100 ZOOLOGY 



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 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 union of the 

 germ cells are described as hereditary. 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 qualities entirely 

 different from their parents and thus present differences 

 among themselves due solely to inheritance. The offspring 

 may also present such a mingling of the qualities inherited 

 from their ancestors as to possess characteristics decidedly 

 new to any of them. Thus likeness to parents and unlike- 

 ness 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 parent 

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

 the environment, are capable of being transmitted. In the 

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

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

 of saying that changes that come to the body cells may not spe- 

 cifically change the germ cells so that they will later rise to the 

 same sort of a change in the body cells which arise from them. 

 It is unquestioned, however, that qualities received by inheritance 

 from the preceding generations are, under favoring circum- 

 stances, capable of being transmitted to the following. 



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

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



