MAN'S ARBOREAL APPRENTICESHIP 291 



But to return to the branches, another arboreal 

 acquisition was a greatly increased power of turning 

 the head from side to side. All ordinary mammals 

 can say "yes," raising and lowering their heads; 

 but the ability to say " no " with emphasis was 

 gained by the arboreal Primates. And it was a 

 great advantage to be able to move the head so as 

 to locate sounds and supplement the mobility of the 

 eyeballs. To arboreal life are also due, according 

 to the anatomist, flat chests and flat backs; and 

 whereas the breathing movements of the ribs count 

 for most in quadrupeds, the movements of the 

 midriff or diaphragm mean most to tree-climbers 

 and modern man. 



Except in special cases, such as marsupials and 

 nest-makers, arboreal mammals tend to have small 

 families, and it could not well be otherwise. In 

 the great majority of Primates only one offspring 

 is born at a time a reproductive economy which 

 would be dangerous were there not a correlated 

 evolution of parental carefulness. All mother- 

 monkeys carry their babies, and hold them in their 

 arms, nursing them " in very human fashion," as 

 Sir Richard Owen said. The fathers, too, have 

 repeatedly been seen carrying the young on their 

 arboreal journeys. Now, the fact that the Simian 

 parent has to carry the very immature baby about 

 implies an intensification of the family relations; it 

 must surely have favored the growth of gentleness. 



On another line is the consideration that arboreal 

 life limits the usefulness of the olfactory organs, 



