158 



EVOLUTION AND GENETICS 



development may be included the evolution of the four limbs for 

 rapid locomotion and the differentiation of the teeth. The first 



is supposed to have been as- 

 sociated with the develop- 

 ment of intelligence and 

 change of location through 

 migration. Differentiation 

 of the teeth would fit the 

 animal for eating different 

 kinds of food, which would 

 tend to stimulate the devel- 

 opment of powers of observa- 

 tion and choice, and con- 

 sequently intelligence. We 

 are unable to associate the 

 warm-blooded condition of 

 the mammals so directly with 

 external factors, but Osborn 

 supposes that it may have 

 appeared in some of the 

 cynodonts; if so, it would no 

 doubt also have favored the 

 evolution of mental powers 

 by maintaining a high and 

 constant rate of metabolism 

 and thus freeing the animals 

 partly from their dependence 

 upon the physical environ- 



IV V F i- .7 



. ment. 

 Fig. 88. — Diagrammatic sections oi vari- 

 ous forms of teeth. I, tusk or incisor The Course of Mammalian 

 of elephant; II, human incisor during Evolution. With independ- 

 development; III^^ completely formed f.^^^ ^f ^j-^g water for repro- 

 duction and the maintenance 

 of body moisture, came 

 greater dependence for ex- 

 istence upon the varied 

 factors of the terrestrial en- 

 vironment. Since food habits estaV)lish the most important con- 

 tact of the individual with his (environment, and the teeth of 

 mammals are specialized for various food habits (Fig. 88), the 



human incisor; IV, human molar; V, 

 molar of ox. In all figures the enamel 

 is black, the dentine shaded with hori- 

 zontal lines, the pulp white, and the 

 cement stippled. (From Parker and 

 Haswell, after Flower and Lydekker.) 



