Pearl’s experiments on the fruit fly[177] show that where long- and short-lived strains are mixed, the first generation they produce is longer-lived than either parent and that for subsequent generations Mendelian laws hold even for longevity, so that there is increased vigor in the hybrid generation due to the mingling of germ plasms that are different. As to bacterial invasion, the stability and resistance of the organism is also a factor, but by rearing insects kept free from all such invasion it appears that “bacteria play but an essentially accidental rôle in determining the length of the span of life in comparison with the influence of heredity.” Pearl criticizes the conclusion of statisticians like Hersch that poverty shortens human life, despite the fact that this is perhaps the most potent single environmental factor affecting civilized man to-day. But we have no real evidence that if the conditions between the rich and poor were reversed the death rate would also be reversed. The influence of high temperature, which is known to accelerate all the metabolic processes, does not interfere with the predominant influence of heredity because it only accelerates life processes exactly in the same way that it accelerates chemical activities and the same is more or less true of the influence of the secretions of the endocrine glands. Pearl concludes[178] that it has already been demonstrated that cells from nearly every part of the metazoan soma are potentially immortal, even in the case of tumors by transplantation, though of course not yet for such exceedingly specialized structures as hair and nails. Under artificial conditions cells from nearly all organs can be made to long outlive the body from which they are taken, just as grafts from apple trees may be passed on indefinitely to successive generations. Thus death is not a necessary inherent consequent of life in even somatic cells but “potential longevity inheres in most of the different kinds of cells for the metazoan body except those which are extremely differentiated for peculiar functions.” The special conditions under which this occurs are often very complex and differ greatly for different tissues and animals, and we shall probably know far more later of the chemico-physical conditions necessary to insure continuous life, for these studies are new, having begun barely twenty years ago. The reason that all these essential tissues are not actually immortal in multicellular animals is that the individual parts do not find in the body the conditions necessary for their continued existence, each part being dependent upon other parts. This view differs from Minot’s that there is a specific inherent lethal process going on within the cells themselves that causes senescence. Pearl concludes “that these visible cytological changes are expressive of effects, not causes, and that they are the effects of the organization of the body as a whole