MECHANICAL CAUSES. 251 
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 causes. 
