36 THE PROBLEMS OF EVOLUTION 



IS responsible for a given response, and in every 

 case to say just how the hereditary substance 

 is able to respond. We know, for example, that a 

 temperature of approximately 39° C. is a neces- 

 sary condition for all of the development of the 

 unhatched chick. We know that a surrounding 

 medium containing oxygen is essential to both life 

 and development of most organisms, and that 

 water and carbon dioxide are also generally neces- 

 sary. We know that exact relationships with a 

 host species are necessary for the development and 

 existence of many parasites. We know that the 

 failure of one step in development may be fatal to 

 the organism or may render it abnormal. Here are 

 conditions representative of every phase of envi- 

 ronment: a physical condition, chemical factors, 

 organic surroundings, and internal associations 

 within the body of the individual. All are essential, 

 responsible for some phase of development. Rosen- 

 heim has shown that some conditions at high alti- 

 tude, probably the intense light, result in the pro- 

 duction of flavones in the sap of edelweiss which 

 appear in greatly reduced quantities in plants 

 raised at London.^ Henderson's thorough analysis 

 of environment shows many ways in which water 

 and carbon dioxide are important to organisms ; ^ 

 it is sufficient to note that they are fundamental 



^ 8 Biochem. Joum., Vol. XII, pp. 283-289, 1918.' 

 ^Fitness of the EnvirouTnent, 1913. 



