Chap. XII.] GEOGRAPHICAL DISTRIBUTION. 321 



I believe, is, that mammals have not been able to migrate, whereas 

 some plants, from their varied means of dispersal, have migrated 

 across the wide and broken interspaces. The great and striking 

 influence of harriers 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 sub-families, very many genera, and a still greater 

 number of sections of genera, are confined to a single region; and 

 it has been observed by several naturalists, that the most natural 

 genera, or those genera in which the species are most closely related 

 to each other, are generally 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 naturalists, 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 con- 

 ditions 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 

 ■climatal changes, which have certainly occurred within recent 

 geological times, must have rendered discontinuous the formerly 

 continuous range of many species. So that we are reduced to con- 

 sider 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 separated 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 distribution of fresh- 

 water productions ; and thirdly, the occurrence of the same terrestrial 

 species on islands »nd 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 



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