The present work treats the problem of the Origin of Species on the same general lines as were adopted by Darwin but from the standpoint reached after nearly thirty years of discussion, with an abundance of new facts and the advocacy of many new or old theories. While not attempting to deal, even in outline with the vast subject of evolution in general, an endeavour has been made to give such an account of the theory of Natural Selec tion as may enable any intelligent reader to obtain a clear conception of Darwin's work, and to understand something of the power and range of his great principle. Darwin wrote for a generation which had not accepted evolution, and which poured contempt on those who upheld the derivation of species from species by any natural law of descent. He did his work so well that descent with modification is now universally accepted as the order of nature in the organic world; and the rising generation of naturalists can hardly realise the novelty of this idea, or that their fathers considered it a scientific heresy to be condemned rather than seriously discu8sed.