The best of Humboldt's work became absorbed long ago into the confused mass of general knowledge. This is the common fate (not by any means an unhappy one) of those who refuse to concentrate upon a single study. Among biologists he is chiefly remembered by his numerous discussions of plant-distribution, which are now considered less remarkable for what they contain than for what they leave out. While his travels were fresh in his mind, Humboldt was impressed by facts of distribution which could not be explained by present physical conditions,[42] but the influence of climate as the more intelligible factor gradually assumed larger and larger proportions in his mind. The writers of text-books, founding their teaching upon Humboldt, often overlooked altogether qualifications which he had shown to be necessary. When Darwin and Wallace pointed out how immensely important is the bearing upon present distribution, not only of the physical history of the great continents, but also of their biological history, and in particular of the interminable conflicts of races of which they have been the scene, naturalists began to perceive how inadequate are horizontal and vertical isothermal zones to explain all the striking facts of distribution, whether of plants or animals (see _infra_ p 129). Premonitions of Biological Evolution. The eighteenth century had done much to impress the minds of men with an orderly development in sun and planets (Kant and Laplace), in the institutions of human societies (Montesquieu), and in the moral aspirations of mankind (Lessing). Many bold attempts had been made to trace a like orderly development in the physical life of plants and animals (Buffon, Erasmus Darwin, etc.), but neither was the proof cogent nor the process intelligible. Cautious people therefore, and those whose prepossessions inclined them to adopt a very different origin for terrestrial life, held during all this time a position of some strength against speculative philosophers who tried to explain the variety and perfection of living nature by unconscious and unintelligent factors. About the year 1840 the doctrine of the fixity of species seemed to be victorious. Cuvier's knowledge and skilful advocacy had a few years before over-powered Geoffroy St. Hilaire's conception of a common plan of structure pervading the whole animal kingdom, and the new _Philosophie Anatomique_ was laid on the shelf, side by side with the _Philosophie Zoologique_ of Lamarck, the _Zoonomia_ of Erasmus Darwin, the _Théorie de la Terre_ of Buffon, and the _Protogæa_ of Leibnitz. Yet even then a spectator who was fully informed and at the same time gifted with uncommon foresight might have satisfied himself that the victory of evolution had become inevitable.