HISTORY OF ZOOLOGY. 63 



an exposition of these theories, but it is to be taken into 

 consideration that, at the present time, we are still in the 

 midst of the reform movement, and it cannot yet be said 

 whether the theories will be able to stand beside the 

 theory of the struggle for existence or will supplant it. 



Migration Theory. To explain how it comes about 

 that characters newly formed by variation become fixed, 

 and do not disappear again through crossing with dif- 

 ferently modified individuals, M. Wagner has proposed 

 tlic (/icory of geographical isolation, or the Migration Theory. 

 New species may arise if a part of the stock of individ- 

 uals of one species should take to wandering, or should be 

 passively transplanted, and thus should come to a new place 

 of residence, in which crossing with the companions of their 

 species who were left behind is not possible. The same 

 might occur, if the region inhabited by a species should be by 

 geological occurrences divided into two parts, between which 

 interchange of forms would be no longer possible. The ani- 

 mals remaining under the old conditions would retain the 

 original characteristics of the species ; the wanderers, on the 

 other hand, would change into a new species. Direct obser- 

 vations speak for the justification of this theory. A litter 

 of rabbits placed at the beginning of the fifteenth century 

 on the island of Porto Santo has in the meantime increased 

 enormously and the descendants have taken on the char- 

 acteristics of a new species. The animals have become 

 smaller and fiercer, have acquired a uniformly reddish 

 color, and no longer admit of pairing with our native rab- 

 bit. A further proof in favor of the theory of geographical 

 isolation is the peculiar faunal character of territories 

 which are separated from adjacent lands by impassable bar- 

 riers, broad rivers or straits, or high mountains (comp. p. 

 5); especially instructive in this regard is the peculiar 

 faunal character of almost every island. The fauna of an 

 island resembles in general the fauna of the mainland 

 from which the island has become separated by geological 

 occurrences; it usually has not only these but also so- 

 called "vicarious species," i.e., species which in certain 



