EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 173 



events in large part, derive their inhabitants from accidental or occa- 

 sional arrivals of wind-blown or water-floated organisms from other 

 countries — especially, of course, from the countries least remote. But, 

 after agreeing upon this point, the two theories must part company in 

 their anticipations. The special-creation theory can have no reason 

 to suppose that a small volcanic island in the midst of a great ocean 

 should be chosen as the theatre of any extraordinary creative activity, 

 or for any particularly rich manufacture of peculiar species to be 

 found nowhere else in the world. On the other hand, the evolution 

 theory would expect to find that such habitats are stocked with more 

 or less peculiar species. For it would expect that when any organisms 

 chanced to reach a wholly isolated refuge of this kind, their descendants 

 should forthwith have started upon an independent course of evolu- 

 tionary history. Protected from intercrossing with any members of 

 their parent species elsewhere, and exposed to considerable changes in 

 their conditions of life, it would indeed be fatal to the general theory 

 of evolution if these descendants, during the course of many genera- 

 tions, were not to undergo appreciable change. It has happened on 

 two or three occasions that European rats have been accidentally 

 imported by ships upon some of these islands, and even already it is 

 observed that their descendants have undergone a slight change of 

 appearance, so as to constitute them what naturalists call local 

 varieties. The change, of course, is but shght, because the time 

 allowed for it has been so short. But the longer the time that a 

 colony of a species is thus completely isolated under changed condi- 

 tions of life the greater, according to the evolution theory, should 

 we expect the change to become. Therefore, in all cases where we 

 happen to know, from independent evidence of a geological kind, that 

 an oceanic island is of very ancient formation, the evolution theory 

 would expect to encounter a great wealth of peculiar species. On the 

 other hand, as I have just observed, the special-creation theory can 

 have no reason to suppose that there should be any correlation 

 between the age of an oceanic island and the number of peculiar species 

 which it may be found to contain. 



Therefore, having considered the principles of geographical distri- 

 bution from the widest or most general point of view, we shall pass to 

 the opposite extreme, and consider exhaustively, or in the utmost 

 possible detail, the facts of such distribution where the conditions are 

 best suited to this purpose — that is, as I have akeady said, upon 

 oceanic islands, which may be metaphoriraTlv regarded as having been 



