Chap. XII.] SINGLE CENTRES OF CREATION. 135 



bringing organisms into new relations with each other 

 and in a lesser degree with the surrounding physical 

 conditions. 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 

 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 geo- 

 logical 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 iden- 

 tically 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 dis- 

 cussed by naturalists, namely, whether species have 

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

 face. Undoubtedly there are many cases of extreme 



