Yet another factor in the intellectual stir and bustle of the time must needs be mentioned even in so short and cursory a sketch as this of the causes which led to the Darwinian crisis. In 1798, Thomas Malthus, a clergyman of the Church of England, published the first edition of his famous and much-debated 'Essay on the Principle of Population.' Malthus was the first person who ever called public attention to the tendency of population to increase up to the utmost limit of subsistence, as well as to the necessary influence of starvation in checking its further development beyond that point. Though his essay dealt only with the question of reproduction in human societies, it was clear that it possessed innumerable analogies in every domain of animal and vegetable life. The book ran through many successive editions with extraordinary rapidity for a work of its class, it was fiercely attacked and bravely defended, it caused an immense amount of discussion and debate, and besides its marvellous direct influence as a germinal power upon the whole subsequent course of politico-economical and sociological thought, it produced also a remarkable indirect influence on the side current of biological and speculative opinion. In particular, as we shall more fully see hereafter, it had an immediate effect in suggesting to the mind of the great naturalist who forms our present subject the embryo idea of 'natural selection.' Such then was the intellectual and social world into which, early in the present century, Charles Darwin found himself born. Everywhere around him in his childhood and youth these great but formless evolutionary ideas were brewing and fermenting. The scientific society of his elders and of the contemporaries among whom he grew up was permeated with the leaven of Laplace and of Lamarck, of Hutton and of Herschel. Inquiry was especially everywhere rife as to the origin and nature of specific distinctions among plants and animals. Those who believed in the doctrine of Buffon and of the 'Zoonomia' and those who disbelieved in it, alike, were profoundly interested and agitated in soul by the far-reaching implications of that fundamental problem. On every side evolutionism, in its crude form, was already in the air. Long before Charles Darwin himself published his conclusive 'Origin of Species,' every thinking mind in the world of science, elder and younger, was deeply engaged upon the self-same problem. Lyell and Horner in alternate fits were doubting and debating. Herbert Spencer had already frankly accepted the new idea with the profound conviction of _a priori_ reasoning. Agassiz was hesitating and raising difficulties. Treviranus was ardently proclaiming his unflinching adhesion. Oken was spinning in metaphysical Germany his fanciful parodies of the Lamarckian hypothesis. Among the depths of Brazilian forests Bates was reading the story of evolution on the gauze-like wings of tropical butterflies.