'So far as natural science can tell us, every quality of sense or
intellect which does _not_ help us to fight, to eat, and to bring up
children, is but a by-product of the qualities which do.'

The pre-rational character of many of our impulses is, however, disguised
by the fact that during the lifetime of each individual they are
increasingly modified by memory and habit and thought. Even the
non-human animals are able to adapt and modify their inherited impulses
either by imitation or by habits founded on individual experience. When
telegraph wires, for instance, were first put up many birds flew against
them and were killed. But although the number of those that were killed
was obviously insufficient to produce a change in the biological
inheritance of the species, very few birds fly against the wires now.
The young birds must have imitated their elders, who had learnt to avoid
the wires; just as the young of many hunting animals are said to learn
devices and precautions which are the result of their parents'
experience, and later to make and hand down by imitation inventions of
their own.

Many of the directly inherited impulses, again, appear both in man and
other animals at a certain point in the growth of the individual, and
then, if they are checked, die away, or, if they are unchecked, form
habits; and impulses, which were originally strong and useful, may no
longer help in preserving life, and may, like the whale's legs or our
teeth and hair, be weakened by biological degeneration. Such temporary
or weakened impulses are especially liable to be transferred to new
objects, or to be modified by experience and thought.

With all these complicated facts the schoolmaster has to deal. In
Macaulay's time he used to be guided by his 'common-sense,' and to