100 ESSAYS OF A BIOLOGIST 



in which the various sciences with which we are con- 

 cerned, of whose relations we had something to say 

 at the beginning of this essay, properly interlock. 



They interlock thus. The physico-chemical 

 sciences are basic to biology. Organisms are made 

 of the same substances as are non-living compounds ; 

 their processes are therefore conformable to certain 

 physico-chemical laws, such as the indestructibility 

 of matter, the conservation of energy, and so forth ; 

 and in so far as we analyse the material aspect of life, 

 physico-chemical concepts are adequate. On the other 

 hand, physico-chemical concepts — or at least our 

 present ones — ^are not all-sufficient. In the first place, 

 the very complicated arrangement of matter which 

 is found in living substance has not been yet sufficiently 

 analysed by physics and chemistry : accordingly we 

 find many processes occurring in biology — such as 

 the directional changes in evolution of which we have 

 spoken — which could not have been foretold on our 

 present physico-chemical knowledge, but must be 

 investigated separately as adding to our store of 

 facts and principles, in the confident hope that a 

 synthesis will one day be possible. Secondly, a whole 

 new category of phenomena, the psychological, is 

 first met with in biology, and to this we cannot as 

 yet apply physical or chemical ideas at all. 



For a combination of these two reasons, biology 

 deals with certain concepts which are not implicit 

 in current physico-chemical ideas. Physics and 



