INTRODUCTION 7 



factor, a rigidly logical interpretation makes it 

 impossible to fall into such an error, although it 

 leaves us ultimately the difficult course proposed 

 by Rabaud: to investigate that which results in 

 the interaction of heritage and environment. 



At this point we come dangerously close to the 

 eternal question, what is life? It would be folly to 

 add to the discussions of this subject which have 

 already appeared; beyond it there is still a profit- 

 able field for inquiry on the basis of known chemi- 

 cal and biological principles. Without knowing 

 exactly what life is, the complexities of heritage 

 and environment furnish an adequate background 

 for the realization that their interaction is suffi- 

 cient to explain the orderly phenomena of life and 

 the complexity of the organic world. This, in itself, 

 is neither a unique nor a difficult concept, but when 

 we introduce the principle of evolutionary change 

 we encounter the difficulties with which the sub- 

 ject of evolution is fraught. It is easier to see that 

 a thing occurs than to determine how it occurs, be 

 it life or evolution. 



Fortunately biological investigations provide an 

 abundance of material from which to draw conclu- 

 sions relating to our problem. In addition to obser- 

 vations and experiments relating directly to the 

 problems of evolution, other branches of biological 

 science disclose many cases in which definite con- 

 ditions produce definite results and changing con- 



