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, 
