Reproduction

Collier's New Encyclopedia (1921)

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 REPRESENTATIVE GOVERN- 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. 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 a bird, 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 is called asexual reproduction. In most cases, however, the parents give origin to spemale cells-which combine and are tocial reproductive elements-egg cells and gether able to grow into a new life. This is called 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 cast-off arm of a starfish may regrow the entire animal with a readiness that suggests a habit; some kinds of worms (e. g., Nemerteans) break into pieces, each of which is able to regrow the whole; large pieces of a sea anemone or of a sponge are sometimes separated off and form new organisms. 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 toward many the Portuguese man-of-war, 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 ger lily, 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 organized as Tunicates. Budding is usually exhibited by comparatively simple and by sedentary animals, and seems indeed to be natural to vegetative organisms. Budding is only possible difwhen the organism is not very highly hly ferentiated, 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 unthe advantage to the species which doubtedly results from the mingling of two life-currents in sexual reproducton. Sexual reproduction in its fully differentiated form involves (a) the distinctness of two parent organisms, (b) kinds the formation of two different of ferent spermato reproductive elements zoa produced by the male and ova by the female, and (c) the fertilization of the egg cell by a male element. Moreover the process of sexual reproduction also sexual union of the two includes the parents, or other ways in which fertilization is secured, while in some cases the fertilized ovum develops in organic relation with the mother organism, from which it is eventually separated as an embryo. 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 favorable to rapid assimilation are necessarily accompanied by by rapidity of growth; but in there is the most favoring circumstances an inevitable limit to the growth in size the rate of a single cell. It occurs when 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 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, operative such as weight, etc., may be definite and varyingly important), a celled animals this limit is reached, or is nearly reached so that starvation begins size of -and in any case the greater the while reproduction is a blossomthe cell the less rapid, in pro 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- , then celled animals. In the Protozoa , 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, though 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 circumstance food supply and favoring , grows rapidly, the growth becoming becomi 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 less favorable to nuconditions become trition through the lessening of the supply of food material, then this rapid organs growth ceases and reproductive are formed and sexual reproduction takes place. Fruit trees are root-pruned in order that the crop of fruit may be abundant; is the reason being that, as nutrition lessened by such pruning, there follows activity an increase of reproductive which takes the form of fruit. If the vegetative activity of the plant be what are one desires, then the flower buds nipped off and sexual activity prevented. cas- A similar result follows from the tration of animals. Other factors matter than the supply of food influence assimilation and reproduction. case of all molecular As in the movements, variations of temperature of change of state. are an obvious cause Reproductive maturity-the blossoming of the individual life-occurs, as has about the time when been shown, In the lower animals growth ceases. 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 limit of size. When in the singleof amphibians, which are reprocouple ductive during larval life, with highly evolved animals, such as the elephants. But, ing of the individual life, it is also in a beginning of death. The flower sense the fruit often end the life of the plant. and processes of rupture It may be that the some of the simplest organby which bulk and multiply their isms reduce their few steps from the more kind are but a of death. It is a fact diffuse dissolution simple animals-e. g. some that in 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. Rate of Reproduction and Increase.- The rate of reproduction depends on 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 on 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 favorable 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 su subsistence, but was met by various checks, afforded suggestions to Darwin and Wallace, who extended the induction of Malthus to pants and animals, recognizing in their increase the fundamental condition of the struggle for existence, and analyzing 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 racepreservative 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. 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 organisms 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 fertilization 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 rationalized. Thus, the shortening of the axis of the flower seems to be the result of a check imposed on the the vegetativ tive system by the reproduction function; thus, the development of gymnosperm into augiosperm 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 endeavors 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, recognizes "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. The increase of reproductive sacrifice which is observed in the evolution of mammals and in the progress through oviparous monotremes, prematurelybearing 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.