64 GENERAL PRINCIPLES OF ZOOLOGY. 



characterises closely resemble the mainland species. Such 

 vicarious species have plainly arisen from the fact that 

 isolated groups of individuals, scattered over the islands, 

 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 by it alone to 

 explain the multiformity of the organic world. In addi- 

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

 possessed an inconceivable capacity for changes ; and 

 yet 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 

 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 clianging conditions of existence, in part in 

 the varying use and disuse of organs, brought about by 

 alterations in the conditions of life. Both principles ought 

 to be sufficient, even without the help of the struggle for 

 existence, to explain the phylogenesis of organisms. 



Influence of Environment. To what extent can the 

 conditions of existence bring about a permanent change in 

 the structure of plants and animals? To decide this is no 

 simple problem, on account of the complexity of the influ- 

 ences entering into the question. 



In cases where the food-supply is altered, organisms 

 change in a very remarkable manner, and within a short 

 time; but yet these changes (Nageli's "Modifications 

 through Nutrition ") seem to have no permanence. Plants 

 which, found in nature in poor soil, are transplanted into 

 rich soil, or vice versa, soon acquire quite a different ap- 



