In developing his conception 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.” 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.” “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.” 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.” 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 vigorous and victorious males.”