DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION I OX 



even those whose kinship would entitle them to be much alike. 

 There is always a disposition among students to feel that the 

 likenesses are due to internal causes and that the unlikenesses are 

 due in some way to the varying external influences. In other 

 words the former are thought to be due to heredity and the latter 

 to the environment. 



Characteristics which animals receive from the reproductive 

 cells from which they arise are described as hereditary. This 

 is equally true whether the. reproduction is sexual or asexual, 

 by means of one cell or two. We ascribe the fact that the 

 fertilized hen's egg produces a fowl and a frog's egg a frog to 

 the action of heredity. No less is the repetition in the child of 

 minute parental peculiarities of feature and form a fact of 

 inheritance. While these likenesses are due to the action of the 

 internal forces of heredity , it must not be deemed that heredity 

 is a purely conservative influence in the life of the organism. 

 The offspring of two parents may inherit from their parents 

 very different qualities and thus present striking variations 

 among themselves due solely to inheritance. The offspring 

 may also present such a mingling of the qualities inherited by 

 way of their ancestors as to possess characteristics decidedly 

 new to any of them. Indeed offspring may inherit features 

 which neither of their parents shows. Thus likeness to parents 

 and unlikeness to parents may equally be due to heredity. It 

 at once perpetuates old qualities and introduces variations from 

 these. 



It was formerly considered that all the characteristics which 

 parents possess are equally subject to inheritance, but it is now 

 denied by many biologists that the qualities which a parent 

 acquires in its own lifetime, as the result of its own actions or of 

 the environment, afe capable of being transmitted. In the 

 light of what was said about the parallel development of pri- 

 mordial germ cells and body cells (§49), this is just another way 

 of saying that changes that come to the body cells may not be 

 passed on specifically to the germ cells so that these will later 

 give rise to the same sort of new qualities in the body cells 

 which in turn arise from them. These statements do not mean 

 that the condition of the body may not modify the character 



