One of the things difficult to explain in the world of thought was the sudden and overmastering impetus given to the theory of development by the publication of the works of Charles Darwin. The main idea of that theory, namely, that the higher orders of life had come into being through evolution from lower types, was by no means new; and there is so much among the plainest features of the world's life to suggest the idea, that one can only wonder why, if the scientific mind was waiting for, and wanting, the theory (as it appears to have been doing), it should not have taken it up long before Darwin's time.