In its bearing on man's place in nature this question is one of the most momentous with which natural science has to deal, and it has occupied the attention of thinking men in every age. I cannot trace its history, but it will be worth our while to place side by side the words of three of the great leaders of modern scientific and philosophic thought. The saying has been attributed to Descartes, "Give me matter and I will construct the world"--meaning by this the living world as well as the non-living; but Descartes specifically excepted the human mind. I do not know whether the great French philosopher actually used these particular words, but they express the essence of the mechanistic hypothesis that he adopted. Kant utterly repudiated such a conception in the following well known passage: "It is quite certain that we cannot become adequately acquainted with organized creatures and their hidden potentialities by means of the merely mechanical principles of nature, much less can we explain them; and this is so certain that we may boldly assert that it is absurd for man even to make such an attempt or to hope that a Newton may one day arise who will make the production of a blade of grass comprehensible to us according to natural laws that have not been ordered by design. Such an insight we must absolutely deny to man." Still, in another place Kant admitted that the facts of comparative anatomy give us "a ray of hope, however faint, that something may be accomplished by the aid of the principle of the mechanism of nature, without which there can be no science in general." It is interesting to turn from this to the bold and aggressive assertion of Huxley: "Living matter differs from other matter in degree and not in kind, the microcosm repeats the macrocosm; and one chain of causation connects the nebulous origin of suns and planetary systems with the protoplasmic foundations of life and organization."