22 THE PROBLEMS OF EVOLUTION 



exists by virtue of the power of its heritage of liv- 

 ing substance to react to the factors of the com- 

 plex environment. To the extent that heritages 

 are similar and the conditions of their environ- 

 ments coincide, we may expect the resulting organ- 

 isms to resemble each other. The mechanism of 

 heredity guarantees to each individual of a species 

 approximately similar heritage, but within the 

 limits characteristic of the species it also guaran- 

 tees diversity. With the exception of identical 

 twins, the chance that two individuals will receive 

 identical heritages is exceedingly remote. And the 

 infinite variation of conditions on the surface of 

 the earth makes it practically impossible for two 

 individuals to live under exactly the same condi- 

 tions. With even slightly different heritages and 

 slightly different environments, the products of 

 interaction, living individuals, cannot fail to be 

 different. Variation is, to this extent, axiomatic. 

 Identical organisms are a practical impossibility 

 although in theory they may exist. 



When we undertake a consideration of the species 

 and of the problems of phylogenetic change, our 

 difficulties multiply. The considerations already 

 stated are not obscure; it is increasingly evident to 

 biologists that individuals must be looked upon as 

 a reaction product, and the principle stated above 

 applies chronologically as well as spatially. If an 

 individual of one generation receives a heritage 



