ential to the understanding of this
thesis that the differentiae of the hereditary factor 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,[163] "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. Still, even in such cases, "it may
be doubted," he says,[164] "whether any one would have thought of
training a dog to point, had not some one dog naturally shown a
tendency in this line ... so that habit and some degree of selection
have probably concurred in civilising by inheritance our dogs." But in
the interpretation of the instincts of domesticated animals, a more
recently suggested hypothesis, that of organic selection,[165] may be
helpful. According to this hypothesis any intelligent modification of
behaviour which is subject to selection is probably coincident in
direction with an inherited tendency to behave in this fashion. Hence
in such behaviour there are two factors: (1) an incipient variation in
the line of such behaviour, and (2) an acquired modification by which
the behaviour is carried further along the same line. Under natural
selection those organisms in which the two factors cooeperate are
likely to survive. Under artificial selection they are deliberately
chosen out from among the rest.

Organic selection has been termed a compromise betwee