Recognizing the fact that the essential cells in our body are inherently capable under proper conditions of living indefinitely, the problem that confronts us is whether environment or heredity has most to do in determining the actual length of life. Pearl[163] concludes that the death rate of the earliest period of life is selective, eliminating the weak and leaving the strong, and that inheritance is “one of the strongest elements, if not indeed the dominating factor, in determining the duration of life of human beings.” The duration of life in animals also depends on the total amount of metabolic activity or work and it has been proven that rats, at least, live longer under conditions so controlled that their activity is lessened, so that the greater the total work done or total energy output, the shorter is the duration of life and _vice versa_, work accelerating the aging process somewhat as rise of temperature does. Pearl says: “The manner in which the environmental forces (of sublethal intensity of course) chiefly act in determining the duration of life appears to be chiefly by changing the rate of metabolism in the individual. Furthermore, one would suggest, on this view, that what heredity does in relation to duration of life is chiefly to determine within fairly narrow limits the total energy output which the individual exhibits in its lifetime.” The duration of life of an animal stands in inverse relation to the total amount of its metabolic activity or, put in other words, to the work in the sense of theoretical mechanics that it as a machine does during its life. Or, to put it in another way, if the total activity of a unit of time is increased by some means other than increased temperature, the same result appears as if the increased activity is caused by increased temperature. Pearl thinks that Steinach’s experiments on the sexual glands, whatever their results for rejuvenation, do not prove “any really significant lengthening of the life span.” Nor does he think that Robertson’s experiments with tethelin from the pituitary gland, whatever its effects upon growth, show that it materially increases the length of life to a degree that has much significance statistically, so that inheritance remains a prime determinant of longevity.