248 



THE ORIGIN AND EVOLUTION OF LIFE 



teins may have a similar number, and that the simpler proteins 

 and the fats and carbohydrates and perhaps other complex 

 organic substances, may each have only a fraction of this 

 number, it can readily be conceived how, primarily by differ- 

 ences in chemical constitution of vital substances, and secon- 



k r -"-1 , .'; ' V. 





Fig. 117. Evolution of Proportion. Adaptation in Length of Neck. 



Short-necked okapi (left), the forest-living giraffe of the Congo, which browses upon the 

 lower branches of trees. 



Long-necked giraffe (right), the plains-living tvpe of the African savannas, which browses 

 on the higher branches of trees. After Lang. 



darily by differences in chemical composition, there might be 

 brought about all of those differences which serve to charac- 

 terize genera, species, and individuals. Furthermore, since the 

 factors which give rise to constitutional changes in one vital 

 substance would probably operate at the same time to cause 

 related changes in certain others, the alterations in one may 

 logically be assumed to serve as a common index to all. 



"In accordance with the foregoing statement it can readily 

 be understood how environment, for instance, might so affect 



