DOCTRINE OF EVOLUTION AND RELATED IDEAS 493 



The following outline may aid the student in grasping some 

 of the important elements of the case. 



1 . Variation makes evolution possible, but only this. We have 

 learned (§60) that body cells and germ cells lead a parallel 

 life in every organism. They are very different in their proc 

 esses and their fate. Both of these classes of cells are in con- 

 tinual action, reaction, and interaction with one another and 

 with the environment. Theoretically, therefore, variations 

 may arise in either of two ways : 



A. Changes may originate in the body cells (aside from what 



was transmitted from the parents). 



a. The body of a given organism may change by the direct 

 action of the environment upon it. 



b. Parts of a body may change through use and disuse. 



c. Parts of a body may change by the interaction of one 

 part upon another, — as in the case of the secretions 

 from the pituitary or the thyroid. 



d. Parts of the body may change from the changed in- 

 fluence of the contained germ cells and the secretions 

 that are related to these, — as in the effects of the 

 gonad secretions upon secondary sex characteristics 

 of the body. 



B. Changes may occur in germ cells. 



a. Germ cells may be modified by the mingling of the two 

 different germ cells, — ovum and sperm, — that originally 

 united to form them. This new compound is different 

 from any ever formed before. 



b. Germ cells may be modified by their environment, — 

 which is chiefly the body in which they are being 

 housed, though it is possible that the general environ- 

 ment also may influence them. 



2. Preservation, repetition, and accumulation of variations from 

 generation to generation after they once appear. Only this can 

 insure evolution. This is the problem of inheritance, of which 

 there are two aspects: 



A. Preservation of fluctuations that have come to the body.; 

 If there is an early separation of germ cells and body cells and 

 they lead a parallel course of development, it becomes a problem 



