Hence all these theories lead to the conception of a medium in which the propagation takes place, and if we admit this medium as an hypothesis, I think we ought to endeavour to construct a mental representation of all the details of its action, and this has been my constant aim in this treatise. ELIE METCHNIKOFF The Nature of Man Elie Metchnikoff, Sub-Director of the Pasteur Institute in Paris, was born May 15, 1845, in the province of Kharkov, Russia, and has worked at the Pasteur Institute since 1888. The greater part of Metchnikoff's work is concerned with the most intimate processes of the body, and notably the means by which it defends itself from the living agents of disease. He is, indeed, the author of a standard treatise entitled "Immunity in Infective Diseases." His early work in zoology led him to study the water-flea, and thence to discover that the white cells of the human blood oppose, consume, and destroy invading microbes. Latterly, Metchnikoff has devoted himself in some measure to more general and especially philosophical studies, the outcome of which is best represented by the notable volume on "The Nature of Man," which was published at Paris in 1903. _I.--Disharmonies in Nature_ Notwithstanding the real advance made by science, it cannot be disputed that a general uneasiness disturbs the whole world to-day, and the frequency of suicide is increased greatly among civilised peoples. Yet if science turns to study human nature, there may be grounds for hope. The Greeks held human nature and the human body in high esteem, and among the Romans such a philosopher as Seneca said, "Take nature as your guide, for so reason bids you and advises you; to live happily is to live naturally." In our own day Herbert Spencer has expressed again the Greek ideal, seeking the foundation of morality in human nature itself. But it has often been taught that human nature is composed of two hostile elements, a body and a soul. The soul alone was to be honoured, while the body was regarded as the vile source of evils. This doctrine has had many disastrous consequences, and it is not surprising that in consequence of it celibacy should have been regarded as the ideal state. Art fell from the Greek ideal until the Renaissance, with its return to that ideal, brought new vigour. When the ancient spirit was born again its influence reached science and even religion, and the Reformation was a defence of human nature. The Lutheran doctrines resumed the principle of a "development as complete as possible of all the natural powers" of man, and compulsory celibacy was abolished. The historical diversity of opinion regarding human nature is what has led me to the attempt to give an exposition of human nature in its strength and in its weakness. But, before dealing with the man himself, we must survey the lower forms of life.