ption of organic evolution Charles Darwin was
of necessity brought into contact with some of the problems of mental
evolution. In _The Origin of Species_ he devoted a chapter to "the
diversities of instinct and of the other mental faculties in animals
of the same class."[153] When he passed to the detailed consideration
of _The Descent of Man_, it was part of his object to show "that there
is no fundamental difference between man and the higher mammals in
their mental faculties."[154] "If no organic being excepting man," he
said, "had possessed any mental power, or if his powers had been of a
wholly different nature, from those of the lower animals, then we
should never have been able to convince ourselves that our high
faculties had been gradually developed."[155] In his discussion of
_The Expression of the Emotions_ it was important for his purpose
"fully to recognise that actions readily become associated with other
actions and with various states of the mind."[156] His hypothesis of
sexual selection is largely dependent upon the exercise of choice on
the part of the female and her preference for "not only the more
attractive but at the same time the more vigourous and vicious
males."[157] Mental processes and physiological processes were for
Darwin closely correlated; and he accepted the conclusion "that the
nervous system not only regulates most of the existing functions of
the body, but has indirectly influenced the progressive development of
various bodily structures and of certain mental qualities."[158]

Throughout his 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 ev