SEXUAL DIFFERENTIATION. 201 arises the many-celled organism. 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). The simpler and earlier 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, earthworms, 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. By this reciprocal action the disadvantages of in-breeding are avoided. 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 nonhermaphrodite (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 parthenogenetic 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 on wide chasm between sexual and non-sexual reproduction, but that both modes of reproduction are directly SEXUAL DIFFERENTIATION. 203 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. 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. Moreover, the parthenogenesis of insects is not an original, primary phenomenon, but a secondary one, which has arisen from a diminution of the male sex; for some reason or another the males became superfluous! At all events, both in plants as well as in animals, sexual propagation-which appears so wonderful a process-arose only at a later period from the earlier form of non-sexual propagation. In both cases inheritance forms a necessary part in the phenomenon of reproduction. The commingling of two homogeneous cells, which in the case of numerous Protista leads to non-sexual propagation by self-division or the formation of spores (sometimes as temporary conjugation, sometimes as permanent copulation), is the first step towards Amphigony. The second step is the heterogeneous development or divergence of the two cells, their division of labour and of form. The smaller and more agile cell becomes the male sperm-cell, the larger and less agile cell the female egg-cell. Both of them, on commingling, transmit their own peculiarities to the common product. This transmission becomes quite intelligible when we examine the whole series of phenomena in connection. CHAPTER IX. LAWS OF TRANSMISSION BY INHERITANCE. Theories of Inheritance.-Difference between Transmission by Inheritance in Sexual and Non-sexual Propagation.-Distinction between Conservative and Progressive Transmission by Inheritance.-Laws of Conservative Transmission: Transmission of Inherited Characters.Uninterrupted or Continuous Transmission.-Interrupted or Latent Transmission.-Alteration of Generations.-Relapse.-Degeneracy.Sexual Transmission.-Secondary Sexual Characters.-Mixed or Amphigonous Transmission.-Hybrids.-Abridged or Simplified Transmis sion.-Laws of Progressive Inheritance: Transmission of Acquired Characters.-Adapted or Acquired Transmission.-Fixed or Established Transmission.-Homochronous Transmission (Identity in time).— Homotopic Transmission (Identity in place).—Molecular Theories of Transmission.-Pangenesis (Darwin).-Perigenesis (Haeckel).-Idioplasma (Nägeli).—Germ-plasma (Weismann).-Intracellular Pangenesis (Vries). THE proper understanding of the two great organic constructive forces of Inheritance and Adaptation are among the most important advances which, during the last thirty years, our modern theory of development has introduced into the general history of nature. The very complicate interaction of these two forces, together with the ever-varying relations of the struggle for existence, is sufficient for producing the whole variety of forms in the organic world. The earlier school of naturalists, at the LAWS OF TRANSMISSION. 205 beginning of our century, did, indeed, recognize the immense importance of this interaction, but they were unable to penetrate further into the mysterious character of the two "constructive forces." However, the grand advances in morphology and physiology, in histology and ontogeny, have now furnished us with a far deeper insight into their true nature, and we know them to be genuine physiology functions, i.e. universal vital forces in organisms themselves; and like other vital processes these two fundamental constructive forces proceed primarily from physical and chemical relations. They certainly at times appear extremely complicate, but can nevertheless be traced back to simple, mechanical causes, to the attraction and repulsion of particles of matter, of molecules and of atoms. As I endeavoured to show in my " General Morphology" (in 1866), we arrive at an understanding of Inheritance from the complicate phenomena of propagation, whereas the phenomena of Adaptation are explained by the elementary conditions of nutrition, more especially by the trophic irritation exercised on the one hand by the direct influence of the external conditions of life, on the other by the peculiar activity of the organs and of the cells of which they are composed. In my last chapter I endeavoured to show that in the case of all the different forms of propagation (and also of inheritance) the most essential point is invariably a detachment from the parental organism of a portion possessing the faculty of leading an individual, independent existence. We may, therefore, in all cases expect that the produced individuals—which are, in fact, as is commonly said, "the flesh and blood of the parents "— will receive the |