THE CELLULAR BASIS 101 



ence of inheritance units of some kind once 

 more became evident, and, without attempting 

 to define what such units are or how they be- 

 have modern students of heredity invariably 

 accept their existence. They are now called 

 determiners or factors or genes, and are usu- 

 ally thought of as elements or units of the 

 germ cells which condition the characters of 

 the developed organism, and which are in a 

 measure independent of one another; though 

 of course neither they nor any other parts of 

 a cell are really independent in the sense that 

 they can exist apart from one another. They 

 are to be thought of as we think of certain 

 chemical radicals which exist only in combina- 

 tion with other chemical elements in the form 

 of molecules, and yet may preserve their iden- 

 tity in many different combinations. 



If there are inheritance units, such as de- 

 terminers or genes, as practically all students 

 of heredity maintain, they must be contained 

 in the germ cells, and it becomes one of the 

 fundamental problems of biology to find out 

 where and what these units are. But whether 

 we assume the existence of these units or not 



