In the last lecture we have noticed the general relations of agnostic speculations with natural science, and have exposed their failure to account for natural facts and laws. We may now inquire into their mode of dealing with the phenomena of life, with regard to the supposed spontaneous evolution of which, and its development up to man himself, so many confident generalizations have been put forth by the agnostic and monistic philosophy. In the earlier history of modern natural science, the tendency was to take nature as we find it, without speculation as to the origin of living things, which men were content to regard as direct products of creative power. But at a very early period--and especially after the revelations of geology had disclosed a succession of ascending dynasties of life--such speculations, which, independently of science, had commended themselves to the poetical and philosophical minds of antiquity, were revived. In France more particularly, the theories of Buffon, Lamarck, and Geoffroy St. Hilaire opened up these exciting themes, and they might even then have attained to the importance they have since acquired but for the great and judicial intellect of Cuvier, which perceived their futility and guided the researches of naturalists into other and more profitable fields. The next stimulus to such hypotheses was given by the progress of physiology, and especially by researches into the embryonic development of animals and plants. Here it was seen that there are homologies and likenesses of plan linking organisms with each other, and that in the course of their development the more complex creatures pass through stages corresponding to the adult condition of lower forms. The questions raised by the geographical distribution of animals, as ascertained by the numerous expeditions and scientific travellers of modern times, tended in the same direction. The way was thus prepared for the broad generalizations of Darwin, who, seizing on the idea of artificial selection as practised by breeders of animals and plants, and imagining that something similar takes place in the natural struggle for existence, saw in this a plausible solution for the question of the progress and the variety of organized beings.