Reproduction

Similarly, when the rate of increase fails to meet the elimina tion, there may be sudden rarity, as with the tile-fish, and even sudden extinction, as with the passenger pigeon. But in most cases there is an automatic adjustment of the balance; thus the sudden decrease may relieve the intra-specific competition, so that the mortality among the young stages is greatly reduced. Slight fluctuations in numbers are much commoner than sudden increases or decreases.

Reference must be made here to Herbert Spencer's thesis that reproductivity tends to decrease in the more highly evolved organ isms. Including under the term "individuation" all the race preserving processes by which the individual life is completed and maintained, and under the term "genesis" all the reproductive processes that lead to the formation of new individuals, Spencer maintained that individuation and genesis vary inversely. Genesis decreases as individuation increases, but not quite so fast ; in other words, progressive evolution in the direction of individuation is correlated with a diminished rate of reproduction.

In support of this conclusion Spencer adduced some general physiological reasons why individuation and genesis should vary inversely, and he brought forward inductively a number of instances of poorly individuated types, like tapeworms, that are very prolific, and of highly individuated types, like golden eagles, that show greatly economized reproductivity. But he did not prove that high individuation directly lessens fertility. What is much more probable is that highly individuated types have re sources which have enabled them to reduce the ratio of elimina tion, and have thus allowed them to vary in the direction of economized reproductivity without decreasing their chances of survival. In mankind the psychological and social factors in individuation may operate directly in lessening pre-occupation with sex-indulgence and in lessening in monogamous married life, the physical incentives thereto, thus resulting in smaller families, but there is no proof that education or the like physiologically lessens fertility.

Reproduction and Death.

While reproduction is concerned with the beginning of new lives, it is not infrequently associated with the death of the parent. In many organisms reproduction is the beginning of death, and the connection may be either direct or indirect. (a) In some Annelida, for instance, the multiplication of germ-cells distends the body and leads to fatal rupture, which is checkmated in the Palolo worms by the sacrifice of the bulk of the body, while the head end remains in a crevice of the coral reef and regrows a new body for the next season. Apart from the

bursting of the body—a somewhat crude nemesis—a fatal strain on the constitution of the animal may result from the amount of nutritive material required for the equipment of the eggs, and from the fatigue involved in liberating either the eggs or the embryos. In the male the tumescence in the reproductive organs and, in vertebrates, the erotization of the body by reproductive hormones may lead to an orgasm so violent that it is sometimes fatal. Both sexes of the fragile butterfly and of the stoutly built marine lamprey pay for their reproduction with their life. Among higher vertebrates there is a marked reduction of the physiological expensiveness of the reproductive process. The too familiar tragedy of the human mother's death in childbirth is an exception, due partly to the increase of brain size in man with consequent enlargement of the infant's head, partly to the unhealthiness of civilized life.

(b) But there is an indirect way in which death has come to be associated with reproduction. There is abundant evidence that the length of life, within certain limits of constitution, is adaptive. In the course of ages of natural selection the duration of life has been automatically adjusted to the survival-welfare of the species, and it is vitally important that reproduction should occur when the organism is in full vigour. It is against the welfare of the species that organisms should reproduce after they are long past their best, and this is one reason why animals die after they reproduce. As Goethe said, death is nature's device for securing abundance of life.

BIBLIOGRAPHY.--Herbert Spencer, Principles of Biology (1864-66) ; P. Geddes and J. Arthur Thomson, The Evolution of Sex (1889) ; Sex (Home University Library, 1914) ; E. Korschelt, Lebensdauer, Altern und Tod (Jena, 1922) ; F. H. A. Marshall, The Physiology of Repro duction (rev. ed., 1922) ; J. Meisenheimer, Geschlecht und Geschlechter im Tierreich (Jena, 1922) ; E. Godlewski, Physiologie der Zeugung Handbuch der vergleichenden Physiologie (ed. H. Winterstein, Jena, 1924, etc.) ; A. Lipschutz, The Internal Secretions of the Sex Glands (1924) ; F. A. E. Crew, Animal Genetics (Edinburgh, 1925) ; John Hammond, Reproduction in the Rabbit (Edinburgh, 1925) ; M. Hart mann, Allgemeine Biologie (Jena, 1927). (J. A. Tee.)