130 Heredity and Environment 



With the publication of Weismann's work on the germ-plasm, 

 in 1892 speculation with regard to these ultra-microscopic units 

 of life and of heredity reached a climax and began to decline, 

 owing to the highly speculative character of the evidence as to 

 the existence, nature and activities of such units. But with the 

 rediscovery of Mendel's principles of heredity the necessity of 

 assuming the existence of inheritance units of some kind once 

 more became evident, and, without being able to define just what 

 such units are or just how they behave, modern students of hered- 

 ity assume their existence. They are now called determiners or 

 factors or genes, and they are usually thought of as units in the 

 germ cells which condition the characters of the developed or- 

 ganism, 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 analogous to chemical 

 radicals which are never independent but exist only in combina- 

 tion with other chemical elements in the form of molecules, and 

 yet preserve their identity in many different combinations. 



It is certain that Mendelian factors are not to be regarded as 

 gemmules or the germs of particular characters. There is not a 

 separate factor for every character, and factors are not "repre- 

 sentatives" or "carriers" of characters. They represent the dif- 

 ferential causes of particular characters just as in the compounds 

 H 2 S0 4 and K„S0 4 the hydrogen and potassium atoms represent 

 the differential causes of the properties manifested by these two 

 substances. 



Location of Inheritance Units. — If there are inheritance units, 

 such as determiners 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. There are many evidences that these genes 

 are located in the chromatin of the nucleus, that they are arranged 

 in a linear series when the chromatin takes the form of threads, or 



