health by reversion to a more primitive state of psychic development, for all these methods might be called rejuvenation cures. Physiological integration, with its increasing stability of the structural substratum, makes senescence cumulative as we go up the scale of evolution, so that it is ever less balanced or offset by rejuvenation, reproduction, or other regressive changes, as is the case with simple organisms whose life cycle consists merely of brief alternating phases of progression and regression, for the large protozoan cell about to divide is old compared with the two smaller daughter cells formed from it. Senescence is retardation and rejuvenescence is the acceleration that works by transforming, readapting, and even sloughing off old and useless structures. It will take long to modify the course of evolutionary processes that are the result of millions of years of alternating progressive and regressive changes, but not only the phenomena of rejuvenescence but “sports” and saltatory mutation, to say nothing of the findings of recent experiments showing how life and even activities of somatic cells separated from the body and given a more favorable environment may be indefinitely prolonged, point toward a vast reservoir of vitality. Thus we come to a new appreciation of the incalculable energy behind all the phenomena of animate existence and the hope is irrepressible that somehow, although we have as yet no idea how or when, we may abate or inhibit the forces that check or repress it and man may emerge into a fuller and even a longer life. * * * * * Jacques Loeb, of the Rockefeller Institute, has devoted himself for many years, with a rare combination of great learning and originality, to problems directly or indirectly bearing upon old age and death.[175] As his studies of tropism show, he is prone to mechanical and chemical interpretations; and since science has more or less eliminated smallpox, typhoid, yellow fever, malaria, rabies, diphtheria, meningitis, etc., the citizens of scientific nations will sometime, he thinks, be guaranteed a pretty fair probability of a much longer duration of life than they now enjoy. If we define life as the sum of all those forces that resist death, which means disintegration, the latter is comparable to digestion, which transforms meat into soluble products by two ferments, pepsin in the stomach and trypsin in the intestine. These ferments break up the mass into molecules small enough to be absorbed by the blood, and both of them exist not merely in digestive organs but probably in all living cells. They do not destroy our body, perhaps because the coöperation of both is required to do so and this is possible only at a certain degree of acidity, which cannot be reached in the living body because respiration is constantly removing acid. Death thus really comes when respiration ceases.