158 HEREDITY AND ENVIRONMENT 



the hereditary continuity between successive 

 generations. But in addition to these specific 

 facts there are certain general considerations 

 which need to be emphasized. 



I. The Specificity of Germ Cells 



The conclusion is inevitable that the germ 

 cells of different species and even those of dif- 

 ferent individuals are not all alike. Every in- 

 dividual difference between organisms must 

 be due to one or more differentiating causes 

 or factors. Specific results come only from 

 specific causes. These causes may be found in 

 the organization of the germ cells or in en- 

 vironmental stimuli, i. e., they may be intrinsic 

 or extrinsic, but as a matter of fact experience 

 has shown that they are generally intrinsic in 

 the germ. In the same environment one egg 

 becomes a chicken and another a duck; one 

 becomes a frog, and another a fish, and an- 

 other a snail; one becomes a black guinea-pig 

 and another a white one; one becomes a male 

 and another a female; one gives rise to a tall 

 man and another to a short man, etc. Since 

 these differences may occur in the same en- 



