Although I regard the doctrine of descent as proved, and hold it to be one of the greatest acquisitions of human knowledge, I must repeat that I do not mean to say that everything is clear in regard to the evolution of the living world. On the contrary, I believe that we still stand merely on the threshold of investigation, and that our insight into the mighty process of evolution, which has brought about the endless diversity of life upon our earth, is still very incomplete in relation to what may yet be found out, and that, instead of being vainglorious, our attitude should be one of modesty. We may well rejoice over the great step forward which the dominant recognition of the Evolution theory implies, but we must confess that the beginnings of life are as little clear to us as those of the solar system. But we can do this at least: we can refer the innumerable and wonderful inter-relations of the organic cosmos to their causes--common descent and adaptation--and we can try to discover the ways and means which have co-operated to bring the organic world to the state in which we know it. When I say that the theory of descent is the most progressive step that has yet been taken in the development of human knowledge, I am bound to give my reasons for this opinion. It is justified, it seems to me, even by this fact alone, that the Evolution idea is not merely a new light on the special region of biological science, zoology and botany, but is of quite general importance. The conception of an evolution of the world of life upon the earth reaches far beyond the bounds of any single science, and influences our whole realm of thought. It means nothing less than the elimination of the miraculous from our knowledge of nature, and the placing of the phenomena of life on the same plane as the other natural processes, that is, as having been brought about by the same forces, and being subject to the same laws. In the domain of the inorganic, no one now doubts that out of nothing nothing can come: energy and matter are from everlasting to everlasting, they can neither be increased or decreased, they can only be transformed--heat into mechanical energy, into light, into electricity, and so on. For us moderns, the lightning is no longer hurled by the Thunderer Zeus on the head of the wicked, but, careless alike of merit or guilt, it strikes where the electric tension finds the easiest and shortest line of discharge. Thus to our mode of thought it now seems clear that no event in the world of the living depends upon caprice, that at no time have organisms been called forth out of nothing by the mighty word of a Creator, but they have been produced at all times by the co-operation of the existing forces of nature, and every species must have arisen just where, and when, and in the form in which it actually did arise, as the necessary outcome of the existing conditions of energy and matter, and of their interactions upon each other. It is