GEOGRAPHICAL DISTRIBUTION 145 



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. 

 Wliat a strange anomaly it would be, if a directly oppo- 

 site 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 migra- 

 tion 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 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 

 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 gen- 

 eral 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 



