Evidence of natural selection was gathered by Karl Pearson from another source and published in 1912. He dealt with material analogous to that of Dr. Snow and showed "that when allowance was made for change of environment in the course of 50 years, a very high association existed between the deaths in the first year of life and the deaths in childhood (1 to 5 years). This association was such that if the infantile death-rate increased by 10% the child death rate decreased by 5.3% in males, while in females the fall in the child death-rate was almost 1% for every 1% rise in the infantile death-rate."
To put the matter in the form of a truism, part of the children born in any district in a given year are doomed by heredity to a premature death; and if they die in one year they will not be alive to die in some succeeding year.
Lately a new mathematical method, which is termed the Variate Difference Correlation method, has been invented and gives more accurate results, in such an investigation as that of natural selection, than any hitherto used. With this instrument Professor Pearson and Miss Elderton have confirmed the previous work. Applying it to the registered births in England and Wales between 1850 and 1912, and the deaths during the first five years of life in the same period, they have again found[57] that "for both sexes a heavy death-rate in one year of life means a markedly lower death-rate in the same group in the following year of life." This lessened death-rate extends in a lessened degree to the year following that, but is not by the present method easy to trace further.
"It is difficult," as they conclude, "to believe that this important fact can be due to any other source than natural selection, i. e., a heavy mortality leaves behind it a stronger population."
To avoid misunderstandings, it may be well to add to this review the closing words of the Elderton-Pearson memoir. "Nature is not concerned with the moral or the immoral, which are standards of human conduct, and the duty of the naturalist is to point out what goes on in Nature. There can now be scarcely a doubt that even in highly organized human communities the death-rate is selective, and physical fitness is the criterion for survival. To assert the existence of this selection and measure its intensity must be distinguished from an advocacy of high infant mortality as a factor of racial efficiency. This reminder is the more needful as there are not wanting those who assert that demonstrating the existence of natural selection in man is identical with decrying all efforts to reduce the infantile death-rate." A further discussion of this point will be found in a later chapter.
The conclusion that, of the infants who die, a large number do so through inherent weakness—because they are not "fit" to survive—is also suggested by a study of the causes of death. From a third to a half of the deaths during the first year of life, and particularly during the first month, are due to what may be termed uterine causes, such as debility, atrophy, inanition, or premature birth. Although in many cases such a death is the result of lack of prenatal care, in still more it must be ascribed to a defect in the parental stock.
In connection with infant mortality, it may be of interest to point out that the intensity of natural selection is probably greater among boys than among girls. There is a steady preponderance of boys over girls at birth (about 105 to 100, in the United States), while among the stillborn the proportion is 158 to 100, if the Massachusetts figures for 1891-1900 may be taken as general in application. Evidently a large number of weak males have been eliminated before birth. This elimination continues for a number of years to be greater among boys than among girls, until in the period of adolescence the death-rates of the two sexes are equal. In adult life the death-rate among men is nearly always higher than that among women, but this is due largely to the fact that men pursue occupations where they are more exposed to death. In such cases, and particularly where deaths are due to accident, the mortality may not only be non-selective, but is sometimes contra-selective, for the strongest and most active men will often be those who expose themselves most to some danger. Such a reversal of the action of natural selection is seen on a large scale in the case of war, where the strongest go to the fray and are killed, while the weaklings stay at home to perpetuate their type of the race.
A curious aspect of the kind of natural selection under consideration,—that which operates by death without reference to the food-supply,—is seen in the evolution of a wide pelvis in women. Before the days of modern obstetrics, the woman born with an unusually narrow pelvis was likely to die during parturition, and the inheritance of a narrower type of pelvis was thus stopped. With the introduction and improvement of instrumental and induced deliveries, many of these women are enabled to survive, with the necessary consequence that their daughters will in many cases have a similarly narrow pelvis, and experience similar difficulty in childbirth. The percentage of deliveries in which instrumental aid is necessary is thus increasing from generation to generation, and is likely to continue to increase for some time. In other words, natural selection, because of man's interference, can no longer maintain the width of woman's pelvis, as it formerly did, and a certain amount of reversion in this respect is probably taking place—a reversion which, if unchecked, would necessarily lead after a long time to a reduction in the average size of skull of that part of the human race which frequently uses forceps at childbirth. The time would be long because the forceps permit the survival of some large-headed infants who otherwise would die.
But it must not be supposed that lethal, non-sustentative selection works only through forms of infant mortality. That aspect was first discussed because it is most obvious, but the relation of natural selection to microbic disease is equally widespread and far more striking.
As to the inheritance of disease as such there is little room for misunderstanding: no biologist now believes a disease is actually handed down from parent to child in the germ-plasm. But what the doctors call a diathesis, a predisposition to some given disease, is most certainly heritable—a fact which Karl Pearson and others have proved by statistics that can not be given here.[58] And any individual who has inherited this diathesis, this lack of resistance to a given disease, is marked as a possible victim of natural selection. The extent to which and the manner in which it operates may be more readily understood by the study of a concrete case. Tuberculosis is, as everyone knows, a disease caused directly by a bacillus; and a disease to which immunity can not be acquired by any process of vaccination or inoculation yet known. It is a disease which is not directly inherited as such. Yet every city-dweller in the United States is almost constantly exposed to infection by this bacillus, and autopsies show that most persons have actually been infected at some period of life, but have resisted further encroachment. Perhaps a fraction of them will eventually die of consumption; the rest will die of some other disease, and will probably never even know that they have carried the bacilli of tuberculosis in their lungs.