In an ordinary population it has been found that the age at death of brothers and sisters furnishes a coefficient of correlation of the order of .3, which shows that heredity does determine the age at which one shall die to considerable extent, but not absolutely.[190]
The index of correlation[191] between the lengths of life within the fraternity in these 100 selected families, furnished a coefficient of-.0163±.0672, practically zero. In other words, if the age is known at which a member of one of these families died, whether it be one month or 100 years, nothing whatever can be predicted about the age at which his brothers and sisters died.
Remembering that longevity is in general inherited, and that it is found in the families of all the people of this study (since one in each fraternity lived to be 90 or over) how is one to interpret this zero coefficient? Evidently it means that although these people had inherited a high degree of longevity, their deaths were brought about by causes which prevented the heredity from getting full expression. As far as hereditary potentialities are concerned, it can be said that all their deaths were due to accident, using that word in a broad sense to include all non-selective deaths by disease. If they had all been able to get the full benefit of their heredity, it would appear that each of these persons might have lived to 90 or more, as did the one in each family who was recorded by the Genealogical Record Office. Genetically, these other deaths may be spoken of as premature.
In an ordinary population, the age of death is determined to the extent of probably 50% by heredity. In this selected long-lived population, heredity appears not to be responsible in any measurable degree whatsoever for the differences in age at death.
The result may be expressed in another, and perhaps more striking, way. Of the 669 individuals studied, a hundred—namely, one child in each family—lived beyond 90; and there were a few others who did. But some 550 of the group, though they had inherited the potentiality of reaching the average age of 90, actually died somewhere around 60; they failed by at least one-third to live up to the promise of their inheritance. If we were to generalize from this single case, we would have to say that five-sixths of the population does not make the most of its physical inheritance.
This is certainly a fact that discourages fatalistic optimism. The man who tells himself that, because of his magnificent inherited constitution, he can safely take any risk, is pretty sure to take too many risks and meet with a non-selective—i.e., genetically, a premature—death, when he might in the nature of things have lived almost a generation longer.
It should be remarked that most of the members of this group seem to have lived in a hard environment. They appear to belong predominantly to the lower strata of society; many of them are immigrants and only a very few of them, to judge by a cursory inspection of the records, possessed more than moderate means. This necessitated a frugal and industrious life which in many ways was doubtless favorable to longevity but which may often have led to overexposure, overwork, lack of proper medical treatment, or other causes of a non-selective death. We would not push the conclusion too far, but we can not doubt that this investigation shows the folly of ignoring the environment,—shows that the best inherited constitution must have a fair chance. And what has here been found for a physical character, would probably hold good in even greater degree for a mental character. All that man inherits is the capacity to develop along a certain line under the influence of proper stimuli,—food and exercise. The object of eugenics is to see that the inherent capacity is there. Given that, the educational system is next needed to furnish the stimuli. The consistent eugenist is therefore an ardent euthenist. He not only works for a better human stock but, because he does not want to see his efforts wasted, he always works to provide the best possible environment for this better stock.
In so far, then, as euthenics is actually providing man with more favorable surroundings,—not with ostensibly more favorable surroundings which, in reality, are unfavorable—there can be no antagonism between it and eugenics. Eugenics is, in fact, a prerequisite of euthenics, for it is only the capable and altruistic man who can contribute to social progress; and such a man can only be produced through eugenics.
Eugenic fatalism, a blind faith in the omnipotence of heredity regardless of the surroundings in which it is placed, has been shown by the study of long-lived families to be unjustified. It was found that even those who inherited exceptional longevity usually did not live as long as their inheritance gave them the right to expect. If they had had more euthenics, they should have lived longer.
But this illustration certainly gives no ground for a belief that euthenics is sufficient to prolong one's life beyond the inherited limit. A study of these long-lived families from another point of view will reveal that heredity is the primary factor and that good environment, euthenics, is the secondary one.