MECHANICAL CAUSES. 25 I 



conditions of nutrition of the organism, in the same way 

 as the phenomena of Inheritance are referable to conditions 

 of reproduction ; but the latter, as well as the former, 

 may further be traced to chemical and physical, that is to 

 mechanical, causes. According to Darwin's Theory of 

 Selection the new forms of organisms, the transformations 

 which artificial selection produces in the state of cultivation, 

 and which natural selection produces in the state of nature, 

 arise solely by the interaction of such causea 



