282 I The Process of Evolution 



to leave more offspring than their cohorts who preferred a strictly 

 terrestrial life. Ascent into the trees meant the penetration of an en- 

 tirely new environment, an environment in which the requirements 

 for survival were strikingly different from those met by terrestrial 

 animals. Many of the trends caused by selection operating on an ar- 

 boreal creature need little explanation. Flexible grasping organs at 

 the ends of the limbs are useful devices for remaining in trees. When 

 leaping from branch to branch it obviously is necessary to be able 

 to judge distances, and genotypes that tended toward good binocular 

 vision were likely to reproduce themselves better than their less 

 fortunate relatives. Genotypes with the eyes rotated toward the front 

 of the head tended to have binocular vision only if a large long 

 snout did not interfere. Both binocular vision and grasping hands 

 and feet lessened the need for a long snout for investigation and 

 manipulation. Shortening of the snout and the resultant reduction 

 of olfactory membranes would be a handicap for a ground-dwell- 

 ing animal, and genotypes having it would probably be selected 

 against. However, in the treetops the loss of sense of smell was less 

 serious and was more than compensated for by improved vision. 



Stereoscopic vision in itself would be of little use without the 

 neural mechanism necessary to evaluate the sensory input and 

 translate it into highly coordinated voluntary movements. Thus, in 

 the arboreal primates, selection resulted in a trend toward high 

 development of the cerebral cortex as a center for evaluation of 

 sensory input and the formulation and initiation of responses to the 

 environmental stimuli received. Because of the arboreal habitat, 

 sight and touch came to override smell and hearing as sources of 

 information. 



Living in trees presents some serious problems in the care of 

 offspring. In primates this gave a selective advantage to individuals 

 that had smaller litters but gave them a high level of care. In many 

 mammals sexual activity is confined to a single season of the year, 

 and the young are born at a time when a suitable food supply is 

 available. In the tropical-forest environment of our distant ances- 

 tors the food supply of fruits and insects was presumably relatively 

 more constant than in temperate zones. Thus there was probably 

 little selection in favor of a single period of sexual activity. In the 

 absence of this factor, selection favored year-round sexuality leading 

 to the continuing presence of males, the establishment of the family 

 group, and increased protection for the smaller litter of helpless 

 young. 



Man is not the only descendant of the shrew-like animals that 

 originally invaded the trees. Like man, baboons have returned to a 

 wholly terrestrial life, while chimpanzees and gorillas spend most, 



