Chap. XII.] SINGLE CENTEES OF CKEATION. 137 



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 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 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 general considerations, 

 that each species has been produced within one area, and 

 has migrated thence as far as it could. It would be 



