Long before the time of Darwin and Huxley, almost at the beginning of recorded thought, philosophers busied themselves with the wonderful diversity of the living world and with speculations as to how it had assumed its present form. From the earliest times to this century, theories as to the living world fell into one or other of two main groups. The key-note of one group was the fixity of species: the belief that from their first appearance species were separate, independent entities, one never springing from another, new species never arising by the modification in different directions of descendants of already existing species. The key-note of the other group of theories was the idea of progressive change: that animals and plants as they passed along the stream of time were continually being moulded by the forces surrounding them, and that the farther back the mind could go in imagination the fewer and simpler species would be; until, in the first beginning, all the existing diverse kinds of living creatures would converge to a single point. It may be that, on the whole, the idea of fixity prevailed more among thinkers with a religious bias; but for the most part the theories were debated independently of the tenets of any faith, Christian or other. There were sceptical defenders of fixity and religious upholders of evolution. However, in Christian countries, from the time of the Reformation onwards, a change in this neutrality of religion to theories of the living world took place. As Pascal prophesied, Protestantism rejected the idea of an infallible Church in favour of the idea of an infallible book, and, because it happened that this book included an early legend of the origin of the world in a form apparently incompatible with evolution, Protestantism and, to a lesser and secondary extent, Catholicism, assumed the position that there was no place for evolution in a Christian philosophy. At the end of last century, and up to the middle of this century, the problem was not raised in any acute form. The chief anatomists and botanists were occupied with the investigation and discovery of facts, and, in an ordinary way, without taking any particular trouble about it, accepted more or less loosely the idea that species were fixed. Now and then an evolutionist propounded his views; but, as a rule, he supported them with a knowledge of facts very much inferior to that possessed by the more orthodox school. Then came Herbert Spencer, reasserting evolution in the old broad spirit, not merely in its application to species, but as the guiding principle of the whole universe from the integrations of nebulæ into systems of suns and planets to the transformations of chemical bodies. Before his marvellous generalisations had time to grip biologists, there came Darwin; and Darwin brought two things: first, a re-statement of the fact of evolution as applied to the living world, supported by an enormous body of evidence, new and old,