treatment, mental evolution was for Darwin incidental
to and contributory to organic evolution. For specialised research
in comparative and genetic psychology, as an independent field of
investigation, he had neither the time nor the requisite training.
None the less his writings and the spirit of his work have exercised a
profound influence on this department of evolutionary thought. And, for
those who follow Darwin's lead, mental evolution is still in a
measure subservient to organic evolution. Mental processes are the
accompaniments or concomitants of the functional activity of specially
differentiated parts of the organism. They are in some way dependent on
physiological and physical conditions. But though they are not physical
in their nature, and though it is difficult or impossible to conceive
that they are physical in their origin, they are, for Darwin and his
followers, factors in the evolutionary process in its physical or
organic aspect. By the physiologist within his special and well-defined
universe of discourse they may be properly regarded as epiphenomena; but
by the naturalist in his more catholic survey of nature they cannot
be so regarded, and were not so regarded by Darwin. Intelligence has
contributed to evolution of which it is in a sense a product.

The facts of observation or of inference which Darwin accepted are
these: Conscious experience accompanies some of the modes of animal
behaviour; it is concomitant with certain physiological processes; these
processes are the outcome of development in the individual and
evolution in the race; the accompanying mental processes undergo a like
development. Into the subtle philosophical questions which arise out
of the naive acceptance of such a creed it was not Darwin's province
to enter; "I have nothing to do," he said ("Origin of Species" (6th
edition), page 205.), "with the origin of the mental powers, any more
than I have with that of life itself." He dealt with the natural history
of organisms, including not only their structure but their modes of
behaviour; with the natural history of the states of consciousness which
accompany some of their actions; and with the relation of behaviour
to experience. We will endeavour to follow Darwin in his modesty and
candour in making no pretence to give ultimate explanations. But we must
note one of the implications of this self-denying ordinance of science.
Development and evolution imply continuity. For Darwin an