of ectodermic, and after about 22 to those of mesodermic origin. The death-rate curve rises at a practically constant rate to extreme old age. From about 60 to the end of life deaths from the breakdown of organs of mesodermic origin lead. The heart generally outwears the lungs and the brain outwears both because evolution is a purely mechanical process instead of being an intelligent one. “It is conceivable that an omnipotent person could have made a much better machine as a whole than the human body which evolution has produced. He would presumably have made an endoderm with as good resisting and wearing qualities as a mesoderm or ectoderm. Evolution by the haphazard process of trial and error which we call natural selection makes each part only just good enough to get by.” All this, the author believes, only strengthens the evidence that the most important part in longevity is played by innate constitutional biological factors. This view so commonly held, that heredity is the chief factor in longevity is doubtless correct in general. But it is fatalistic and directly tends to lessen the confidence of hygienists and physicians in the efficacy of all their methods of prolonging life in the aged. There is, we think, good reason to believe that there is a great and now rapidly growing number of exceptions to this so-called law, cases in which by conformity to right rules of living, age has been increased many years beyond that which our forbears attained. Indeed, the very fact of the gradual prolongation of life shows that the hereditary predisposition to die at a certain age can be, to a great extent, overcome. The psychological effect of this dogma of the prepotence of heredity in determining the length of life is itself not only depressing but may readily become, as psychologists can best understand, a dangerous lethal agent with the old and cause those who have reached the span of years at which their forbears died to succumb to their troubles with less resistance. Indeed, it is one of the chief purposes of this volume to show that the old-age problem is not merely economic, philanthropic, social, or even medical, but also, when all is said and done, perhaps chiefly psychological and that the future welfare of the race depends upon the development of an old age due not chiefly to heredity but to better knowledge and control of the conditions of this state of life.