From observations made originally on water-fleas, he was led to discover the functions of the white corpuscles of human blood. He showed by what mechanism these made perpetual war against the intruding microbes of disease, and he laid the foundations of knowledge as to the agencies that weaken and the modes of strengthening these guardians of our health. In a series of investigations into the phenomena of inflammation in men and lower animals, he carried his observations into new fields, and explained the relations of the white corpuscles to the juices that attract and repel them (chemotaxis). It was he, for instance, who discovered that these corpuscles, under certain circumstances, migrate into the hairs and absorb and remove the pigment, so producing the blanching of old age. Although popularly the most interesting this was far from being the most important of the changes of senile decay that he found to be due to the activity of the wandering cells of the body. And, as will be seen in the present volume, the actions and interactions of the bacteria harboured in the body, the white corpuscles that are a natural part of the body, and the various juices or serums produced naturally or introduced by accident or design, are concerned in life itself and the decay of life. Metchnikoff is an expert of experts in the science of life, and has gained the right to a hearing by forty years of patient devotion and brilliant research. In the volume that he has now given to the public, he has addressed himself to the gravest and the most serious problems of humanity, to life and sex and death and the fear of death. From the earliest days when man could spare time from the satisfaction of his immediate wants to reflect upon his nature and destiny, these problems and the invention of fantastic solutions or evasive anodynes have absorbed his attention. The folklore and philosophy, the religion and poetry of all races and of all stages of culture, from savage barbarism to decadent refinement, revolve round these obsessions of the mind, and, as Metchnikoff most plainly shows, no enduring comfort has yet been found. Now for the first time in the history of thought, the exact methods of science have been brought to the statement of the problems.