iv ENVIRONMENT AND PHYSIOLOGY 201 



on successive generations, important sexual changes 

 occur which are correlated with the changes in growth 

 and size ; instead of being as usual hermaphrodite 

 the individuals (of Lymnaea megasoma] become uni- 

 sexual and are exclusively female. At the same time 

 also the hepatic gland undergoes a process of partial 

 atrophy. These influences of external agents on in- 

 ternal viscera, such as the sexual glands and liver, 

 whether directly or indirectly exerted, are very inter- 

 esting, and more cases of a similar nature may be 

 discovered if more attention is paid to the process. It 

 is merely a matter of historical interest to recall that 

 the father of natural science, Aristotle, many centuries 

 ago noticed the variations in growth and dimensions 

 while comparing animals of the same species in Egypt 

 and in Greece. 



The preceding facts show that external influences 

 may react on the general growth of organisms, and 

 perhaps on their internal constitution. 



In some cases, however, where variation in environ- 

 ment might be expected on a priori grounds to inter- 

 fere greatly with internal constitution, we perceive no 

 such interference, and we are compelled to draw the 

 conclusion that the organism admits of a greater 

 amount of physiological elasticity than we expected. 

 I find a good instance of such elasticity in the result 

 of recent experiments which have been conducted by 



