152 HEREDITY AND ENVIRONMENT 



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 ^gg 

 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- 

 vironment they must be due to differences in 

 the germ cells concerned. 



On the other hand different environmental 

 conditions may be associated with similar de- 

 velopmental results. Loeb and others have 

 found that artificial parthenogenesis may be 

 induced by a great variety of environmental 

 stimuli, viz., by salt solutions, by acids and 

 alkalis, by fatty acids and fat solvents, by 

 alkaloids and cyanides, by blood serum and 



