HISTORY OF ZOOLOGY. 53 



species should take to wandering, or should be transplanted, and 

 thus come to a new place, 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 by 

 geological changes be divided into two parts, between which inter- 

 change of forms would be no longer possible. The animals 

 remaining under the old conditions would retain the original 

 characteristics; the wanderers, on the other hand, would change 

 into a new species. Direct observations support this theory. A 

 litter of rabbits placed at the beginning of the fifteenth century 

 on the island of Porto Santo has in the mean time increased 

 enormously and the descendants have taken on the characteristics 

 of a new species. The animals have become smaller and fiercer, 

 have acquired a uniformly reddish color, and no longer pair with 

 the European rabbit. A further proof in favor of the theory of 

 geographical isolation is the peculiar faunal character of regions 

 separated from adjacent lands by impassable barriers, broad rivers 

 or straits, or high mountains (comp. p. 42) ; 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 che island has become separated by 

 geological changes; it usually has not only these but also so-called 

 ' vicarious species/ i.e., species which in certain characteristics 

 closely resemble the species of the mainland. Such vicarious 

 species have plainly arisen from the fact that isolated groups of 

 individuals, scattered over the islands, have taken on a develop- 

 ment divergent from the form from which they started. With all 

 due recognition of the migration theory, it will never be possible 

 by it alone to explain the multiformity of the organic world. In 

 addition, it must be assumed that formerly the earth's surface 

 possessed an enormous capacity for change; but the more recent 

 investigations make it probable that the distribution of land and 

 water has not varied to the degree that was formerly believed. 

 The experience of botanists, too, teaches that several varieties can 

 arise in the same locality and become constant. 



Lamarckism. While the migration theory agrees with Dar- 

 winism in this, that the new characters appearing through varia- 

 tion are to be regarded as the products of chance, yet it is just 

 this part of the theory which has been subjected to searching 

 criticism. Many zoologists have again adopted the causal founda- 

 tion of the descent theory proposed by Lamarck and believe that 

 the cause of species formation is to be found in part in the 



