In propagation characterized by unlikeness of the successive generations, there is asexual genesis with occasionally-recurring sexual genesis; in other words—agamogenesis interrupted more or less frequently by gamogenesis. If we set out with a generation of perfect males and females, then, from their ova arise individuals which are neither males nor females, but which produce the next generation from buds. By this method of multiplication many individuals originate from a single fertilized germ. The product of development is organized round more than one centre or axis. The simplest form of heterogenesis is that seen in most uniaxial plants. If, as we find ourselves obliged to do, we regard each separate shoot or axis of growth as a distinct individual, homogenesis is seen in those which have absolutely terminal flowers; but in all other uniaxial plants, the successive individuals are not represented by the series A, A, A, A, &c., but they are represented by the series A, B, A, B, A, B, &c. For in the majority of plants which were classed as uniaxial ([§ 50]), and which may be conveniently so distinguished from other plants, the axis which shoots up from the seed, and substantially constitutes the plant, does not itself flower but gives lateral origin to flowering axes. Though in ordinary uniaxial plants the fructifying apparatus appears to be at the end of the primary, vertical axis; yet dissection shows that, morphologically considered, each fructifying axis is an offspring from the primary axis. There arises from the seed a sexless individual, from which spring by gemmation individuals having reproductive organs; and from these there result fertilized germs or seeds that give rise to sexless individuals. That is to say, gamogenesis and agamogenesis alternate: the peculiarity being that the sexual individuals arise from the sexless ones by continuous development. The Salpæ show us an allied form of heterogenesis in the animal kingdom. Individuals developed from fertilized ova, instead of themselves producing fertilized ova, produce, by gemmation, strings of individuals from which fertilized ova again originate. In multiaxial plants, we have a succession of generations represented by the series A, B, B, B, &c., A, B, B, B, &c. Supposing A to be a flowering axis or sexual individual, then, from any fertilized germ it casts off, there grows up a sexless individual, B; from this there bud-out other sexless individuals, B, and so on for generations more or less numerous, until at length, from some of these sexless individuals, there bud-out seed-bearing individuals of the original form A. Branched herbs, shrubs, and trees, exhibit this form of heterogenesis: the successive generations of sexless individuals thus produced being, in most cases, continuously developed, or aggregated into a compound individual, but being in some cases discontinuously developed. Among animals a kind of heterogenesis represented by the same succession of letters, occurs in such compound polypes as the Sertularia, and in those of the Hydrozoa which assume alternately the polypoid form and the form of the Medusa. The chief differences presented by these groups arise from the fact that the successive generations of sexless individuals produced by budding, are in some cases continuously developed, and in others discontinuously developed; and from the fact that, in some cases, the sexual individuals give off their fertilized germs while still growing on the parent-polypedom, but in other cases not until after leaving the parent-polypedom and undergoing further development. Where, as in all the foregoing kinds of agamogenesis, the new individuals bud out, not from any specialized reproductive organs but from unspecialized parts of the parent, the process has been named, by Prof. Owen, metagenesis. In most instances the individuals thus produced grow from the outsides of the parents—the metagenesis is external. But there is also a kind of metagenesis which we may distinguish as internal. Certain entozoa of the genus Distoma exhibit it. From the egg of a Distoma there results a rudely-formed creature known as a sporocyst and from this a redia. Gradually, as this divides and buds, the greater part of the inner substance is transformed into young animals called Cercariæ (which are the larvæ of Distomata); until at length it becomes little more than a living sac full of living offspring. In the Distoma pacifica, the brood of young animals thus arising by internal gemmation are not Cercariæ, but are like their parent: themselves becoming the producers of Cercariæ, after the same manner, at a subsequent period. So that now the succession of forms is represented by the series A, B, A, B, &c., now by the series A, B, B, A, B, B, &c., and now by A, B, B, C, A. Both cases, however, exemplify internal metagenesis in contrast with the several kinds of external metagenesis described above. That agamogenesis which is carried on in a reproductive organ—either an ovarium or the homologue of one—has been called, by Prof. Owen, parthenogenesis. It is the process familiarly exemplified in the Aphides. Here, from the fertilized eggs laid by perfect females there grow up imperfect females, in the ovaria of which are developed ova that though unfertilized, rapidly assume the organization of other imperfect females, and are born viviparously. From this second generation of imperfect females, there by-and-by arises, in the same manner, a third generation of the same kind; and so on for many generations: the series being thus symbolized by the letters A, B, B, B, B, B, &c., A. Respecting this kind of heterogenesis it should be added that, in animals as in plants, the number of generations of sexless individuals produced before the re-appearance of sexual ones, is indefinite; both in the sense that in the same species it may go on to a greater or less extent according to circumstances, and in the sense that among the generations of individuals proceeding from the same fertilized germ, a recurrence of sexual individuals takes place earlier in some of the diverging lines of multiplication than in others. In trees we see that on some branches flower-bearing axes arise while other branches are still producing only leaf-bearing axes; and in the successive generations of Aphides a parallel fact has been observed. Lastly has to be set down that kind of heterogenesis in which, along with gamogenesis, there occurs a form of agamogenesis exactly like it, save in the absence of fecundation. This is called true parthenogenesis—reproduction carried on by virgin mothers which are in all respects like other mothers. Among silk-worm-moths this parthenogenesis is exceptional rather than ordinary. Usually the eggs of these insects are fertilized; but if they are not they are still laid, and some of them produce larvæ. In certain Lepidoptera, however, of the groups Psychidæ and Tineidæ, parthenogenesis appears to be a normal process—indeed, so far as is known, the only process; for of some species the males have never been found.
A general conception of the relations among the different modes of Genesis, thus briefly described, will be best given by the following tabular statement.
| Genesis is |  | Homogenesis, which is usually Gamogenesis |  | Oviparous or Ovo-viviparous or Viviparous | ||
| or | ||||||
Heterogenesis, which is | | Gamogenesis alternating with Agamogenesis |  | Parthenogenesis | ||
| or Metagenesis | | Internal or External | ||||
This, like all other classifications of such phenomena, presents anomalies. It may be justly objected that the processes here grouped under the head agamogenesis, are the same as those before grouped under the head of discontinuous development ([§ 50]): thus making development and genesis partially coincident. Doubtless it seems awkward that what are from one point of view considered as structural changes are from another point of view considered as modes of multiplication.[[29]] There is, however, nothing for us but a choice of imperfections. We cannot by any logical dichotomies accurately express relations which, in Nature, graduate into one another insensibly. Neither the above, nor any other scheme, can do more than give an approximate idea of the truth.
§ 76. Genesis under every form is a process of negative or positive disintegration; and is thus essentially opposed to that process of integration which is the primary process in individual evolution. Negative disintegration occurs in those cases where, as among the compound Hydrozoa, there is a continuous development of new individuals by budding from the bodies of older individuals; and where the older individuals are thus prevented from growing to a greater size, or reaching a higher degree of integration. Positive disintegration occurs in those forms of agamogenesis where the production of new individuals is discontinuous, as well as in all cases of gamogenesis. The degrees of disintegration are various. At the one extreme the parent organism is completely broken up, or dissolved into new individuals; and at the other extreme each new individual forms but a small deduction from the parent organism. Protozoa and Protophyta show us that form of disintegration called spontaneous fission: two or more individuals being produced by the splitting-up of the original one. The Volvox and the Hydrodictyon are plants which, having developed broods within themselves, give them exit by bursting; and among animals the one lately referred to which arises from the Distoma egg, entirely loses its individuality in the individualities of the numerous Distoma-larvæ with which it becomes filled. Speaking generally, the degree of disintegration becomes less marked as we approach the higher organic forms. Plants of superior types throw off from themselves, whether by gamogenesis or agamogenesis, parts that are relatively small; and among superior animals there is no case in which the parent individuality is habitually lost in the production of new individuals. To the last, however, there is of necessity a greater or less disintegration. The seeds and pollen-grains of a flowering plant are disintegrated portions of tissue; as are also the ova and spermatozoa of animals. And whether the fertilized germs carry away from their parents small or large quantities of nutriment, these quantities in all cases involve further negative or positive disintegrations of the parents.
Except in spore-producing plants, new individuals which result from agamogenesis usually do not separate from the parent-individuals until they have undergone considerable development, if not complete development. The agamogenetic offspring of those lowest organisms which develop centrally, do not, of course, pass beyond central structure; but the agamogenetic offspring of organisms which develop axially, commonly assume an axial structure before they become independent. The vegetal kingdom shows us this in the advanced organization of detached bulbils, and of buds that root themselves before separating. Of animals, the Hydrozoa, the Trematoda, and the Salpæ, present us with different kinds of agamogenesis, in all of which the new individuals are organized to a considerable extent before being cast off. This rule is not without exceptions, however. The statoblasts of the Plumatella (which play the part of winter eggs), developed in an unspecialized part of the body, furnish a case of metagenesis in which centres of development, instead of axes, are detached; and in the above-described parthenogenesis of moths and bees, such centres are detached from an ovarium.
When produced by gamogenesis, the new individuals become (in a morphological sense) independent of the parents while still in the shape of centres of development, rather than axes of development; and this even where the reverse is apparently the case. The fertilized germs of those inferior plants which are central, or multicentral, in their development, are of course thrown off as centres; and the same is usually the case even in those which are uniaxial or multiaxial. In the higher plants, of the two elements that go to the formation of the fertilized germ, the pollen-cell is absolutely separated from the parent-plant under the shape of a centre, and the egg-cell, though not absolutely separated from the parent, is still no longer subordinate to the organizing forces of the parent. So that when, after the egg-cell has been fertilized by matter from the pollen-tube, the development commences, it proceeds without parental control: the new individual, though remaining physically united with the old individual, becomes structurally and functionally separate: the old individual doing no more than supply materials. Throughout the animal kingdom, the new individuals produced by gamogenesis are obviously separated in the shape of centres of development wherever the reproduction is oviparous: the only conspicuous variation being in the quantity of nutritive matter bequeathed by the parent at the time of separation. And though, where the reproduction is viviparous, the process appears to be different, and in one sense is so, yet, intrinsically, it is the same. For in these cases the new individual really detaches itself from the parent while still only a centre of development; but instead of being finally cast off in this state it is re-attached, and supplied with nutriment until it assumes a more or less complete axial structure.
§ 77. As we have lately seen, the essential act in gamogenesis is the union of two cell-nuclei, produced in the great majority of cases by different parent organisms. Nearly always the containing cells, often called gametes, are unlike: the sperm-cell being the male product, and the germ-cell the female. But among some Protozoa and many of the lower Algæ and Fungi, the uniting cells show no differentiation. Sexuality is only nascent.
There are very many modes and modifications of modes in which these cells are produced; very many modes and modifications of modes by which they are brought into contact; and very many modes and modifications of modes by which the resulting fertilized germs have secured to them the fit conditions for their development. But passing over these divergent and re-divergent kinds of sexual multiplication, which it would take too much space here to specify, the one universal trait is this coalescence of a detached portion of one organism with a more or less detached portion of another.
Such simple Algæ as the Desmidieæ, which are sometimes called unicellular plants, show us a coalescence, not of detached portions of two organisms, but of two entire organisms: the entire contents of the individuals uniting to form the germ-mass. Where, as among the Confervoideæ, we have aggregated cells whose individualities are scarcely at all subordinate to that of the aggregate, the gamogenetic act is often effected by the union "of separate motile protoplasmic masses produced by the division of the contents of any cell of the aggregate. These free-swimming masses of protoplasm, which are quite similar to (but generally smaller than) the agamogenetic 'zoospores' of the same plants, and to the free-swimming individuals of many Protophyta, are apparently the primitive type of gametes (conjugating cells); but it is noteworthy that such a gamete nearly always unites with one derived from another cell or from another individual. The same fact holds with regard to the gametes of the Protophytes themselves, which are formed in the same way from the single cell of the mother individual. In the higher types of Confervoideæ, and in Vaucheria, we find these equivalent, free-swimming, gametes replaced by sexually differentiated sperm- and germ-cells, in some cases arising in different organs set apart for their production, and essentially representing those found in the higher plants. Transitional forms, intermediate between these and the cases where equivalent gametes are formed from any cell of the plant are also known."