migrate and to lay her eggs in other birds' nests. An
action, which we ourselves require experience to enable us to perform,
when performed by an animal, more especially by a very young one,
without experience, and when performed by many individuals in the same
way, without their knowing for what purpose it is performed, is
usually said to be instinctive." And in the summary at the close of
the chapter he says,[161] "I have endeavoured briefly to show that the
mental qualities of our domestic animals vary, and that the variations
are inherited. Still more briefly I have attempted to show that
instincts vary slightly in a state of nature. No one will dispute that
instincts are of the highest importance to each animal. Therefore
there is no real difficulty, under changing conditions of life, in
natural selection accumulating to any extent slight modifications of
instinct which are in any way useful. In many cases habit or use and
disuse have probably come into play."

Into the details of Darwin's treatment there is neither space nor need
to enter. There are some ambiguous passages; but it may be said that
for him, as for his followers to-day, instinctive behaviour is wholly
the result of racial preparation transmitted through organic heredity.
For the performance of the instinctive act no individual preparation
under the guidance of personal experience is necessary. It is true
that Darwin quotes with approval Huber's saying that "a little dose of
judgment or reason often comes into play, even with animals low in the
scale of nature."[162] But we may fairly interpret his meaning to be
that in behaviour, which is commonly called instinctive, some element
of intelligent guidance is often combined. If this be conceded the
strictly instinctive performance (or part of the performance) is the
outcome of heredity and due to the direct transmission of parental or
ancestral aptitudes. Hence the instinctive response as such depends
entirely on how the nervous mechanism has been built up through
heredity; while intelligent behaviour, or the intelligent factor in
behaviour, depends also on how the nervous mechanism has been modified
and moulded by use during its development and concurrently with the
growth of individual experience in the customary situations of daily
life. Of course it is essential to the Darwinian thesis that what Sir
E. Ray Lankester has termed "educability," not less than instinct, is
hereditary. But it is also ess