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I have shown that when the period of maximum rate of growth is early, the whole growth proceeds at a rapid rate from early years on, at least as far as our material permits us to follow it; the onset of the puberty spurt is early, its intensity great, its duration short. Conversely, when the period of maximum rate of growth is late its intensity is slight, its duration long. This is expressed by the values of the maximum rates of growth which are low for those who reach maturity late, high for those who reach it early.

It is a most important question to decide whether this unity of the tempo of development that prevails until maturity has been reached will extend over later life. As far as I am aware the only reliable material that shows an interrelation of phenomena of aging and of the life span is that discovered by Dr. Felix Bernstein, who has proved that early presbyopia indicates an early death by senile degeneration. All others are more or less impressions of medical practitioners who believe, for instance, that early calcification of the larynx indicates an early onset of arterial degeneration. It seems most important that by organized effort the life history of individuals should be followed up in order to show whether the rate at which physiological development and decay occur are constitutionally determined. Studies on longevity also suggest that this is the case, but they can never be quite convincing.

The observations here discussed refer to individual development. We had to investigate next whether the conditions that determine the speed of the life cycle are hereditary or determined by environment. If they are hereditary we may expect that members of fraternities are alike in their tempo, provided the social conditions of the whole community investigated are fairly uniform. The best material at my disposal is contained in the records of the Hebrew Orphan Asylum in New York, for here the conditions of nutrition, shelter and mode of life are as uniform as can be obtained. I have already shown that there is a fairly strong positive correlation between the dates of first menstruation of sisters.^[61] Unfortunately it was not possible to classify the material relating to growth in the same manner as was done for other series because, on account of gaps in the series of observations, the moment of maximum rate of growth cannot be determined for all members of each family and the material would have been too scanty. On account of the asymmetries of the increment curve, particularly during adolescence, I did not use correlations, but classified the material in three groups, about equal in numbers—children tall, medium and short at a given age—and compared the curves of growth of their brothers and sisters. Then it appeared that the brothers and sisters of the tall ones, who include many of those with rapid tempo of development, will also have a rapid tempo, an early time for the maximum rate of growth, a rapid rate and an early termination of growth, while the brothers and sisters of the short ones, who include many of those with sluggish tempo of development, have a slow rate of growth of less intensity and longer duration (Fig. 1). Since the conditions under which these children live are unusually uniform, we may conclude that proof for the heredity of the tempo of growth has been given. This agrees with the results obtained by Pearl by experimentation with animals and with observations on plants of short and long vegetative periods. An analogous study of children in the Horace Mann School gave the same result. In this case it might be suspected that similar home environment is a contributory cause to the similarity of the growth curve of members of each fraternity (Fig. 2).

Fig. 1. Annual growth of brothers and sisters, tall, medium-sized and short, at the selected ages of 7, 9, 11, and 13 years. Continuous observations. Hebrew Orphan Asylum.

Fig. 2. Annual growth of brothers and sisters, tall, medium-sized and short, at the selected ages of 7, 9, 11, and 13 years. Continuous observations. Horace Mann School.

It is obvious that a phenomenon of such complexity as length of body and tempo of development must be governed by many hereditary factors and that we are dealing with a phenomenon of general organization of the body and that a search for genes would not be advisable. Is not there some danger anyway, that the number of genes will depend rather upon the number of investigators than upon their actual existence?

[60]	Proceedings of the National Academy of Sciences, vol. 21 (1935), pp. 414-416.

[61]	Human Biology, vol. 4, no. 3 (1932), p. 308.