854 SINGLE CENTRES OF CREATION. 



and will be taken advantage of by natural selection, only so 

 far as it profits each individual in its complex struggle for 

 life, so the amount of modification in different species will 

 be no uniform quantity. If a number of species, after having 

 long competed with each other in their old home, were td 

 migrate in a body into a new and afterward isolated country, 

 they would be little liable to modification ; for neither migra 

 tion nor isolation in themselves effect any thing. Thes« 

 principles come into play only by bringing organisms info 

 new relations with each other and in a lesser degree wit\ 

 the surrounding physical conditions. As we have seen ii* 

 the last chapter 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 

 d.'stant quarters of the world, must originally have pro- 

 ceeded 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 climatical changes which have super- 

 vened 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 individ- 

 uals 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 



