GEOGRAPHICAL DISTRIBUTION 327 



that some forms have retained nearly the same character from an 

 enormously remote geological period, so certain species have 

 migrated over vast spaces, and have not become greatly or at all 

 modified. 



According to these views, it is obvious that the several species 

 of the same genus, though inhabiting the most distant quarters of 

 the world, must originally have proceeded from the same source, 

 as they are descended from the same progenitor. In the case of 

 those species which have undergone, during whole geological pe- 

 riods, little modification, there is not much difficulty in believing 

 that they have migrated from the same region; for during the 

 vast geographical and climatical changes which have supervened 

 since ancient times, almost any amount of migration is possible. 

 But in many other cases, in which we have reason to beHeve that 

 the species of a genus have been produced within comparatively 

 recent times, there is great difficulty on this head. It is also ob- 

 vious that the individuals of the same species, though now in- 

 habiting distant and isolated regions, must have proceeded from 

 one spot, where their parents were first produced: for, as has been 

 explained, it is incredible that individuals identically the same 

 should have been produced from parents specifically distinct. 



SINGLE CENTRES OF SUPPOSED CREATION 



We are thus brought to the question which has been largely 

 discussed by naturalists, namely, whether species have been 

 created at one or more points of the earth's surface. Undoubtedly 

 there are many cases of extreme difficulty in understanding how 

 the same species could possibly have migrated from some one 

 point to the several distant and isolated points where now found. 

 Nevertheless the simplicity of the view that each species was first 

 produced within a single region captivates the mind. He who re- 

 jects it, rejects the vera causa of ordinary generation with subse- 

 quent migration, and calls in the agency of a miracle. It is univer- 

 sally admitted, that in most cases the area inhabited by a species is 

 continuous ; and that when a plant or animal inhabits two points so 

 distant from each other, or with an interval of such a nature, that 

 the space could not have been easily passed over by migration, the 

 fact is given as something remarkable and exceptional. The in- 

 capacity of migrating across a wide sea is more clear in the case 

 of terrestrial mammals than perhaps with any other organic be- 

 ings; and, accordingly, we find no inexplicable instances of the 

 same mammals inhabiting distant points of the world. No geol- 

 ogist feels any difficulty in Great Britain possessing the same 



