194 EVOLUTION AND GENETICS 



wide range of stimuli would also be brought to bear upon the ani- 

 mal by its increased ability to investigate other things. 



Mind and Environment. Conditions at present show us that 

 only tropical forests are favorable to the existence of such arboreal 

 types. Elsewhere they would find food too scarce, and would be 

 forced to seek it on the ground as well as in the trees. In the tropi- 

 cal forests, however, there is a constant supply of fruits, which 

 are well adapted to mastication by their rather primitive teeth. 

 Insects are also available as a part of their diet. We have already 

 seen in the heterodont dentition of more primitive mammals and 

 of the cynodont reptiles an initial stimulus to development of in- 

 telligence through the variety of reactions in securing food. Such 

 food habits as those of the primates might well be correlated with 

 this type of development, but in other ways as well the tropical 

 forests would provide diversity of contacts. Their richness in 

 all forms of Ufe is unsurpassed by any other terrestrial environ- 

 ment. 



From the Trees to the Ground. Anything which might force 

 these arboreal creatures to the ground, such as scarcity of food or 

 the thinning of the forests, would find them well able to meet com- 

 petition. They would be less specialized for attack or defense 

 than the carnivores, less specialized for flight from their enemies 

 than the herbivores, and consequently unable to meet either of 

 these groups in direct competition within their limited fields of 

 activity. However, they could easily escape from terrestrial 

 enemies by climbing, and through their omnivorous habits would 

 find an abundance of food without the limitations imposed upon 

 the more specialized animals. 



On the ground the animal would have a new set of tools at his 

 disposal. His hind-limbs, developed for perching upon branches, 

 would find the ground a more stable substratum, and his hands 

 would then be freed from all but occasional use in locomotion. 

 Their grasping power, turned to new uses, might readily accom- 

 plish many things. The use of stones and clubs as weapons sug- 

 gests itself as a simple result of his inferiority in competition with 

 carnivores, but whatever might be the beginning of his use of im- 

 plements it could hardly fail to open up new possibilities. 



All of the things which might logically follow upon descent from 

 the trees point toward ever increasing diversity of activities and 

 stimuli. Once the ability is acquired to make use of other things 



