r's saying that "a little dose of judgment
or reason often comes into play, even with animals low in the scale of
nature." (Ibid. page 205.) 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 essential to the understanding of this thesis that the differentiae
of the hereditary factors should be clearly grasped.

For Darwin there were two modes of racial preparation, (1) natural
selection, and (2) the establishment of individually acquired habit. He
showed that instincts are subject to hereditary variation; he saw that
instincts are also subject to modification through acquisition in the
course of individual life. He believed that not only the variations but
also, to some extent, the modifications are inherited. He therefore held
that some instincts (the greater number) are due to natural selection
but that others (less numerous) are due, or partly due, to the
inheritance of acquired habits. The latter involve Lamarckian
inheritance, which of late years has been the centre of so much
controversy. It is noteworthy however that Darwin laid especial emphasis
on the fact that many of the most typical and also the most
complex instincts--those of neuter insects--do not admit of such an
interpretation. "I am surprised," he says ("Origin of Species"
(6th edition), page 233.), "that no one has hitherto advanced this
demonstrative case of neuter insects, against the well-known doctrine of
inherited habit, as advanced by Lamarck." None the less Darwin admitted
this doctrine as supplementary to that which was more distinctively his
own--for example in the case of the instincts of domesticated animals.