Chap. XII.] SINGLE CENTRES OP CREATION. 135 



ing organisms into new relations with each other and in 

 a lesser degree with the surrounding physical condi- 

 tions. As we have seen in 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 sev- 

 eral 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 periods 

 little modification, there is not much difficulty in be- 

 lieving 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 spe- 

 cies of a genus have been produced within comparative- 

 ly recent times, there is great difficulty on this head. 

 It is also obvious that the individuals of the same spe- 

 cies, 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 dis- 

 tinct. 



Single Centres of supposed Creation. — We are thus 

 brought to the question which has been largely dis- 

 cussed by naturalists, namely, whether species have 

 been created at one or more pointy of the earth's sur- 

 face. Undoubtedly there are many cases of extreme 



