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It is for the same reason that no insect has ever become even moder-
ately intelligent. The fixed pathways of instinct, however elaborate,
require far fewer nerve-cells than the multiple switchboards that
underlie intelligence. It appears to be impossible to build a brain
mechanism for flexible behaviour with less than a quite large mini-
mum of neurones; and no insect has reached a size to provide this
Thus only the land vertebrates are left. The reptiles shared bio-
logical dominance with the insects in the Mesozoic. But while the
insects had reached the end of their blind alley, the reptiles showed
themselves capable of further advance. Temperature regulation is a
necessary basis for final progress, since without it the rate of bodily
function could never be stabilized, and without such stabilization,
higher mental processes could never become accurate and de-
Two reptilian lines achieved this next step, in the guise of the birds
and the mammals. The birds soon, however, came to a dead end,
chiefly because their forelimbs were entirely taken up in ihe special-
ization for flight. The sub-human mammals made another funda-
mental advance, in the shape of internal development, permitting the
young animal to arrive at a much more advanced stage before it was
called upon to face the world. They also (like the birds) developed
true family life.
Most mammalian lines, however, cut themselves off from indefinite
progress by one-sided evolution, turning their limbs and jaws into
specialized and therefore limited instruments. And, for the most
part, they relied mainly on the crude sense of smell, which cannot
present as differentiated a pattern of detailed knowledge as can sight.
Finally, the majority continued to produce their young several at a
time, in litters. As J. B. S. Haldane has pointed out, this gives rise
to an acute struggle for existence in the prenatal period, a consider-
able percentage of embryos being aborted or resorbed. Such intra-
uterine selection will put a premium upon rapidity of growth and
differentiation, since the devil takes the hindmost; and this rapidity
of development will tend automatically to be carried on into postnatal
As everyone knows, man is characterized by a rate of development
which is abnormally slow as compared with that of any other mam-
mal. The period from birth to the first onset of sexual maturity
comprises nearly a quarter of the normal span of his life, instead of an
eighth, a tenth or twelfth, as in some other animals. This again is in
one sense a unique characteristic of man, although from the evolu-