INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 93^ 



related; and second, that there are interesting differences 

 among even those whose kinship would entitle them to great 

 likeness. 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 in- 

 fluences. In other words the former are thought to be due 

 to heredity and the latter to the environment. 



Characteristics which animals receive from their ancestors 

 near or remote are described as hereditary. We ascribe the fact 

 that the 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 entirely 

 different qualities from their parents and thus present differ- 

 ences among themselves due solely to inheritance. The off- 

 spring may also present such a mingling of the qualities in- 

 herited from their ancestors as to possess characteristics 

 decidedly new to any of them. Thus likeness to parents and 

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

 once perpetuates old qualities and introduces variations. 



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 par- 

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

 or of the environment, are capable of being transmitted. It 

 is unquestioned however that qualities received from the par- 

 ents are, under favoring circumstances, capable of being trans- 

 mitted to offspring. 



127. The Bearers of Heredity. It follows from the fact 

 that the adult organism is produced from the union of the male 

 and female elements that these two cells are in some way en- 

 dowed to carry the parental qualities. There are strong evi- 

 dences that the chromatic elements, or chromosomes, in the 



