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.”

It is difficult to draw a sharp line between philosophy and natural science. The naturalist who introduces a new principle, or demonstrates a fact which throws a new light on existence, not only renders an important service to philosophy but is himself a philosopher in the broader sense of the word. The aim of philosophy in the stricter sense is to attain points of view from which the fundamental phenomena and the principles of the special sciences can be seen in their relative importance and connection. But philosophy in this stricter sense has always been influenced by philosophy in the broader sense. Greek philosophy came under the influence of logic and mathematics, modern philosophy under the influence of natural science. The name of Charles Darwin stands with those of Galileo, Newton, and Robert Mayer—names which denote new problems and great alterations in our conception of the universe.

First of all we must lay stress on Darwin's own personality. His deep love of truth, his indefatigable inquiry, his wide horizon, and his steady self-criticism make him a scientific model, even if his results and theories should eventually come to possess mainly an historical interest. In the intellectual domain the primary object is to reach high summits from which wide surveys are possible, to reach them toiling honestly upwards by the way of experience, and then not to turn dizzy when a summit is gained. Darwinians have sometimes turned dizzy, but Darwin never.

There is scarcely any subject to which Darwin devoted so much time and work as to his researches into the biology of flowers, or, in other words, to the consideration of the question to what extent the structural and physiological characters of flowers are correlated with their function of producing fruits and seeds. We know from his own words what fascination these studies possessed for him. We repeatedly find, for example, in his letters expressions such as this:— “Nothing in my life has ever interested me more than the fertilisation of such plants as Primula and Lythrum, or again Anacamptis or Listera.”

Expressions of this kind coming from a man whose theories exerted an epoch-making influence, would be unintelligible if his researches into the biology of flowers had been concerned only with records of isolated facts, however interesting these might be. We may at once take it for granted that the investigations were undertaken with the view of following up important problems of general interest, problems which are briefly dealt with in this essay.

Darwin published the results of his researches in several papers and in three larger works, (i) On the various contrivances by which British and Foreign Orchids are fertilised by insects (First edition, London, 1862; second edition, 1877 ; popular edition, 1904). (ii) The effects of Cross and Self fertilisation in the vegetable kingdom (First edition, 1876 ; second edition, 1878).