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I venture to bring to your attention a number of observations regarding the life span which I hope may be of interest to you.

The life span is usually understood to be expressed by tables of expectation of life; but this is not what I mean to discuss. I wish to exclude all deaths due to infections, contagious diseases and accidents, and consider only the life span that we should find, if the strength of the organism were allowed to exhaust itself and death were to occur as a result of senility.

Even here certain allowances have to be made, for we may distinguish between an hereditary, purely biologically determined element and another one that depends upon conditions of life. Ample or deficient nutrition, more or less exhausting daily labor, abuse of the body, greater or lesser nervous strains are elements that modify the life span as it may be determined by heredity. Even geographical conditions may have their influence. Tropical or temperate climate, the degree of humidity, altitude, all have their influence upon the life span. It is, therefore, impossible to speak of the life span of an individual as determined absolutely by hereditary constitution. It must always be understood as the result of a hereditary constitution subject to a given set of environmental conditions. Therefore, even in a population of the same descent, the life span will depend upon social and economic conditions.

A closer examination of the problem shows that the simple statement that a certain length of life may be expected for an individual of known hereditary character and living in a known environment does not exhaust it. The life span is the result of physiological processes that go on throughout life and that have to be observed from the time of birth until death. When we study the distribution of moments of the occurrence of definite physiological changes, it appears that the variability of the time of occurrence increases with great rapidity during life. Measured by standard variations, the period of pregnancy has a variability of a few days, the appearance of the first teeth of a few weeks, the time when puberty is reached varies by more than a year, the time of menopause by several years, and death by arteriosclerosis by more than seven years. These rapid increases are not the same for different types of physiological phenomena. The teeth, for instance, behave quite differently from the skeleton. All, however, show the characteristic rapid increase in variability. This may be due to one of two causes. Either the increase of variability may be due to a high degree of variability of the changes which occur during a given interval without any relation to the time when a previous stage is reached, or the given interval may have a marked correlation with the time when the previous stage has been reached. If the former is the case, it would be impossible to predict the future, if the latter we may be able to predict the course of the life span. It is, therefore, of fundamental importance for the understanding of the life span to determine whether there is any correlation between the rapidity of physiological processes during life.

Unfortunately it is very difficult without a somewhat rigid organization to follow individuals from birth to death. Continuous observations on individuals are most easily obtained during school life, and I have asked myself the question whether during youth there is any evidence of a consistent speed of physiological changes. The observations show clearly that such consistency prevails. Following young children of six years up to maturity it may be observed that growth is completed earlier for tall ones than for short ones, both for boys and girls. During adolescence all children show a decided increase in the speed of development. Among girls this occurs earlier than among boys. This sudden spurt is followed by a rapid decrease in the rate of growth. Among young children, those who are tall have the spurt earlier than those who are short. Tall girls have their first menstruation, on the average, earlier than short ones. The criterion of size of young children is not as clear as might be desired, because some children are tall because they are accelerated in development, others are tall because by heredity they belong to a tall strain. It is, therefore, more instructive to compare all those who have the period of maximum rate of growth at the same time. Then it appears that the whole growth period for those who mature early is condensed.^[63] The bodily growth occupies a short period and proceeds with great energy. The reverse is true of those with a late spurt. They develop slowly and the whole period of growth is extended. The same observation may be made on girls arranged according to the time when they reach maturity, but it is not quite so clearly defined, because the relation between sexual maturity and bodily growth is more indirect. It is, however, evident from all the material collected that the period of bodily development is a unit which in some individuals proceeds rapidly, in others slowly.

The next important question to be decided is whether this unity of the rate at which the physiological life process runs on is determined by heredity or by the influence of outer conditions. The latter may be proved by a number of observations. Many investigators have shown that the average stature of European populations has increased considerably since the middle of the past century. Previous studies do not show us how this change comes about, whether it is a result of speeding the process of development and of an incidental final increase, or whether it is a general rise of the standards for each age. So far as the material collected to date allows us to judge, there is a speeding up of growth which brings about very great differences during the growth period. These differences decrease when growth begins to slow up, but result in a somewhat higher stature of the adult. The groups compared were measured, the one in 1909, the other in 1935. It would seem that the changed conditions result in a change of the tempo of development. In other words, we find here proof that the tempo of the life cycle in youth may be strongly modified by conditions of life. I do not venture to speculate on the causes that may underlie these changes, for it is not apparent that the social and economic conditions of the groups concerned have changed noticeably during the interval of twenty-five years. The only other series known to me is one of measurements of children in Jena^[64] in Germany; one taken from 1878-1880, the other in 1921. This shows also a considerable increase in stature among the children measured in 1921 notwithstanding the malnutrition of the preceding years. It is not convincing because during the interval Jena had become an industrial center which attracted people from a distance. Since the native Thuringian population is markedly short, that of the wider environment taller, I was inclined to ascribe the difference to the differing ancestry of the two series. The phenomenon observed here in New York in a more homogeneous group indicates, however, that conditions similar to those prevailing here may have contributed to the increase in stature.

These observations conform with the experimental results of observations on rats. Between 1912 and 1919 Gudernatsch^[65] administered dried endocrine glands to successive generations of white rats. He observed that the feeding of dried thymus gland brought it about that the animals treated were healthy, had numerous pregnancies, large litters and long life. Recent work by L. G. Rowntree, J. H. Clark, and A. M. Hanson^[66] showed that injections of thymus extract (Hanson) accelerated the rate of growth and development, hastened the onset of adolescence in the offspring of the treated rats and increased the fertility of parent rats. It is still more interesting to note that the acceleration is much greater in later litters of the second generation and is more marked in each succeeding generation under treatment. Omission of the injection in one generation caused the loss of all these changes. Analogous observations were made by Dr. Otto Roth.^[67] There is still some doubt as to the active principle that causes the acceleration. Both the experiments on rats and the observations on man show clearly that the tempo of development and the ultimate size may be influenced by outer conditions.

Nevertheless, the importance of hereditary determinants may not be neglected. Many attempts have been made to investigate the correlations between the ages at death of parents and children, and it has been found that a fairly marked positive correlation exists. The same is true for members of a fraternity. The material is not quite convincing because it is difficult to eliminate complex social causes and to confine the cases strictly to death due to senile degeneration. I have investigated the question in how far the tempo of development of one member of a fraternity may be repeated among other members of the same fraternity. The data prove that a child tall for its own age will have brothers and sisters who mature early, while others who are short for their own age will have such of a slow tempo of development. These data are from an orphan asylum, where all the children were under the same environmental conditions, so that external influences, if any, were very slight. This also agrees with observations made on animals. Pearl^[68] particularly has raised from a mixed series strains differing materially in life span.

All these observations may be summarized in the statement that each individual has by heredity a certain tempo of development that may be modified by outer conditions. The gross, generalized observations available at the present time suggest that in a socially uniform group the tempo of development may be considered as an hereditary characteristic of individuals.

The data which we have at our disposal end with the completion of growth and the important question arises whether the characteristic tempo of the individual extends over later periods of life; whether a rapid tempo of growth will also be associated with rapid decay and earlier death, or whether other types of relation exist. Unfortunately it is quite impossible at the present time to obtain adequate data which, as you will readily observe, must be based on long continued observations of the same individual. If we can obtain the coöperation of the proper authorities such data might easily be secured from the officer corps of the Army and Navy and in similar organizations that require periodic health examinations.

We owe the knowledge of data in regard to later life to Dr. Felix Bernstein^[69] who proved by means of life insurance records that an early onset of presbyopia is associated with other early degenerative processes which lead to an earlier death by arteriosclerosis. I do not doubt that many records of death in the archives of life insurance companies could by appropriate search be associated with the growth curves of individuals. Private schools which keep such records remain in touch with their graduates, and by means of proper organization, policyholders among them could be found in sufficient number to give the required information. Furthermore, since we know that the tempo of development is hereditary we might investigate the degree of presbyopia among the parents of children whose growth curve is known. This might also be secured with the help of private schools.

The general problem of the tempo of physiological processes in relation to the life span is certainly not only of theoretical interest, but may also enable us to predict with increased accuracy the expectation of life even in early years.

[62]	Read at the 46th Annual Meeting of the Association of Life Insurance Medical Directors of America, October 17-18, 1935.

[63]	See p. 118.

[64]	Robert Rössle and Herta Böning, “Das Wachstum der Schulkinder,” Veröffentlichungen aus der Kriegs- und Konstitutions Pathologie, vol. 4, part 1 (1924).

[65]	Max Hirsch, Handbuch der inneren Sekretion (1930). Chapter: Entwicklung und Wachstum.

[66]	Archives of Internal Medicine, vol. 56 (1935), no. 1, pp. 1-29.

[67]	Zeitschrift für Morphologie und Anthropologie, vol. 33 (1935), pp. 409-439.

[68]	See for instance “The Biology of Death VI,” The Scientific Monthly (1921), pp. 143-162.

[69]	Zeitschrift für die gesammte Versicherungs-Wissenschaft, vol. 31 (1931), p. 150.