Man has risen from the ape chiefly through the action of natural selection. Any scheme of conscious race betterment, then, should carefully examine nature's method, to learn to what extent it is still acting, and to what extent it may better be supplanted or assisted by methods of man's own invention.

Natural selection operates in two ways: (1) through a selective death-rate and (2) through a selective birth-rate. The first of these forms has often been considered the whole of natural selection, but wrongly. The second steadily gains in importance as an organism rises in the scale of evolution; until in man it is likely soon to dwarf the lethal factor into insignificance. For it is evident that the appalling slaughter of all but a few of the individuals born, which one usually associates with the idea of natural selection, will take place only when the number of individuals born is very large. As the reproductive rate decreases, so does the death-rate, for a larger proportion of those born are able to find food and to escape enemies.

When considering man, one realizes at once that relatively few babies or adults starve to death. The selective death-rate therefore must include only those who are unable to escape their enemies; and while these enemies of the species, particularly certain microÖrganisms, still take a heavy toll from the race, the progress of science is likely to make it much smaller in the future.

The different aspects of natural selection may be classified as follows:

The lethal factor is the one which Darwin himself most emphasized. Obviously a race will be steadily improved, if the worst stock in it is cut off before it has a chance to reproduce, and if the best stock survives to perpetuate its kind. "This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called natural selection, or the survival of the fittest," Darwin wrote; and he went on to show that the principal checks on increase were overcrowding, the difficulty of obtaining food, destruction by enemies, and the lethal effects of climate. These causes may be conveniently divided as in the above diagram, into sustentative and non-sustentative. The sustentative factor has acquired particular prominence in the human species, since Malthus wrote his essay on population—that essay which both Darwin and Wallace confess was the starting point of their discovery of natural selection.

There is a "constant tendency in all animated life to increase beyond the nourishment prepared for it," Malthus declared. "It is incontrovertibly true that there is no bound to the prolific plants and animals, but what is made by their crowding and interfering with each others' means of subsistence." His deduction is well known: that as man tends to increase in geometrical ratio, and can not hope to increase his food-supply more rapidly than in arithmetical ratio, the human race must eventually face starvation, unless the birth-rate be reduced.

Darwin was much impressed by this argument and ever since his time it has usually been the foundation for any discussion of natural selection. Nevertheless it is partly false for all animals, as one of the authors showed[52] some years ago, since a species which regularly eats up all the food in sight is rare indeed; and it is of very little racial importance in the present-day evolution of man. Scarcity of food may put sufficient pressure on him to cause emigration, but rarely death. The importance of Malthus' argument to eugenics is too slight to warrant further discussion.

When the non-sustentative forms of lethal selection are considered, it is seen very clearly that man is not exempt from the workings of this law. A non-sustentative form of natural selection takes place through the destruction of the individual by some adverse feature of the environment, such as excessive cold, or bacteria; or by bodily deficiency; and it is independent of mere food-supply. W. F. R. Weldon showed by a long series of measurements, for example, that as the harbor of Plymouth, England, kept getting muddier, the crabs which lived in it kept getting narrower; those with the greatest frontal breadth filtered the water entering their gills least effectively, and died.

But, it was objected, man is above all this. He has gained the control of his own environment. The bloody hand of natural selection may fall on crabs: but surely you would not have us think that Man, the Lord of Creation, shares the same fate?

Biologists could hardly think otherwise. Statisticians were able to supply the needed proof. A selective death-rate in man can not only be demonstrated but it can be actually measured.

"The measure of the selective death-rate." says[53] Karl Pearson, to whom this achievement is due, "is extraordinarily simple. It consists in the fact that the inheritance of the length of life between parent and offspring is found statistically to be about one-third of the average inheritance of physical characters in man. This can only be due to the fact that the death of parent or of offspring in a certain number of cases is due to random and not to constitutional causes." He arrived at the conclusion[54] that 60% of the deaths were selective, in the Quaker families which he was then studying. The exact proportion must vary in accordance with the nature of the material and the environment, but as A. Ploetz found at least 60% of the deaths to be selective in the European royal families and nobility, where the environment is uniformly good, there is no reason to think that Professor Pearson's conclusion is invalid.

Dr. Ploetz[55] investigated the relation between length of life in parents, and infant mortality, in about 1,000 families including 5,500 children; half of these were from the nobility and half from the peasantry. The results were of the same order in each case, indicating that environment is a much less important factor than many have been wont to suppose. After discussing Professor Pearson's work, he continued:

In general, then, one may believe that more than a half of the persons who die nowadays, die because they were not fit by by nature (i. e., heredity) to survive under the conditions into which they were born. They are the victims of lethal natural selection, nearly always of the non-sustentative type. As Karl Pearson says, "Every man who has lived through a hard winter, every man who has examined a mortality table, every man who has studied the history of nations has probably seen natural selection at work."

There is still another graphic way of seeing natural selection at work, by an examination of the infant mortality alone. Imagine a thousand babies coming into the world on a given day. It is known that under average American conditions more than one-tenth of them will die during the first year of life. Now if those who die at this time are the inherently weaker, then the death-rate among survivors ought to be correspondingly less during succeeding years, for many will have been cut down at once, who might otherwise have lingered for several years, although doomed to die before maturity. On the other hand, if only a few die during the first year, one might expect a proportionately greater number to die in succeeding years. If it is actually found that a high death-rate in the first year of life is associated with a low death-rate in succeeding years, then there will be grounds for believing that natural selection is really cutting off the weaker and allowing the stronger to survive.

E. C. Snow[56] analyzed the infant mortality registration of parts of England and Prussia to determine whether any such conclusion was justified. His investigation met with many difficulties, and his results are not as clear-cut as could be desired, but he felt justified in concluding from them that "the general result can not be questioned. Natural selection, in the form of a selective death-rate, is strongly operative in man in the early years of life. We assert with great confidence that a high mortality in infancy (the first two years of life) is followed by a correspondingly low mortality in childhood, and vice-versa.... Our work has led us to the conclusion that infant mortality does effect a 'weeding out' of the unfit."

"Unfitness" in this connection must not be interpreted too narrowly. A child may be "unfit" to survive in its environment, merely because its parents are ignorant and careless. Such unfitness makes more probable an inheritance of low intelligence.

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.

Of a group of men picked at random from the population, why will some eventually die of tuberculosis and the others resist infection? Is it a matter of environment?—are open-air schools, sanitary tenements, proper hygiene, the kind of measures that will change this condition? Such is the doctrine widely preached at the present day. It is alleged that the white plague may be stamped out, if the open cases of tuberculosis are isolated and the rest of the population is taught how to live properly. The problem is almost universally declared to be a problem of infection.

Infection certainly is the immediate problem, but the biologist sees a greater one a little farther back. It is the problem of natural selection.

To prove this, it is necessary to prove (1) that some people are born with less resistance to tuberculosis than others and (2) that it is these people with weak natural resistance who die of phthisis, while their neighbors with stronger resistance survive. The proof of these propositions has been abundantly given by Karl Pearson, G. Archdall Reid and others. Their main points may be indicated. In the first place it must be shown that the morbidity from tuberculosis is largely due to heredity—a point on which most medical men are still uninformed. Measurement of the direct correlation between phthisis in parent and child shows it to be about .5, i. e., what one expects if it is a matter of heredity. This is the coefficient for most physical and mental characters: it is the coefficient for such pathological traits as deafness and insanity, which are obviously due in most cases to inheritance rather than infection.

But, one objects, this high correlation between parent and child does not prove inheritance,—it obviously proves infection. The family relations are so intimate that it is folly to overlook this factor in the spread of the disease.

Very well, Professor Pearson replied, if the relations between parent and child are so intimate that they lead to infection, they are certainly not less intimate between husband and wife, and there ought to be just as much infection in this relationship as in the former. The correlation was measured in thousands of cases and was found to lie around .25, being lowest in the poorer classes and highest in the well-to-do classes.

At first glance this seems partly to confirm the objection—it looks as if there must be a considerable amount of tubercular infection between husband and wife. But when it is found that the resemblance between husband and wife in the matter of insanity is also .25, the objection becomes less formidable. Certainly it will hardly be argued that one of the partners infects the other with this disability.

As a fact, a correlation of .25 between husband and wife, for tuberculosis, is only partly due to infection. What it does mean is that like tends to mate with like—called assortative mating. This coefficient of resemblance between husband and wife in regard to phthisis is about the same as the correlation of resemblance between husband and wife for eye color, stature, longevity, general health, truthfulness, tone of voice, and many other characters. No one will suppose that life partners "infect" each other in these respects. Certainly no one will claim that a man deliberately selects a wife on the basis of resemblance to himself in these points; but he most certainly does so to some extent unconsciously, as will be described at greater length in Chapter XI. Assortative mating is a well-established fact, and there is every reason to believe that much of the resemblance between husband and wife as regards tuberculosis is due to this fact, and not to infection.[59]

Again, it is objected that the infection of children is not a family matter, but due to tuberculous cows' milk: how then does it appear equally among the Japanese, where cows are not tuberculous and cow's milk rarely used as an infant food: or among such people as the Esquimaux and Polynesians, who have never seen a cow?

But, it is argued, at any rate bad housing and unsanitary conditions of life will make infection easier and lower the resistance of the individual. Perhaps such conditions may make infection easier, but that is of little importance considering how easy it is for all city dwellers—for the population as a whole. The question remains, will not bad housing cause a greater liability to fatal phthisis? Will not destitution and its attendant conditions increase the probability that a given individual will succumb to the white plague?

Most physicians think this to be the case, but they have not taken the pains to measure the respective rÔles, by the exact methods of modern science. S. Adolphus Knopf of New York, an authority on tuberculosis, recognizes the importance of the heredity factor, but says that after this, the most important predisposing conditions are of the nature of unsanitary schools, unsanitary tenements, unsanitary factories and workshops. This may be very true; these conditions may follow after heredity in importance—but how near do they follow? That is a matter capable of fairly accurate measurement, and should be discussed with figures, not generalities.

Taking the case of destitution, which includes, necessarily, most of the other evils specified, Professor Pearson measured the correlation with liability to phthisis and found it to be .02. The correlation for direct heredity—that is, the resemblance between parent and offspring—it will be remembered, is .50. As compared with this, the environmental factor of .02 is utterly insignificant. It seems evident that whether or not one dies from tuberculosis, under present-day urban conditions, depends mainly on the kind of constitution one has inherited.

There is no escape, then, from the conclusion that in any individual, death from tuberculosis is largely a matter of natural selection. But by taking a longer view, one can actually see the change to which natural selection is one of the contributors. The following table shows the deaths from consumption in Massachusetts, per 10,000 population:

F. L. Hoffman further points out[60] that in Massachusetts, Rhode Island, and Connecticut, 1872-1911, the decline in the death-rate from tuberculosis has been about 50%. "The evidence is absolutely conclusive that actually as well as relatively, the mortality from tuberculosis in what is the most intensely industrial area of America has progressively diminished during the last 40 years."

It will be noted that the great increase in death from consumption in this area began in the decade following 1840, when the large Irish immigration began. The Irish are commonly believed to be particularly susceptible to phthisis. Crowded together in industrial conditions, they rapidly underwent infection, and their weak racial resistance led to a high death-rate. The weak lines of heredity were rapidly cut off; in other words, the intensity of natural selection was great, for a while. The result was to leave the population of these New England states much more resistant, on the average, than it was before; and as the Irish immigration soon slowed down, and no new stocks with great weakness arrived, tuberculosis naturally tended to "burn itself out." This seems to be a partial explanation of the decline in the death-rate from phthisis in New England during the last half century, although it is not suggested that it represents the complete explanation: improved methods of treatment and sanitation doubtless played their part. But that they are the sole cause of the decline is made highly improbable by the low correlation between phthisis and environmental factors, which was mentioned above, and by all the other biometric study of tuberculosis, which has proved that the results ascribed to hygiene, including sanitorium treatment, are to some degree illusory.

That tuberculosis is particularly fatal to the Negro race is well known. Even to-day, after several centuries of natural selection in the United States, the annual death-rate from consumption among Negroes in the registration area is 431.9 per 100,000 population (census of 1900) as compared with 170.5 for the whites; in the cities alone it is 471.0. That overcrowding and climate can not be the sole factors is indicated by the fact that the Negro race has been decimated, wherever it has met tuberculosis. "In the years 1803 and 1810 the British government imported three or four thousand Negroes from Mozambique into Ceylon to form into regiments, and of these in December, 1820, there were left just 440, including the male descendants. All the rest had perished mainly from tuberculosis, and in a country where the disease is not nearly so prevalent as in England."[61] Archdall Reid has pointed out[62] that the American, Polynesian and Australian aborigines, to whom tuberculosis was unknown before the advent of Europeans, and who had therefore never been selected against it, could not survive its advent: they were killed by much smaller infections than would have injured a European, whose stock has been purged by centuries of natural selection.

These racial histories are the most important evidence available to the student of natural selection in man. The conclusion to be drawn from them seems plain. Natural selection, which has in the past never had an opportunity to act upon the Negro race through tuberculosis, is now engaged in hastening, at a relatively rapid rate, the evolution of this race toward immunity from death by tuberculosis. The evolution of the white race on this line is, as the figures show, going on simultaneously, but having begun centuries earlier, it is not now so rapid. The weakest white stocks were cut off hundreds of years ago, in Great Britain or Europe; those of the black race are only now going. Despite all the efforts of medicine and sanitation, it is likely that the Negro death-rate from phthisis will continue high for some years, until what is left of the race will possess a degree of resistance, or immunity, not much inferior to that of the whites among whom they live. The blacks in North America now must be already more resistant than their ancestors; the mulattoes descended of normal healthy unions should be more resistant than the pure Negroes, although no statistics are available on the point; but were a new immigration to take place from Africa to-day, and the immigrants to be put into villages with their Americanized brethren, the high death-rate would result.

While the Negroes were thus undergoing the radical surgery of natural selection, what was happening to the aborigines of America? The answer of history is unmistakable; they were meeting the same fate, in an even more violent form. Not tuberculosis alone, but small-pox, measles, alcohol and a dozen other importations of the conquerors, found in the aborigines of the New World a stock which had never been selected against these diseases.

It is the custom of sentimentalists sometimes to talk as if the North American Indian had been killed off by the white man. So he was,—but not directly: he was killed off by natural selection, acting through the white man's diseases and narcotics. In 1841 Catlin wrote, "Thirty millions of white men are now scuffling for the goods and luxuries of life over the bones of twelve millions of red men, six millions of whom have fallen victims to small-pox." Small-pox is an old story to the white race, and the death of the least resistant strains in each generation has left a population that is fairly resistant. It was new to the natives of America, and history shows the result. Alcohol, too, counted its victims by the thousand, for the same reason. The process of natural selection among the North American Indians has not yet stopped; if there are a century from now any Indians left, they will of necessity belong to stocks which are relatively resistant to alcohol and tuberculosis and the other widespread and fatal diseases which were unknown upon this continent before Columbus.

The decrease of natives following the Spanish conquest of tropical America has long been one of the most striking events of history. Popular historians sometimes speak as if most of the native population had been killed off by the cruelty of the conquistadores. Surely such talk could not proceed from those who are familiar with the action of natural selection. It is obvious that when the Spaniard brought the natives together, making them work in mines and assemble in churches, he brought them under conditions especially favorable for infection by the new diseases which he had brought. The aborigines of the New World, up to the time the Spaniards came, had undergone no evolution whatever against these diseases; consequently the evolution began at so rapid a rate that in a few centuries only those who lived in out-of-the-way places remain unscathed.

The same story is repeated, in a survey of the history of the Pacific Islands. Even such a disease as whooping-cough carried off adults by the hundred. Robert Louis Stevenson has left a graphic picture[63] of natural selection at work:

"The tribe of Hapaa," he writes, "is said to have numbered some four hundred when the small-pox came and reduced them by one-fourth. Six months later a woman developed tubercular consumption; the disease spread like fire about the valley, and in less than a year two survivors, a man and a woman, fled from the newly-created solitude.... Early in the year of my visit, for example, or late the year before, the first case of phthisis appeared in a household of 17 persons, and by the end of August, when the tale was told me, one soul survived, a boy who had been absent at his schooling."

In Tasmania is another good illustration of the evolution of a race proceeding so rapidly as to be fatal to the race. When the first English settled on the island, in 1803, the native population consisted of several thousand. Tuberculosis and many other new diseases, and, most of all, alcohol, began to operate on the aborigines, who were attracted to the settlements of the whites. In a quarter of a century there were only a few hundred left. Many, of course, had met violent deaths, but an enlightened perusal of any history of the period,[64] will leave no doubt that natural selection by disease was responsible for most of the mortality. By 1847 the number of native Tasmanians was reduced to 44, who were already unmistakably doomed by alcohol and bacteria. When the last full-blood Tasmanian died in 1876, a new chapter was written in the story of the modern evolution of the human race.

No such stories are told about the white settlements on this continent, even before the days of quarantine and scientific medicine. There is no other adequate explanation of the difference, than that the two races have evolved to a different degree in their resistance to these diseases. It is easily seen, then, that man's evolution is going on, at varying rates of speed, in probably all parts of the human race at the present time.

We do not mean, of course, to suggest that all the natives who have died in the New World since the landing of Columbus, have died because the evolution of their race had not proceeded so far in certain directions as had that of their conquerors. But the proportion of them who were eliminated for that reason is certainly very large. In the more remote parts of South America the process is still going on. Recent press dispatches have carried the account of the University of Pennsylvania's Amazon Expedition, under the direction of William C. Farrabee. In a letter dated March 16, 1916, the leader told of the discovery of the remains of the tribe of Pikipitanges, a once populous tribe of which a chief, six women and two boys alone are left. The tribe had been almost wiped out, Dr. Farabee reported, by an epidemic of influenza!

If the aborigines of the New World succumb to the diseases of the European, it is not less true that the European succumbs to diseases against which his race has not been selected. The deadliness of yellow fever to Americans in the tropics, and the relative immunity of Negroes, is familiar; so too is the frequently fatal result of the African tropical fevers on the white man, while the natives suffer from them much less, having been made more resistant by centuries of natural selection.

This long discussion may now be summarized. We dealt with lethal selection, that form of natural selection which operates by prematurely killing off the less fit and leaving the more fit to survive and reproduce their kind. It is of course understood that the word "fit" in this connection does not necessarily mean morally or mentally superior, but merely fit for the particular environment. In a community of rascals, the greatest rascal might be the fittest to survive. In the slums of a modern city the Jewish type, stringently selected through centuries of ghetto life, is particularly fit to survive, although it may not be the physical ideal of an anthropologist.

Two forms of lethal selection were distinguished, one depending on starvation and the other on causes not connected with the food supply. Direct starvation is not a factor of importance in the survival of most races during most of the time at the present day so far as the civilized portion of the world is concerned. But disease and the other lethal factors not connected with the food-supply, through which natural selection acts, are still of great importance. From a half to two-thirds of all deaths are of a selective character, even under favorable conditions.

It is also to be noted, however, that with the progress of medicine, and the diminution of unfit material, this kind of natural selection will tend to become less and less widespread. For a long time, natural selection in man has probably done little to cause marked change in his physical or mental characteristics. Man's interference has prevented. In recent centuries natural selection has probably done no more on the whole than keep the race where it was: it is to be feared that it has not even done that. It is doubtful if there is any race to-day which attains the physical and mental average of the Athenians of 2,500 years ago.

Lethal natural selection, then, has been and still is a factor of great importance in the evolution of the race, but at present it is doing little or nothing that promises to further the ideal of eugenics—race betterment.

But lethal natural selection is only half the story. It is obvious that if the constitution of a race can be altered by excess of deaths in a certain class, it can equally be altered by excess of births in a certain class. This is reproductive selection, which may appear in either one of two forms. If the individual leaves few or no progeny because of his failure to mate at the proper time, it is called sexual selection; if, however, he mates, yet leaves few or no progeny (as compared with other individuals), it is called fecundal selection.

Even in man, the importance of the rÔle of reproductive selection is insufficiently understood; in the lower animals scientists have tended still more to undervalue it. As a fact, no species ordinarily multiplies in such numbers as to exhaust all the food available, despite the teaching of Malthus and Darwin to the contrary. The rate of reproduction is the crux of natural selection; each species normally has such a reproduction rate as will suffice to withstand the premature deaths and sterility of some individuals, and yet not so large as to press unduly upon the food supply. The problem of natural selection is a problem of the adjustment between reproductive rate and death-rate, and the struggle for subsistence is only one of several factors.

While the reproductive rate must be looked upon as a characteristic which has its adaptations like other characteristics, it has one peculiarity—its increase is always opposed by lethal selection. The chances of life are reduced by reproducing, inasmuch as more danger is entailed by the extra activities of courtship, and later, in bearing and caring for the young, since these duties reduce the normal wariness of individual life. The reproductive rate, therefore, always remains at the lowest point which will suffice for the reproductive needs of the species. For this reason alone the non-sustentative form of selection might be expected to be the predominant kind.

J. T. Gulick and Karl Pearson have pointed out that there is a normal conflict between natural selection and fecundal selection. Fecundal selection is said by them to be constantly tending to increase the reproductive rate, because fecundity is partly a matter of heredity, and the fecund parents leave more offspring with the same characteristic. Lethal selection, on the contrary, constantly asserts its power to reduce the reproductive rate, because the reproductive demands on the parents reduce their chances of life by interference with their natural ability of self-protection. This is quite true, but the analysis is incomplete, for an increased number of progeny not only decreases the life chances of the parents, but also of the young, by reducing the amount of care they receive.

In short, lethal selection and reproductive selection accomplish the same end—a change in the constitution of the species—by different means; but they are so closely linked together and balanced that any change in the operation of one is likely to cause a change in the operation of the other. This will be clearer when the effect of reproductive selection is studied in man.

Recalling the truism that most human characters have a hereditary basis, it is evident that the constitution of society will remain stable from generation to generation, only if each section of society is reproducing at the same rate as every other (and assuming, for the moment, that the death-rate remains constant). Then if the birth-rate of one part of the population is altered, if it is decreased, for example, the next generation will contain proportionately fewer representatives of this class, the succeeding generation fewer still, and so on indefinitely—unless a selective death-rate is operating at the same time. It is well known not only that the death-rate varies widely in different parts of the population, as was pointed out in the earlier part of this chapter, but that the birth-rate is rarely the same in any two sections of the population. Evidently, therefore, the make-up of society must necessarily be changing from generation to generation. It will be the object of the rest of this chapter to investigate the ways in which it is changing, while in the latter half of the book we shall point out some of the ways in which it might be changed to better advantage than it is at present.

Sexual selection, or differential success in marrying, will be discussed at some length in Chapter XI; here it may be pointed out that the number who fail to marry is very much greater than one often realizes. It has already been noted that a large part of the population dies before it reaches the age of marriage. Of 1,000 babies born in the United States, only 750 will reach the average age of marriage; in some countries half of the thousand will have fallen by that time. These dead certainly will leave no descendants; but even of the survivors, part will fail to marry. The returns of the thirteenth U. S. census showed that of the males 45-64 years of age, 10% were single, while 11% of the females, 35-44 years old, were single. Few marriages will take place after those ages. Add the number who died unmarried previous to those ages, but after the age of 20, and it is safe to say that at least one-third of the persons born in the United States die (early or late) without having married.

The consideration of those who died before the age of marriage properly comes under the head of lethal selection, but if attention is confined to those who, though reaching the age of marriage, fail to marry, sexual selection still has importance. For instance, it is generally known (and some statistical proof will be given in Chapter XI) that beauty is directly associated with the chance of marriage. The pretty girls in general marry earlier as well in larger percentage; many of the ugly ones will never find mates. Herbert Spencer argued ingeniously that beauty is associated with general mental and moral superiority, and the more exact studies of recent years have tended to confirm his generalization. A recent, but not conclusive, investigation[65] showed beauty to be correlated with intelligence to the extent of .34. If this is confirmed, it offers a good illustration of the action of sexual selection in furthering the progressive evolution of the race. Miss Gilmore, studying a group of normal school graduates, found a direct correlation between intelligence (as judged by class marks) and early marriage after graduation. Anyone who would take the trouble could easily investigate numerous cases of this sort, which would show the effect of sexual selection in perpetuating desirable qualities.

But sexual selection no longer has the importance that it once had, for nowadays the mere fact of marriage is not a measure of fecundity, to the extent that it once was. In the old days of unlimited fecundity, the early marriage of a beautiful, or intelligent, woman meant a probable perpetuation of her endowments; but at present, when artificial restraint of fertility is so widespread, the result does not follow as a matter of course: and it is evident that the race is little or not at all helped by the early marriage of an attractive woman, if she has too few or no children.

Fecundal selection, then, is becoming the important phase of reproductive selection, in the evolution of civilized races. The differential birth-rate is, as we have often insisted, the all-important factor of eugenics, and it merits careful consideration from all sides.

Such consideration is made difficult by the inadequate vital statistics of the United States (which ranks with Turkey and China in this respect); but there is no doubt that the birth-rate as a whole is low, as compared with that of other countries; although as a whole it is not dangerously low and there is, of course, no necessary evil in a low birth-rate, of itself, if the quality be satisfactory. The U. S. Census tabulation for 1915 gives the following comparison of the number of babies born alive each year, per 1,000 population, in various countries:

The United States birth-rate may, on its face, appear high enough; but its face does not show that this height is due largely to the fecundity of immigrant women. Statistics to prove this are given in Chapter XIII, but may be supplemented here by some figures from Pittsburgh.

Ward 7, in that city, contains the homes of many well-to-do, and contains more representatives of the old American stock than any other ward in the city, having 56.4% of residents who are native born of native parents while the majority of the residents in nearly all the other wards in the city are either themselves foreign-born, or the offspring of foreign-born parents.

Ward 7 has the lowest birth-rate and the lowest rate of net increase of any ward in the city.

With this may be contrasted the sixth ward, which runs along the south bank of the Allegheny river. It is one of the great factory districts of the city, but also contains a large number of homes. Nearly 3,000 of its 14,817 males of voting age are illiterate. Its death-rate is the highest in the city. Almost nine-tenths of its residents are either foreigners or the children of foreigners. Its birth-rate is three times that of the seventh ward.

Taking into account all the wards of the city, it is found that the birth-rate rises as one considers the wards which are marked by a large foreign population, illiteracy, poverty and a high death-rate. On the other hand, the birth-rate falls as one passes to the wards that have most native-born residents, most education, most prosperity—and, to some extent, education and prosperity denote efficiency and eugenic value. For 27 wards there is a high negative correlation (-.673), between birth-rate and percentage of native-born of native parents in the population. The correlation between illiteracy and net increase[66] is +.731.

The net increase of Pittsburgh's population, therefore, is greatest where the percentage of foreign-born and of illiterates is greatest.

The significance of such figures in natural selection must be evident. Pittsburgh, like probably all large cities in civilized countries, breeds from the bottom. The lower a class is in the scale of intelligence, the greater is its reproductive contribution. Recalling that intelligence is inherited, that like begets like in this respect, one can hardly feel encouraged over the quality of the population of Pittsburgh, a few generations hence.

Of course these illiterate foreign laborers are, from a eugenic point of view, not wholly bad. The picture should not be painted any blacker than the original. Some of these ignorant stocks, in another generation and with decent surroundings, will furnish excellent citizens.

But taken as a whole, it can hardly be supposed that the fecund stocks of Pittsburgh, with their illiteracy, squalor and tuberculosis, their high death-rates, their economic straits, are as good eugenic material as the families that are dying out in the more substantial residence section which their fathers created in the eastern part of the city.

And it can hardly be supposed that the city, and the nation, of the future, would not benefit by a change in the distribution of births, whereby more would come from the seventh ward and its like, and fewer from the sixth and its like.

Evidently, there is no difficulty about seeing this form of natural selection at work, and at work in such a way as greatly to change the character of one section of the species. For comparison, some figures are presented from European sources. In the French war budget of 1911 it appears that from 1,000 women between the ages of 15 and 50, in different districts of Paris, the number of yearly births was as follows:

Disregarding the last class altogether, it is yet evident that while the mother in a wealthy home bears two children, the mother in the slums bears four. It is evident then that in Paris at the present time reproductive selection is changing the mental and moral composition of the population at a rapid rate, which can not be very materially reduced even if it is found that the death-rate in the poorer districts is considerably greater than it is on the more fashionable boulevards.

J. Bertillon has brought together[67] in a similar way data from a number of cities, showing the following birth-rates:

Obviously, in all these cases reproductive selection will soon bring about such a change in the character of the population, that a much larger part of it than at present will have the hereditary characteristics of the poorer classes and a much smaller part of it than at present the hereditary characteristics of the well-to-do classes.

David Heron and others have recently studied[68] the relation which the birth-rate in different boroughs of London bears to their social and economic conditions. Using the correlation method, they found "that in London the birth-rate per 1,000 married women, aged 15 to 54, is highest where the conditions show the greatest poverty—namely, in quarters where pawnbrokers abound, where unskilled labor is the principal source of income, where consumption is most common and most deadly, where pauperism is most rife, and, finally, where the greatest proportion of the children born die in infancy. The correlation coefficients show that the association of these evil conditions with the relative number of children born is a very close one; and if the question is put in another way, and the calculations are based on measures of prosperity instead of on measures of poverty, a high degree of correlation is found between prosperity and a low birth-rate.

"It must not be supposed that a high rate of infant mortality, which almost invariably accompanies a high birth-rate, either in London or elsewhere, goes far toward counteracting the effects of the differential birth-rate. Where infant mortality is highest the average number of children above the age of two for each married woman is highest also, and although the chances of death at all ages are greater among the inhabitants of the poorer quarters, their rate of natural increase remains considerably higher than that of the inhabitants of the richer.

"From the detailed study of the figures made by Newsholme and Stevenson, conclusions essentially the same as those of Heron can be drawn.... Their first step was to divide the London boroughs into six groups according to the average number of domestic servants for 100 families in each. This is probably as good a measure of prosperity as any other. They then determined the total birth-rate of the population in each group, and arrived at the following figures:

"In order to find out how far the differences shown by these figures are due to differences in the percentage of women who marry in each group and the age at which they marry, they corrected the figures in such a way as to make them represent what the birth-rates would be in each group, if the proportion of wives of each age to the whole population comprising the group was the same as it is in the whole of England and Wales. The corrected birth-rates thus obtained were as follows:

"It will readily be seen that the effect of the correction has been to reduce the difference between the two extreme groups by about one-third, showing that to this extent it is due to the way in which they differ as to the average age and number of the women who marry. Further, Groups II, III, IV and V have all been brought to about the same level, with a corrected birth-rate about halfway between the highest and the lowest. This shows that there is no gradual decrease in fertility associated with a gradually increasing grade of prosperity, but that three sharply divided classes may be distinguished: a very poor class with a high degree of fertility, to which about a quarter of the population of London belong, a rich class with a low degree of fertility, and a class intermediate in both respects."

"Eugenics is less directly concerned with this side of the question that with the relative rate of increase of the different classes. This may be found for the six groups in the usual way by deducting the death-rate from the birth-rate. The following figures for the rate of natural increase are then obtained:

"The figures show in a manner which hardly admits of any doubt that in London at any rate the inhabitants of the poorest quarters—over a million in number—are reproducing themselves at a much greater rate than the more well-to-do."

A research on similar lines by S. R. Steinmetz[69] in Holland shows that the average number of children in the lowest class families is 5.44. People in industry or small trade, skilled mechanics and professors of theology have five children to the family; in other classes the number is as follows:

It is not hard to see that the next generation in Holland is likely to have proportionately fewer gifted individuals than has the present one.

Fortunately, it is very probable that the differential birth-rate is not of such ominous import in rural districts as it is in cities, although some of the tribes of degenerates which live in the country show birth-rates of four to six children per wife.[70] But in the more highly civilized nations now, something like a half of the population lives in urban districts, and the startling extent to which these urban populations breed from the bottom involves a disastrous change in the balance of population within a few generations, unless it is in some way checked.

Just how great the change may be, statistically, has been emphasized by Karl Pearson, who points out that "50% of the married population provide 75% of the next generation," owing to the number of deaths before maturity, the number of celibates and the number of childless marriages. "The same rule may be expressed in another way: 50% of the next generation is produced by 25% of the married population." At this rate in a few generations the less efficient and socially valuable, with their large families, will overwhelm the more efficient and socially valuable, and their small families.

Fecundal selection is at work to-day on a large scale, changing the character of the population, and from a eugenic point of view changing it for the worse. Fortunately, it is not impossible to arrest this change.

But, it may be objected, is not this change merely "the survival of the fittest?" In a sense, yes; and it is necessary that the more intelligent classes should make themselves "fitter" to survive, by a change of attitude toward reproduction. But the dying-out of the intellectually superior part of the population is a pathological condition, not a part of normal evolution; for barring artificial interference with the birth-rate, fertility has been found to go hand in hand with general superiority. This demonstration is due to F. A. Woods' study[71] of 608 members of the royal families of Europe, among whom, for reasons of state, large families are desired, and among whom there has probably been little restraint on the birth-rate. Averaging the ratings of his individuals from grade 1, the mentally and physically very inferior, to grade 10, the mentally and physically very superior, he found that the number of children produced and brought to maturity increased in a fairly direct ratio. His figures are as follows:

Investigations of Karl Pearson and Alexander Graham Bell[72] show that fecundity and longevity are associated. It follows that the mentally and morally superior, who are the most fecund, are also the longest-lived; and as this longevity is largely due to inheritance it follows that, under natural conditions, the standard of the stratum of society under consideration would gradually rise, in respect to longevity, in each generation.

Such is probably one of the methods by which the human race has gradually increased its level of desirable characters in each generation. The desirable characters were associated with each other, and also with fecundity. The desirable characters are still associated with each other, but their association with fecundity is now negative. It is in this change that eugenics finds justification for its existence as a propaganda. Its object is to restore the positive correlation between desirable characters and fecundity, on which the progressive evolution of the race depends.

The bearing of natural selection on the present-day evolution of the human race, particularly in the United States of America, must be reviewed in a few closing paragraphs.

Selection by death may result either from inadequate food supply, or from some other lethal factor. The former type, although something of a bugaboo ever since the time of Malthus, has in reality relatively little effect on the human race at present. Non-sustentative lethal selection in man is operating chiefly through zymotic diseases and the bad hygiene of the mentally inferior.

Reproductive selection is increasingly effective and its action is such as to cause grave alarm both through the failure of some to marry properly (sexual selection) and the failure of some to bear enough children, while others bear too many (fecundal selection). It is obvious that the racial result of this process will depend on what kind of people bear and rear the most children; and it has been shown that in general the larger families are in the section of the population that makes fewer contributions to human prosperity and happiness, while those endowed with great gifts, who ought to be transmitting them to their children, are in many cases not even reproducing their own number.

Natural selection raised man from apehood to his present estate. It is still operating on him on a large scale, in several ways, but in none of these ways is it now doing much actually to improve the race, and in some ways, owing to man's own interference, it is rapidly hastening race degeneracy.