iv ENVIRONMENT AND DEVELOPMENT 197 



to live to the adult condition. At all events, it is 

 certainly more interesting for us to see that slight 

 external causes may determine considerable effects 

 than it would be to meet with sheer impossibility of 

 any action or influence. The facts that we are now 

 acquainted with go to show that the influence of 

 environment upon the development of the egg is 

 considerable from the very first stages of the 

 building of the embryo, from the segmentation process 

 itself. This has been ascertained by H. W. Conn in 

 reference to the eggs of Thalassema mellita, where 

 segmentation is regular or irregular according to the 

 mode of life free or protected ; by Pfliiger, in 

 reference to amphibian eggs, by O. and R. Hertwig, 

 and by Marcacci ; while Tichomiroff has witnessed 

 parthenogenesis artificially induced in insects by 

 chemical stimuli. 



Many facts also go to prove that external influences 

 are very efficient in accelerating or retarding 

 the developmental processes, as every naturalist has 

 certainly noticed when comparing the development of 

 eggs from the same Lymnaea or Planorbis for 

 instance, under different conditions of heat or light ; 

 and other facts show that environment also exerts a 

 marked influence on the general metabolism of the 

 body. 



This influence is illustrated by experiments on the 



