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.