SINGLE CENTERS OF OBEATION. 379 



of dispersal, have migrated across the wide and broken 

 interspaces. The great and striking influence of barriers 

 of all kinds, is intelligible only on the view that the great 

 majority of species have been produced on" one side, and 

 have not been able to migrate to the opposite side. Some 

 few families, many subfamilies, very many genera, a still 

 greater number of sections of genera, are confined to a 

 single region; and it has been observed by several natural- 

 ists that the most natural genera, or those genera in which 

 the species are most closely related to each other, are gen- 

 erally confined to the same country, or if they have a wide 

 range that their range is continuous. What a strange 

 anomaly it would be if, a directly opposite rule were to 

 prevail when we go down one step lower in the series, 

 namely, to the individuals of the same species, and 

 these had not been, at least at first, confined to some one 

 region ! 



Hence, it seems to me, as it has to many other natu- 

 ralists, that the view of each species having been produced 

 in one area alone, and having subsequently migrated from 

 that area as far as its powers of migration and subsistence 

 under past and present conditions permitted, is the most 

 probable. Undoubtedly many cases occur in which we 

 cannot explain how the same species could have passed from 

 one point to the'other. But the geographical and climatical 

 changes which have certainly occurred within recent geo- 

 logical times, must have rendered discontinuous the for- 

 merly continuous range of many species. So that we are 

 reduced to consider whether the exceptions to 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 sep- 

 arated points, nor do I for a moment pretend 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 distri- 



