GENETICS 91 



be provided a practically uniform environment 

 within the body. This condition is all the more 

 striking when we consider that reproduction in 

 the natural state occurs when conditions are 

 optimum for the individual, when the external 

 environment affords the best opportunity for the 

 internal environment to maintain a normal state. 

 But this is a matter of degree. It does not mean 

 that germ cells, chromosomes, or genes are wholly 

 independent of external environmental conditions 

 or that they encounter uninterrupted constancy of 

 conditions in the body. Every organic structure 

 acts toward this end but organisms are imperfect 

 and no part of them can be wholly independent of 

 any phase of the environment. Germinal cells 

 may be affected by moderate changes of tempera- 

 ture to such a degree that even the normal body 

 temperature of homoiothermal animals may be 

 inimical to their production. It has been shown 

 that the descent of the testes into a scrotal pouch 

 in mammals, either periodically or permanently, 

 provides a lower temperature than that of the 

 body for the normal production of spermatozoa, 

 and that forced retention of a testis in the ab- 

 dominal cavity results in the prompt breaking 

 down of the germinal epithelium and the resorp- 

 tion of the fully developed spermatozoa.^^ The 



" Lawrence, Walter, Biol. Bull., Vol. LI, pp. 129-152, 1926. See also 

 comment in Cunningham, J. T., Modern Biology, p. 162, 1928. 



