exceptional cases of the same species, 

 now living at distant and separated points; nor do I 

 for a moment pretend that any explanation could be 

 offered of many such cases. 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 fresh- 

 water productions ; and thirdly, the occurrence of the 

 same terrestrial species on islands and on the 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 ignor- 

 ance with respect to former climatal and geographical 

 changes and various occasional means of transport, the 

 belief that this has been the universal 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 distinct species of a genus, 

 which on my theory have all descended from a common 

 progenitor, can have migrated (undergoing modification 

 during some part of their migration) from the area 

 inhabited by their progenitor. If it can be shown to 

 be almost invariably the case, that a region, of which 

 most of its inhabitants are closely related to, or belong- 

 to the same genera with the species of a second region, 



