4 HEREDITY AND EUGENICS 



misconceptions need to be pointed out first. The 

 question is often asked whether heredity or environ- 

 ment is more important in connection with develop- 

 ment. But the question cannot rightly be asked 

 in this way, because any organism is the result of 

 continuous complicated reactions and interactions, 

 not onl}^ between the developing germ and its environ- 

 ment, but also between the different parts of the 

 organism itself. Moreover, it is quite incorrect to 

 assume that the organic germ and the environment 

 mutually react with each other in any simple way. 

 A particular change in the environment ma}^ con- 

 spicuously affect one part of the developing organism 

 without visibly affecting other parts. Thus Stockard 

 (1909) showed that when magnesium chloride is 

 added to the sea water in which certain fish embryos 

 are developing, c3xlopean fishes are produced, with 

 one median eye instead of two lateral ones. This is 

 a surprising reaction of the organism, and more 

 particularly of the nervous system, to a definite 

 environmental stimulus.* Some differences in the 

 environment will therefore produce very marked 

 effects on the developing organism. 



On the other hand, organisms developing in the 

 same environment may show marked differences, 

 because they have inherited different characters. 

 Tw^o hen's eggs in an incubator, under the same con- 

 ditions of temperature, moisture, etc., may develop 

 birds, one with a rose comb and the other with a 

 single comb, or one with white feathers and the other 

 with brow^n. Obvioush^ the environment is not a 

 differential, but the difference was in the original 

 eggs and is inherited from the previous generation. 



* Stockard 's result has recently been shown to be due to 

 differential destruction of the nerve plate in the embryo, the 

 destruction beginning at the anterior end, as in Child's experi- 

 ments with potassium cyanide. 



