cH. XIX Influence of Habits and Surroundings 311 



shape and size of the tail and in the respiratory appendages 

 borne by the legs ; they are not found together, but live in 

 pools of different degrees of saltness. Now Schmankewitsch 

 took specimens of ^4. salina which live in the less salt water, 



Fig. fiti. — Tail-Iobes oi Ariemia salina. (to the left) and oi Artemia milhausejtii 

 (to the right) ; between these four stages in the transformation of the one into 

 the other, (From Chambers's Encyclop. ; after Schmankewitsch.) 



added salt gradually to the medium in which they were 

 living, and in the course of generations turned them into 

 A: milhciusenii. He also reversed the process by freshening 

 the water little by little. Moreover, he accustomed A. salina 

 to entirely fresh water, and then found that the form had 

 changed towards that of a related genus, Branchipus. This 

 last step has been adversely criticised, but it is allowed that 

 one species of brine-shrimp was changed into another. 



Many interesting experiments have been made on the 

 effect of chemical reagents on cells, but these are perhaps 

 of most interest to the student of drugs. Still the fact that 

 the form of a cell and its predominant phase of activity 

 may be entirely changed in this way is important, especially 

 when we remember that it was in single cells that life first 

 began, and is now continued. Even Weismann agrees with 

 Spencer's conclusion that " the direct action of the medium 

 was \h& primordial factor of organic evolution." 



To Claude Bernard, the main problem of evolution 

 seemed to be concerned with variations in nutrition : 

 " L'evolution, c'est I'ensemble constant de ces alternatives 

 de la nutrition ; c'est la nutrition consideree dans sa 

 reality, embrassde d'un coup d'oeil k travers le temps." 

 John Hunter and others have shown how the walls of the 



