CHAP. XII.] SINGLE CENTRES OF CREATION. 301 



trial mammals than perhaps with any other organic beings ; and, 

 accordingly, we find no inexplicable instances of the same 

 mammals inhabiting distant points of the world. No geologist 

 feels any difliculty in Great Britain possessing the same quad- 

 rupeds with the rest of Europe, for they were no doubt once 

 united. But if the same species can be produced at two separate 

 points, why do we not find a single mammal common to Europe 

 and Australia or South America? The conditions of life are 

 nearly the same, so that a multitude of European animals and 

 plants have become naturalised in America and Australia; and 

 some of the aboriginal plants are identically the same at these 

 distant points of the northern and southern hemispheres ? The 

 answer, as I believe, is, that mammals have not been able to 

 migrate, whereas some plants, from their varied means of dis- 

 persal, 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 greac 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, con- 

 fined 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 

 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 separated points, nor do I for a moment pretend 



