Reproduction

REPRODUCTION. The general term Reproduction in cludes the whole sequence of processes or events by which new individuals arise and life is continued from generation to genera tion. It is often and rightly said that the major activities of organisms centre round the contrasted functions of nutrition and reproduction, using both terms widely. Yet it is evident that nutrition and reproduction are not necessarily two sharply circumscribed single functions, but may imply the direction of numerous activities towards two particular ends, the pres ervation of the individual, on the one hand, and the continuance of the race, on the other. In studying higher animals, it is impos sible to consider either reproduction or nutrition apart from the functions of moving and feeling, or apart from circulation and the hormones. Many functions may be ancillary to reproduction, which means much more than the activity of the reproductive or gans or gonads. Moreover in the higher reaches of life, reproduc tion has its psychological as well as its physiological aspect.

The antithesis between nutrition and reproduction, however, is one of the fundamental ideas in biology. Nutrition not only implies fuel for immediate consumption, it implies increase of capital, whether in growth or reserves. It has emphatically a plus sign, whereas reproduction is always minus, since it means parting with some of the living material, and the sacrifice is sometimes enormous. Yet the antithesis must not be pressed too hard. As Haeckel emphasized, reproduction may be regarded as a form of discontinuous growth, specialized for multiplication ; and growth is the outcome of nutrition. ]n asexual modes there is a separation of surplus material accumulated by antecedent nutritive processes. Even in sexual reproduction an elaborate nutritive preparation is often necessary, as in the equipment of a huge number of eggs with yolk.

What Reproduction Implies.

In the simplest case repro duction is complete in one act—the division of a microbe into two. But this is not typical of higher forms. (a) Very generally repro duction implies the differentiation of two sexes, the egg-producing female, and the sperm-producing male ; and with this many de grees of sex-dimorphism may be associated. Sexual fusion, how ever, is primarily not reproductive (see SEx). (b) At a certain age or under certain environmental, e.g., seasonal, stimulation the reproductive organs become active,—a process which may be linked to other physiological changes in the body generally. But

biology is still far from an understanding of the physiological processes which lead, for instance, to the activation of a wild bird's gonads in spring. For most of the year they are in abey ance, sometimes hardly visible on dissection, but suddenly they become large and the seat of rapid multiplication of cells. In some animals a special diet is required to activate the gonads; thus in some Diptera a meal of blood is necessary. Especially as regards Algae and small crustaceans, much is known in regard to the environmental conditions that bring on reproductive ac tivity, but generalization is still very difficult. (c) While the gonads are influenced by the body, there is also a converse influ ence. For hormones which are produced by the testis or by the ovary are distribut6d by the blood throughout the body, and serve to provoke new growths, such as antlers, or to excite previously inactive organs, such as milk-glands. (d) In a great variety of animals there is a well-marked phase in which the sexes become aware of one another as desirable, or as opportunities for satis faction, and seek to secure sexual union, sometimes coercively, but often by evoking mutual interest and excitement. This is a prelude to actual pairing, and it often attains to some artistic subtlety (see COURTSHIP OF ANIMALS). (e) The outcome is the actual liberation of the sperms on the male's part, which may or may not be simultaneous with the liberation of ova on the female's part, as in the cases of frogs and bees respectively. But all sorts of modes occur. The sexes may not see or touch one another, yet there may be simultaneous liberation of eggs and sperms, as in sea-urchins, where the actual fertilization in the water is very fortuitous. In diverse fishes, though there is no physical contact between the sexes, the proximity of the spawn ing female is necessary as liberating stimulus to the male's emis sion of sperm. On the other hand, at many different levels, as from dragon-flies and crabs to birds and mammals, there is some sort of sexual embrace or amplexus in the course of which the sperms pass from male to female. As in other successions of events among organisms, there may be in reproduction an entire suppression of a chapter that is more or less typical of the ordinary trajectory of life. Thus most marine fishes suppress insemination—a term which should be restricted to the transference of sperms from the male to the female, or from one hermaphrodite to another, as in earthworms and snails.