an explanation began to seem very improbable, and it was completely discredited by the fact that many kinds of plants and animals have persisted with little or no change during several successive periods, side by side with other kinds in which there has been extensive variation and extinction. It was further observed that between the forms of successive periods in the same geographical regions there was a manifest family likeness, indicating that the later were connected with the earlier through the ordinary bonds of physical descent. A host of facts from comparative morphology and embryology went to confirm this inference; and so, when after nearly twenty years of incubation Mr. Darwin was ready to plant the seeds of his remarkable theory, he found the soil very thoroughly prepared and fertilized in which to plant them. All that men were waiting for was the discovery of a vera causa, All that was wanted was to be able to point to some one agency, similar to agencies now in operation and therefore intelligible, which could be proved to be capable of making specific changes in plants and animals. Mr. Darwin's solution of the problem was so beautiful, it has become so generally accepted and so deeply interfused into all the thinking of our time, it seems now so natural and so inevitable, that we may be in danger of forgetting that the problem was really one of the most complicated and abstruse that the scientific mind has ever grappled with. Starting from the known experiences of breeders of domestic animals and cultivated plants, and duly considering the remarkable and sometimes wonderful changes that are wrought by the simple process of selection, the problem before Mr. Darwin was to detect among the multifarious phenomena of organic nature any agency capable of accomplishing what man thus accomplishes by selection. In detecting the agency of natural selection, working perpetually through the preservation of favored individuals and races in the struggle for existence, Mr. Darwin found the vera causa for which men were waiting. With infinite patience and caution he applied his method of explanation to one group of organic phenomena after another, meeting in every quarter with fresh and often unexpected verification. He had the satisfaction of living to see pretty much the whole contemporary world of zoölogists, botanists, and palæontologists pursuing the lines of investigation which he had laid down and in general agreement as to the fundamental principle. There was a general acquiescence in natural selection as an agency capable of working specific changes, while further speculation and investigation in all directions were employed in ascertaining the precise character of its work and determining the limits of its efficacy. That all the phenomena of the organic world can be accounted for by natural selection, Mr. Darwin never at any time supposed; nor was he ever so silly as to suppose that all
