Sexual or amphigonic propagation (Amphigonia) is the usual method of propagation among all higher animals and plants. It is evident that it has only developed, at a very late period of the earth’s history, from non-sexual propagation, and apparently in the first instance from the method of propagation by germ-cells. In the earliest periods of the organic history of the earth, all organisms propagated themselves in a non-sexual manner, as numerous lower organisms still do, especially all those which are at the lowest stage of organization, and which, strictly speaking, can be considered neither as animals nor as plants, and which therefore, as primary creatures, or Protista, are best excluded from both the animal and vegetable kingdoms. In the case of the higher animals and plants, the increase of individuals, as a rule, is at present brought about in the majority of cases by sexual propagation.
In all the chief forms of non-sexual propagation mentioned above—in fission, in the formation of buds, germ buds, and germ cells—the separated cell or group of cells was able by itself to develop into a new individual, but in the case of sexual propagation the cell must first be fructified by another generative substance. The fructifying male sperm must first mix with the female germ-cell (the egg) before the latter can develop into a new individual. These two different generative substances, the male sperm and the female egg, are either produced by one and the same individual hermaphrodite (Hermaphroditismus), or by two different individuals (sexual separation, Gonochorismus) (Gen. Morph. ii. 58, 59).
The simpler and more ancient form of sexual propagation is through double-sexed individuals (Hermaphroditismus). It occurs in the great majority of plants, but only in a minority of animals, for example, in the garden snails, leeches, earth-worms, and many other worms. Every single individual among hermaphrodites produces within itself materials of both sexes—eggs and sperm. In most of the higher plants every blossom contains both the male organ (stamens and anther) and the female organs (style and germ). Every garden snail produces in one part of its sexual gland eggs, and in another part sperm. Many hermaphrodites can fructify themselves; in others, however, copulation and reciprocal fructification of both hermaphrodites is necessary for causing the development of the eggs. This latter case is evidently a transition to sexual separation.
Sexual separation (Gonochorismus,) which characterizes the more complicated of the two kinds of sexual reproduction, has evidently been developed from the condition of hermaphroditism at a late period of the organic history of the world. It is at present the universal method of propagation of the higher animals, and occurs, on the other hand, only in the minority of plants (for example, in many aquatic plants, e.g. Hydrocharis, Vallisneria; and in trees, e.g. Willows, Poplars). Every organic individual, as a non-hermaphrodite (Gonochoristus), produces within itself only one of two generative substances, either the male or the female. The female individuals, both in animals and plants, produce eggs or egg-cells. The eggs of plants in the case of flowering plants (Phanerogama), are commonly called “embryo sacs”; in the case of flowerless plants (Cryptogama), “fruit spores.” In animals, the male individual secretes the fructifying sperm (sperma); in plants, the corpuscles, which correspond to the sperm. In the Phanerogama, these are the pollen grains, or flower-dust; in the Cryptogama, a sperm, which, like that of most animals, consists of floating vibratile cells actively moving in a fluid—the zoosperms, spermatozoa, or sperm-cells.
The so-called virginal reproduction (Parthenogenesis) offers an interesting form of transition from sexual reproduction to the non-sexual formation of germ-cells (which most resembles it); it has been demonstrated to occur in many cases among Insects, especially by Siebold’s excellent investigations. In this case germ-cells, which otherwise appear and are formed exactly like egg-cells, become capable of developing themselves into new individuals without requiring the fructifying seed. The most remarkable and most instructive of the different partheno-genetic phenomena are furnished by those cases in which the same germ-cells, according as they are fructified or not, produce different kinds of individuals. Among our common honey bees, a male individual (a drone) arises out of the eggs of the queen, if the egg has not been fructified; a female (a queen, or working bee), if the egg has been fructified. It is evident from this, that in reality there exists no wide chasm between sexual and non-sexual reproduction, but that both modes of reproduction are directly connected. The parthenogenesis of Insects must probably be regarded as a relapse from the sexual mode of propagation (possessed by the original parents of the insects) to the earlier condition of non-sexual propagation. (Gen. Morph. ii. 86.) In any case, however, sexual reproduction, both in plants and animals, which seems such a wonderful process, has only arisen at a later date out of the more ancient process of non-sexual reproduction. In both cases heredity is a necessary part of the phenomenon.
In all the different modes of propagation the essential point of the process is invariably a detachment of a portion of the parental organism possessing the capability of leading an individual, independent existence. We may, therefore, in all cases expect, à priori, that the produced individuals—which are, in fact, as is commonly said, “the flesh and blood” of the parents—will receive the vital characteristics and qualities of form which the parental individuals possess. It is simply a larger or smaller quantity of the parental material, in fact of its albuminous protoplasm, or cell-substance, which passes to the produced individual. But together with the material, its vital properties—that is, the molecular motions of the plasma—are transmitted, which then manifest themselves in its form. Inheritance by sexual breeding loses very much of the mysterious and wonderful character which it at first sight possesses for the uninitiated, if we consider the above-mentioned series of the different modes of propagation, and their connection one with another. It at first appears exceedingly wonderful that in the sexual propagation of man, and of all higher animals, the small egg, the minute cell, often invisible to the naked eye, is able to transfer to the produced organism all the qualities of the maternal organism, and, no less mysterious, that at the same time the essential qualities of the paternal organism are transferred to the offspring by means of the male sperm, which fructifies the egg-cell by means of a viscid substance in which minute thread-like cells or zoosperms move about. But as soon as we compare the connected stages of the different kinds of propagation, in which the produced organism separates itself more and more as a distinct growth from the parental individual, and more or less early enters upon its independent career; as soon as we consider, at the same time, that the growth and development of every higher organism only depends upon the increase of the cells composing it—that is, upon their simple propagation by division—it becomes quite evident that all these remarkable processes belong to one series.
The life of every organic individual is nothing but a connected chain of very complicated material phenomena of motion. These motions must be considered as changes in the position and combination of the molecules, that is, of the smallest particles of animated matter (of atoms placed together in the most varied manner). The specific, definite tendency of these orderly, continuous, and inherent motions of life depends, in every organism, upon the chemical mingling of the albuminous generative matter to which it owes its origin. In man, as in the case of the higher animals which propagate themselves in a sexual manner, the individual vital motion commences at the moment in which the egg-cell is fructified by the spermatic filaments of the seed, in which process both generative substances actually mix; and here the tendency of the vital motion is determined by the specific, or more accurately, by the individual nature of the sperm as well as of the egg. There can be no doubt as to the purely mechanical material nature of this process. But here we stand full of wonder and astonishment before the infinite and inconceivable delicacy of this albuminous matter. We are amazed at the undeniable fact that the simple egg-cell of the maternal organism, and a single paternal sperm-thread, transfer the molecular individual vital motion of these two individuals to the child so accurately, that afterwards the minutest bodily and mental peculiarities of both parents reappear in it.
Here we stand before a mechanical phenomenon of nature of which Virchow, whose genius founded the “cellular pathology,” says with full justice: “If the naturalist cared to follow the custom of historians and preachers, and to clothe phenomena, which are in their way unique, with the hollow pomp of ponderous and sounding words, this would be the opportunity for him; for we have now approached one of those great mysteries of animal nature, which encircle the region of animal life as opposed to all the rest of the world of phenomena. The question of the formation of cells, the question of the excitation of a continuous and equable motion, and, finally, the questions of the independence of the nervous system and of the soul—these are the great problems on which the human mind can measure its strength.” To comprehend the relation of the male and female to the egg-cell is almost as much as to solve all those mysteries. The origin and development of the egg-cell in the mother’s body, the transmission of the bodily and mental peculiarities of the father to it by his seed, touch upon all the questions which the human mind has ever raised about man’s existence. And, we add, these most important questions are solved, by means of the Theory of Descent, in a purely mechanical and purely monistic sense!
There can then be no further doubt that, in the sexual propagation of man and all higher organisms, inheritance, which is a purely mechanical process, is directly dependent upon the material continuity of the producing and produced organism, just as is the case in the simplest non-sexual propagation of the lower organisms. However, I must at once take this opportunity of drawing attention to an important difference which inheritance presents in sexual and non-sexual propagation. It is a fact long since acknowledged, that the individual peculiarities of the producing organism are much more accurately transmitted to the produced organism by non-sexual than by sexual propagation. Gardeners have for a long time made use of this fact in many ways. When, for instance, a single individual of a species of tree with stiff, upright branches accidentally produces down-hanging branches, a gardener, as a rule, cannot transmit this peculiarity by sexual, but only by non-sexual propagation. The twigs cut off such a weeping tree and planted as cuttings or slips, afterwards produce trees having likewise hanging branches, as, for example, the weeping willows and beeches. Seedlings, on the other hand, which have been reared out of the seed of such a weeping tree, generally have the original stiff and upright form of branches possessed by their ancestors. The same may be observed in a very striking manner in the so-called “copper-coloured trees,” that is, varieties of trees which are characterized by a red or reddish brown colour of the leaves. Off-shoots from such copper-coloured trees (for example, the copper beech), which have been propagated by cuttings in a non-sexual manner, show the peculiar colour and nature of the leaves which distinguished the parental individual, while others reared from seeds of such a copper-coloured tree return to the green-coloured condition of leaf.
This difference in inheritance will seem very natural when we consider that the material connection between the producing and produced individuals is much closer and lasts much longer in non-sexual than in sexual propagation. The special tendency of the molecular motion of life can therefore fix itself much longer and more thoroughly in the filial organism, and be more strictly transmitted by non-sexual than by sexual propagation. All these phenomena, considered in connection, clearly prove that the transmission of bodily and mental peculiarities is a purely material and mechanical process. By propagation a greater or lesser quantity of albuminous particles, and together with them the individual form of motion inherent in these molecules of protoplasm, are transmitted from the parental organism to the offspring. As this form of motion remains continuous, the more delicate peculiarities inherent in the parental organism must sooner or later reappear in the filial organism.