CONTINUITY AND PERPETUATION Sexual Reproduetiohi.—If organisms did not reproduce and multiply, they would sooner or later be extinguished, if not by natural, at all events by accidental death. For this purpose they must separate off some portion of their body capable of growing into a new individual. This gives rise to some method of vegeta tive or asexual reproduction, or to sexual reproduction. We have already seen that unicellular organism can multiply by mere fission into two ; similarly in the simpler multicellular plants, such as the fresh-water alga Vaucheria or the fungus Mucor, every cell is capable of growing into a new plant. But in many forms special reproductive cells (spores) or multicellular bodies (gemmules and buds) are set apart for the purpose, often in vast numbers. Propa gation by shoots developed on runners or tubers is common among the higher plants, and a very similar method of bud-formation is widely distributed among animals such as sponges, polyps and sea-squirts. In the Bacteria, indeed, fission and spore-formation seems to be the only method of multiplication; but in all other groups of plants and animals, sexual reproduction occurs, though the asexual method may also be retained.
The typical sexual reproduction of plants and animals takes place by special cells of two kinds set apart and liberated for the purpose, and called the reproductive cells, germ-cells or gametes. Each gamete of one kind gives rise by fusion with a gamete of the other kind in the process of fertilization, to a single cell, the zygote, which grows into a new individual. Usually the two kinds of cell become differentiated along divergent lines adapted to the functions they have to perform. One, the ovum, is quiescent, large, stored with f ood-material for the nutrition of the develop ing embryo to which it will give rise. The other, the spermato zoon of animals or spermatozoid of plants, is small, motile, and usually furnished with a head containing the nuclear material, and a vibratile flagellum or whip-like "tail," with the help of which it swims towards and bores its way into the ovum. An individual bearing ova is of the female sex (9); one bearing spermatozoa is of the male sex ( 6). But the terms male and female are often con veniently extended to the gametes themselves. Hermaphrodites give rise to both kinds of germ-cells.
In fertilization one male gamete fuses with one female gamete; and not only do their cell-bodies unite, but their nuclei also com bine into one nucleus. Thus the nucleus of the resulting zygote contains chromatin derived from two individuals, since the gam etes usually come from different parents ; for it is only exception ally that hermaphrodites are self-fertilizing. Unsuccessful male gametes, which do not reach an ovum, perish ; and likewise unfer tilized ova die, except in those rare cases where parthenogenesis (q.v.) occurs (among lower plants, insects, etc.).
The significance of fertilization is great : on the one hand it provides a stimulus to the ovum for renewed activity and de velopment, on the other it combines two streams of hereditary substance.
The evolutionary history of sex (q.v.) is not yet known. Among unicellular forms sexual reproduction may be of a much simpler kind than that described above. For instance the gametes of the two sexes may differ little or not at all from each other, and among unicellular forms two ordinary individuals may act as gametes coming together and fusing to form a zygote. These simpler modes of fertilization may not represent actual stages in the evolution of sex, but give some notion of how sexual reproduction may have developed. Since typical fertilization occurs among both plants and animals it is probably a very ancient process, already fully developed in the common ancestors of all these organisms. It may have originated at a remote time in the history of proto plasm, when it was possibly beneficial for two masses of living material of slightly different constitution to mix and combine their properties.
Among the lowest multicellular plants, the Algae, there are forms in which every cell, or almost every cell can act as a gamete or give rise to gametes, and so share in the making of a new gen eration. But there is an increasing tendency among higher plants, and especially in animals, to restrict the reproductive function to certain cells, which may be set apart at a very early stage in the embryo and often become localized in special organs or gonads. Thus arises a distinction between germ-cells, destined to give rise to gametes, and body or somatic cells devoted to vegetative functions. On such facts is based Weismann's theory of the continuity of the germ-plasm, the most valuable conclu sion of which is that germ-plasm, that substance which is passed on from generation to generation by the gametes, can be traced back through an unbroken lineage of relatively undifferentiated cells to the original zygote, and to a certain limited extent can be said to be independent of the body or soma which harbours it. Whereas the rest of the multicellular organism, the soma, under goes differentiation and eventually dies, the germ-cells continue for ever giving rise to new generations. Such considerations bring us to the subject of the origin and biological significance of death.
Natural death, then, does not necessarily occur among uni cellular organisms if the essential mechanism of life, and more especially the nucleus, is continuously or periodically repaired. They are potentially immortal, and the same may be said of the germ-cells of higher forms. Death appears among living organisms when the soma becomes differentiated from the germ cells. The soma dies, but the germ-cells continue to live and multiply, transmitted from one mortal parent to the next. But why, it may be asked, should the soma die? Even the soma might conceivably go on living for ever provided the conditions were favourable and the wear and tear of life continually re paired. That somatic cells are capable of indefinite growth and multiplication seems proved by recent experiments on their arti ficial culture. Fragments of animal tissues have been kept alive for countless cell-generations by periodic transplantation to fresh nutritive medium (see TISSUE CULTURE), and it is well known that plants can be propagated indefinitely from cuttings. But it is one of the penalties of specialization that organisms in adaptation to particular modes of life tend to acquire a definite, suitable shape and size. However well-regulated they may be, those essential proportions between surface and volume can hardly be so well preserved, that nice co-ordination of parts, that power of repairing waste and injuries, can hardly be so accurately adjusted as to continue working smoothly for ever. To reach the condition of sexual maturity, to preserve the life of the individual long enough to ensure reproduction, is all that is essential for the continuance of the race. And so we find that the length of life of an organism becomes adapted to its needs, and the energies of the individual are exhausted in securing the success in life of the next generation. For instance, many animals in the colder regions live only for one season, leaving behind them their eggs to survive the winter and develop next year. Frequently the male sex dies as soon as fertilization has been accomplished. A definite relation can be traced between the length of life of the individuals of a species, the number of young produced and the care required to bring them up. Thus the longevity of an organism is bound up in evolution with its method and rate of reproduction.
If we have insisted in the foregoing paragraphs that living organisms are the products of a continuous uninterrupted stream of ever-changing and ever-growing substance, that every organ ism is derived from a pre-existing organism, every cell from a pre-existing cell, every nucleus from a pre-existing nucleus, and indeed, all chromatin from pre-existing chromatin, it is because this principle of continuity is the very foundation rock on which is built the modern doctrine of organic evolution.
As a matter of history it was not this principle which led, in the last century, to the widespread acceptance of the evolutionary interpretation; at that time, indeed, most of the evidence for it had not yet been discovered. Rather was it the cumulative, but indirect, evidence derived from the study of the classification, comparative anatomy, embryology, geographical distribution and palaeontology of animals and plants, as well as from the study of variation and heredity, subjects then first coming into prom inence.