6 WHAT EVOLUTION IS 



strictly inorganic. They have, in ad- 

 dition to the chemistry and physics 

 of lifeless nature, a chemistry and 

 physics more or less their own. It is 

 in this way that organic evolution 

 differs from simple cosmic evolution, 

 for organic evolution is a general op- 

 eration among plants and animals 

 some aspects of which are not to be 

 met with in the inorganic. 



Organic evolution, though a well- 

 unified field in biology, can be profit- 

 ably treated under two heads. The 

 first of these has to do with the doc- 

 trine of descent with modification — 

 the belief that plants and animals of 

 particular kinds have descended by 

 gradual modification from preexist- 

 ing plants and animals of very dif- 

 ferent kinds. This belief, which is 

 often spoken of as if it were the 

 whole of evolution, is supplemented 

 by what may be treated under a sec- 



