still have to say that, much more than a million years
after his departure from the chimpanzee level, man had merely advanced
far enough to chip stone implements; because we find no other trace
whatever of intelligence than this until near the close of the
Palaeolithic period. If there is any mystery, it is in the slowness of
man's development.

Let us further recollect that it is a common occurrence in the calendar
of life for a particular organ to be especially developed in one
member of a particular group more than in the others. The trunk of the
elephant, the neck of the giraffe, the limbs of the horse or deer, the
canines of the satire-toothed tiger, the wings of the bat, the colouring
of the tiger, the horns of the deer, are so many examples in the mammal
world alone. The brain is a useful organ like any other, and it is easy
to conceive that the circumstances of one group may select it just as
the environment of another group may lead to the selection of speed,
weapons, or colouring. In fact, as we saw, there was so great and
general an evolution of brain in the Tertiary Era that our modern
mammals quite commonly have many times the brain of their Tertiary
ancestors. Can we suggest any reasons why brain should be especially
developed in the apes, and more particularly still in the ancestors of
man?

The Primate group generally is a race of tree-climbers. The appearance
of fruit on early Tertiary trees and the multiplication of carnivores
explain this. The Primate is, except in a few robust cases, a
particularly defenceless animal. When its earliest ancestors came in
contact with fruit and nut-bearing trees, they developed climbing power
and other means of defence and offense were sacrificed. Keenness of
scent and range of hearing would now be of less moment, but sight would
be stimulated, especially when soft-footed climbing carnivores came on
the scene. There is, however, a much deeper significance in the adoption
of climbing, and we must borrow a page from the modern physiology of the
brain to understand it.

The stress laid in the modern education of young children on the use of
the hands is not merely due to a feeling that they should handle objects
as well as read about them. It is partly due to the belief of many
distinguished physiologists that the training of the hands has a direct
stimulating effect on the thought-centres in the brain. The centre in
the cerebrum which controls the use of the hands is