46 GENERAL PRINCIPLES OF ZOOLOGY 



modified individuals, Moritz Wagner has proposed the Theory of Geo- 

 graphical Isolation, or the Migration Theory. New species may arise if 

 a part of the individuals of a species should wander to a new place, in which 

 crossing with the others of their species who were left behind is not possible. 

 The same might occur, if geological changes should divide the region 

 inhabited by a species into two parts, between which interchange of 

 forms would no longer be 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. A litter of rabbits placed at the beginning of the fifteenth 

 century on the island of Porto Santo has increased enormously and the 

 descendants have taken on the characteristics of a new species. They 

 have become smaller and fiercer, have acquired a uniformly reddish color, 

 and no longer pair with the European rabbit. A further proof 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. 34) ; especially instructive is the pecul- 

 iar 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 changes; it usually has not only these but 

 also '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 island, have taken on a development divergent from the form 

 from which they started. With all due recognition of the migration 

 theory, it will never be possible to explain the multiformity of the organic 

 world by it alone. It must be assumed that formerly the earth's surface 

 possessed an enormous capacity for change; but the more rescent investi- 

 gations 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. \Vhile the migration theory agrees with Darwinism 

 in this, that the new characters appearing through variation 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 foundation of the descent theory proposed by 

 Lamarck and believe that the cause of species formation is to be found in 

 part in the immediate influence of changing environment, in part in the 

 varying use and disuse of organs, brought about by alterations in the con- 

 ditions of life. Both principles, they say, are sufficient, even without the 



