Progress may be defined as continuous adaptation. In modern phrase, this is called evolution. The continuity makes the difference between evolution and revolution. Both are natural. Both occur in the organic, the social, and the intellectual sphere. Evolution is the orderly progress of the organism or group of organisms, by which it becomes more and more in harmony with surrounding conditions. If the conditions become more and more complex, the organism will progress in complexity; but if the conditions be more and more simple, progress (if such it may still be called) will be towards simplicity of structure, unnecessary complexity being eliminated, or, in any case, disappearing. Hence, in parasites and some forms of life which live under simple conditions, we have the phenomena of degeneration, or a passage from a more complex to a more simple condition. Revolution in organic life is the destruction of one organism or group of organisms, and the replacement in its stead of a wholly different organism or group of organisms. During hard times there may be much revolution, or replacement of one set of organic forms by another set of organic forms. It was by revolution that the dominant reptiles of the Mesozoic epoch were replaced by the dominant mammals of Kainozoic times. It was by revolution that pterodactyls were supplanted by birds. Revolution has exterminated many a group in geological ages. On the other hand, it was by evolution that the little-specialized Eocene ungulates gave rise to the horse, the camel, and the deer; by divergent evolution that the bears and dogs were derived from common ancestors. Palæontology testifies both to evolution and revolution.[BC] That history does the same, I need not stay to exemplify. The same laws also apply to systems of thought. Darwinism has revolutionized our conceptions of nature. Darwin placed upon a satisfactory basis a new order of interpretation of the organic world. By it other interpretations have been supplanted. And now this new conception is undergoing evolution, not without some divergence.