g 2 o Geographical Distribution. Chap. xii. 



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 periods little 

 modification, there is not much difficulty in believing that they have 

 migrated from the same region ; for during the vast geographical 

 and climatal 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 believe that the species of a genus 

 have been produced within comparatively recent times, there is 

 great difficulty on this head. It is also obvious that the individuals 

 of the same species, though now inhabiting 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 rejects it, rejects the vera causa of ordinary 

 generation with subsequent migration, and calls in the agency of a 

 miracle. It is universally 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 incapacity of migrating across a wide sea is 

 more clear in the case of terrestrial 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 difficulty in Great Britain possessing 

 the same quadrupeds 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 



