Reproduction is the term applied to the whole process whereby life is continued from generation to generation. One of the characteristics of life is its continuity; the races of animals and the orders of plants live on without marked change for centuries; by slow modifications they may be enriched or impoverished, increased or thinned, but there is no breach of continuity. All the forms of life seem to evolutionists like twigs on one many-branched tree; they are genetically related by near or distant bonds of kinship, and in a very real sense each generation is continuous with those which come before and after it. As an evergreen tree replaces by fresh growth those leaves which it loses, so, throughout the world, by various forms of reproduction the continuance of life is secured.
As reproduction is a fundamental fact of life, it cannot be discussed apart from other facts, such as growth, at the limit of which reproduction usually occurs, or development, in which the germ grows into the likeness of its parent, or the occurrence of two sexes producing complementary and mutually dependent reproductive elements. A theory of reproduction must be consistent with the facts of growth and development, and merges into theories of sex and of heredity—the latter being based on a study of the precise relations between successive generations. See EMBRYOLOGY, HEREDITY, SEX.
Modes of Reproduction.—Separated fragments of a sponge or cuttings from the rose, the buds of a hydra, or the bulbils of a lily, the eggs of birds, and the seeds of plants are alike able to grow into new organisms; and thus we see that the common fact about all kinds of reproduction is that parts of one organism are separated to form or to help to form new lives. In many cases what is separated from the parent life is simply part of its body, an overgrowth or a definite bud, which, being set free, is able to reproduce the whole of which it is a representative sample. This we call asexual reproduction. In most cases, however, the parents give origin to special reproductive elements—egg-cells and male-cells—which combine and are together able to grow into a new life. This we call sexual reproduction.
The simplest forms of reproduction are found among the single-celled plants and animals. There we may find an organism like Schizogenes, multiplying by breakage, reproducing by rupture, presumably when the cell has overgrown its normal size; in others numerous buds are liberated at once, as in Arcella and Pelomyxa; in many, familiarly in the yeast-plant, one bud is formed at a time; in most the cell divides into two or many daughter-cells. The formation of many daughter-cells or spores is little more than ordinary division taking place repeatedly in rapid succession, and within the substance of the parent-cell—in other words, in limited time and space.
We have seen that reproduction begins among single-celled organisms in a kind of rupture; but even among the more complex forms of life an equally crude mode of reproduction sometimes occurs. The cast-off arm of a starfish may re-grow the entire animal with a readiness that suggests a habit; some kinds of worms (e.g. Neimerteans) break into pieces, each of which is able to re-grow the whole; large pieces of a sea-anemone or of a sponge are sometimes separated off and form new organisms. It is easy to show experimentally that parts cut from a hydra, a sponge, or a sea-anemone, from a seaweed, a moss, or a tree, may in certain conditions grow into an entire organism.
But the usual mode of asexual reproduction is by the formation of definite buds. When these buds remain continuous, colonial organisms result, like many sponges, most hydroids, Siphonophora like the Portuguese man-of-war, many corals, almost all the Polyzoa, and many Tunicates. The runners of a strawberry and the suckers which grow around a rose-bush illustrate the same state. But in a few plants, like the liverwort and the tiger-lily, a kind of bud may be detached, and thus begin a new life. It is among animals, however, that the liberation of buds is best illustrated, for this mode of reproduction occurs in hydra and many hydroids, in some 'worms,' and in Polyzoa, and even in animals as highly organised as Tunicates. Budding is usually exhibited by comparatively simple and by sedentary animals, and seems indeed to be natural to vegetative organisms. It is easy to understand why asexual reproduction is among many-celled animals always associated with sexual reproduction, and entirely replaced by it in the higher forms. For budding is only possible when the organism is not very highly differentiated, or when part of the body retains many indifferent units; moreover, it is an expensive way of securing the continuance of generation, and is without the advantage to the species which undoubtedly results from the mingling of two life-currents in sexual reproduction.
Sexual reproduction in its fully differentiated form involves (a) the distinctness of two parent organisms, (b) the formation of two different kinds of reproductive elements—e.g. spermatozoa produced by the male and ova by the female, and (c) the fertilisation of the egg-cell by a male element. Moreover, the process of sexual reproduction also includes the sexual union of the two parents, or other ways in which fertilisation is secured, while in some cases the fertilised ovum develops in organic relation with the mother-organism, from which it is eventually separated as an embryo. But, while many organisms exhibit fully differentiated sexual reproduction, and while the essentials of the process are always the same, there are not a few important variations in detail—witness the occurrence of hermaphroditism, parthenogenesis, and alternation of generations, the first and last of which are discussed in separate articles.
Physiology of Reproduction.—All growth is, in a certain sense, of the nature of reproduction. It is an increase in the amount of protoplasm and its attendant train of substances. Abundance of food material and conditions favourable to rapid assimilation are necessarily accompanied by rapidity of growth; but in the most favouring circumstances there is an inevitable limit to the growth in size of a single cell. It occurs when the rate of assimilation of the constantly increasing mass of protoplasm becomes equal to the highest possible rate of absorption. Since absorption can only take place through the surfaces, and since, with any given figure of cell, the ratio of volume to surface is a perfectly definite one, and one which increases at a definite rate as the cell grows, there must be for any given figure of cell a perfectly definite limit of size. For any mass of cells arranged in any manner there must be, for similar reasons (though other factors, such as weight, &c., may be operative and varyingly important), a definite limit of size. When in the single-celled animals this limit is reached, or is nearly reached, so that starvation begins—and in any case the greater the size of the cell the less rapid, in proportion to volume, must be the absorption, unless at a certain point other factors at present unknown occur—then division of the cell takes place, by which means, the volume remaining the same, the surface is doubled, so that the ratio of volume to surface and therefore of assimilation to absorption is lowered, and growth is once more possible. This law (first clearly stated by Spencer and by Leuckart) is evidently the expression of a factor concerned in the initiation of cell division and therefore of the Metazoa or many-celled animals. In the Protozoa, then, reproduction is related to, and in a certain sense caused by, a diminution in the possible rate of assimilation, which, to the protoplasm concerned, bears the aspect of an impaired nutrition. In the Metazoa, although reproduction is not so entirely a mere process of cell division as in the Protozoa, a connection between nutrition and reproduction is observable. The common hydra, with an abundant food-supply and favouring circumstance, grows rapidly, the growth becoming a process of asexual reproduction and taking the form of the production of numerous buds, which may themselves produce a crop of secondary buds. But if the conditions become less favourable to nutrition through the lessening of the supply of food material, or, in terms of the more definite generalisation emphasised above, less favourable to assimilation through, say, a fall of temperature, then this rapid growth ceases and reproductive organs are formed and sexual reproduction takes place. Planarian worms in good nutritive conditions form asexual chains of daughter worms. A check to nutrition is followed by the separation and sexual maturity of the links.
Fruit trees are root-pruned in order that the crop of fruit may be abundant; the reason being that, as nutrition is lessened by such pruning, there follows an increase of reproductive activity which takes the form of fruit. If the vegetative activity of the plant be what one desires, then the flower buds are nipped off and sexual activity prevented. A similar result follows from the castration of animals. The position of the flower at the end of the vegetative axis is an expression of the fact that at that point the food-supplies are more scanty than at any other point along the axis. This distribution of food matter is shown again in such plants as the tiger-lily, which have a mode of asexual reproduction, one that is of continuous growth, by the development of little bulbils which occur in the axis of the leaves, such bulbils being only found on the lower part of the stem. Other factors than the supply of food-matter influence assimilation and reproduction. As in the case of all molecular movements, variations of temperature are an obvious cause of change of state. For every animal—i.e. for every peculiar form of 'protoplasm'—there is a particular temperature which, other things being constant, is most favourable to rapidity of assimilation. This point is widely different in the various forms of life. In every case it is probable that a rise of temperature up to a certain point is followed by a feverish state of body and a tendency to hasten sexual maturity and reproduction. If our conception of the relation of assimilation to reproduction be correct, then, as already suggested, a fall of temperature below that most favourable to assimilation ought to be followed by an increasing tendency to sexual reproduction.
Reproductive maturity—the blossoming of the individual life—occurs, as we have seen, about the time when growth ceases. In the lower animals sexual maturity is attained relatively sooner than in the higher forms; but there are many strange cases of precocious and retarded reproduction. Thus we may contrast our common annuals and the 'century plant' or American aloe, or some midges, worms, and even a couple of amphibians, which are reproductive during larval life, with highly evolved animals, such as the elephants. The physiology of reproduction must take account of that profound reaction which affects the whole system as sexual maturity is attained, of the various ways in which the reproductive elements are separated from the parents, of the relation which, alike in plant and animal, may be established between the fertilised egg-cell and the mother-organism, and of the way in which an embryo thus nurtured eventually becomes independent. Moreover, there are often highly evolved psychical activities associated with reproduction—notably the love between mates and between parents and offspring.
But, while reproduction is a blossoming of the individual life, it is also in a sense the beginning of death. The flower and fruit often end the life of the plant. It may be that the processes of rupture by which some of the simplest organisms reduce their bulk and multiply their kind are but a few steps from the more diffuse dissolution of death. It is a fact that in some simple animals—e.g. some 'worms'—the parent, and especially the mother, ruptures and dies in liberating the reproductive elements. So, among higher forms, not a few insects—mayflies, locusts, butterflies—die a few hours after reproduction. The exhaustion is fatal, and the males are sometimes victims as well as their mates. In higher organisms the fatality of the reproductive sacrifice has been greatly lessened, yet death may tragically occur, even in human life, as the direct nemesis of reproduction. In short, the process by which new lives begin, by which the continued life of the species is secured, tends to be antagonistic to the life of the parent individuals. The old leaves fall off the tree, and their places are filled by others.
The Rate of Reproduction and Increase.—The rate of reproduction depends upon the constitution of the individual organism and on its immediate environment and nutrition. The rate of increase, which is much more difficult to estimate, depends upon the wide and complex conditions of life which are often included in the phrase 'the struggle for existence.' While it is true that organisms sometimes exhibit an extraordinary increase in numbers in favourable areas and seasons, and while we know of many forms and even of whole races which have dwindled away and become extinct, the fluctuations in the numbers of plants and animals seem for the most part to be imperceptibly gradual. Their rate of reproduction is adjusted to the conditions of their life; the rise or fall of the population is seldom emphatic. The essay of Malthus (1798), in which he showed that the increase of human population tended to outrun the means of subsistence, but was met by various checks, afforded suggestions to Darwin and Wallace, who extended the induction of Malthus to plants and animals, recognising in their increase the fundamental condition of the struggle for existence, and analysing the checks as various forms of natural selection. But Herbert Spencer's analysis of the laws of multiplication was even more penetrating. Including under the term individuation all those race-preservative processes by which individual life is completed and maintained, and under the term genesis all those processes aiding the formation and perfecting of new individuals, he showed both inductively and deductively 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 associated with a diminishing rate of reproduction.
The Importance of Reproduction in Evolution.—As almost every individual life begins in the intimate union of two living units—the male-cell and the egg-cell—there is in the nature of the organism's beginning an evident possibility of variation. The two cells, and more especially the nuclei of the two cells, are intermingled; and in the vital combination which results new characteristics may be evolved, old features may be strengthened, peculiarities may be averaged off. On fertilisation as a source of variation, emphasis has been laid by Treviranus, Galton, Brooks, and others, while Hatschek regards the intermingling as an important counteractive of disadvantageous individual peculiarities, and Weismann finds in it the sole source of transmissible variations in many-celled animals.
In the individual life the antithesis between the reproductive and the nutritive functions has many expressions, and in terms of this antithesis not a few lines of variations can be rationalised. Thus, the shortening of the axis of the flower seems to be the result of a check imposed upon the vegetative system by the reproduction function; thus, the development of gymnosperm into angiosperm suggests a continuous subordination of the reproductive carpellary leaf; thus, in almost every natural alliance of phanerogams may be read a contrast between more and less vegetative types, such as is seen within the limits of a single species in the transitions between the leafy kale and the cauliflower. Among animals the antithesis is expressed in different ways—as in the varied degree in which the reproductive individuals of a hydroid colony are differentiated from the nutritive members.
In considering the evolution of animals great importance is always—and rightly—attached to the self-preserving struggles and endeavours which secure the satisfaction of nutritive needs; but the species-maintaining activities of reproduction have been not less important. Thus, Darwin insisted on the importance of sexual selection as a factor in evolution, and, though the criticisms of Wallace and others have lessened the cogency of Darwin's argument, there can be little doubt that courtship has aided in the evolution of the psychical life of animals. Romanes, too, in his insistence on the importance of isolation, recognises 'the reproductive factor in evolution.' For by variations in the reproductive system a species may be divided into mutually sterile sets, which, prevented from intercrossing by this physiological barrier, are free to develop along divergent paths. In a very different connection, Robert Chambers emphasised the importance of 'prolonged gestation,' and Fiske has directed attention to the progressive influence of prolonged infancy, while Miss Buckley has well pointed out that an increase of parental care and sacrifice as seen in birds and mammals has surely been a factor in, as well as a result of, the general ascent of animals.
The increase of reproductive sacrifice which we observe in the evolution of mammals and in the progress through oviparous monotremes, prematurely-bearing marsupials, and various grades of placentals; the growth of parental care, and the frequent subordination of self-preserving to species-maintaining ends; and finally, the rise of sociality from foundations based in organic kinship, are well-known facts of animal life which suggest the importance of the reproductive factor in evolution.
See EMBRYOLOGY, HEREDITY, SEX; H. Spencer, Principles of Biology (Lond. 1864-66); E. Haeckel, Generelle Morphologie (Berlin, 1866); V. Hensen, Physiologie der Zeugung, in Hermann's Handbuch der Physiologie (Leip. 1881); article 'Reproduction,' by P. Geddes and S. H. Vines, in Ency. Brit.; A. Weismann, Papers on Heredity, &c. (Oxford, 1889); P. Geddes and J. A. Thomson, Evolution of Sex (Lond. 1889).