that form 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. In the 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 larvae. In certain LepidojHeirt, however, of the groups Psijchidw and f^KNESIS.
Tinoidw., 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.
{'Oviparous or Genesis is Homogenesis, which is Gamogenesis < Ovo-viviparous or I Viviparous or Heterogenesis, which is < Gamogenesis alternating with ["Parthenogenesis or Agamogenesis < Pseudo-parthenogenesis or r Internal ^ Metagenesis <^ or [ External This, like all other classifications of such phenomena, pre- sents 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 develop- ment (§ 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.* * Prof. Huxley avoids this difficulty by making every kind of Genesis a mode of development. His classification, which suggested the one given above, is as follows: — Continuous (Growth Metamorphosis Development^ / Metagenesis 216 THE INDUCTIONS OF BIOLOGY.
There is, however, nothing for us but a choice of imperfec- tions. We cannot by any logical dichotomies, accurately express relations which, in Nature, graduate into each other 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 one element of individual evolution. Negative disintegration occurs in those cases where, as among the compound Hydrozoa, there is a con- tinuous 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 reach- ing a higher degree of integration. Positive disintegration occurs in those cases of agamogenesis where the formation of new individuals is discontinuous, and in all cases of gamo- genesis. 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, the new individual forms but a small deduction from the parent organism. Protozoa and Protojphyta, show us that form of disintegration called spontaneous fission: two or four individuals being produced by the splitting-up of the original one. The Vohox and the Hydrodictyon^ are plants which, having developed broods of young plants within themselves, give them exit by bursting; and among animals, the one lately referred to, which arises from the Distoma ^gg, entirely loses its individuality in the individ- ualities of the numerous Distoma-larYse with which it be- comes filled. Speaking generally, the degree of disintegration becomes less marked, as we approach the higher organic forms. Plants of advanced types throw off from themselves, whether by gamogenesis or agamogenesis, parts that are relatively small; and among the higher animals, there is no case in which the parent individuality is habitually GENESIS. 217 lost, in tlio production of new individualities/ To the last, however, there is of necessity a greater or less disinte- gration. 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 of nutriment in all cases involve further negative or positive disintegrations of the parents.
'New individuals that result from agamogenesis, usually do not separate from the parent-individuals, until they have undergone considerable development, if not complete develop- ment. The agamogenetic offspring of those lowest organisms which develop centrally, do not, of course, pass beyond cen- tral structure; but the agamogenetic offspring of organisms that 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^ the Salpce, and the ApMdes, 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 excep- tions, however. The winter-eggs of the Plumatella, developed in an unspecialized part of the body, present us with a case of metagenesis, in which centres of development, instead of axes, are detached; and in the above-described parthenogene- sis of moths and bees, such centres are detached from an ovarium.
When produced by gamogenesis, the new individuals be- come independent of the parents while 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 (3f those inferior plants which are central, or multicen- tral, in their development, are of course thrown off as centres. In the higher plants, of the two elements that go to the form- ation of the fertilized germ, the pollen-cell is absolutely 218 THE INDUCTIONS OF BIOLOGY.
» separated from the parent-plant under the shapa of a centre; and the embryo -cell, though not absolutely separated from the parent, is still no longer subordinate to the organizing forces of the parent. So that when, the embryo-cell having been fertilized by matter from the pollen-tube, the develop- ment commences, it proceeds without parental control: the new individual, though remaining physically united with the old individual, becomes structurally and functionally separate while still only a centre of development; and takes on its axial form by processes of its own — 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 to the new centre of de- velopment, at the time of its 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 develop- ment; 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. Under all its various forms, the essential act in gamo- genesis, is the union of two centres or cells, produced by different parent organisms: the sperm-cell being the male product, and the germ-cell the female. 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 th^ result- ing fertilized fferms have secured to them the fit conditions for their development. But passing over these many diver- gent and rc-divergent kinds of sexual multiplication, which GENESIS. 219 it would take too much space here to specify, the one uni- versal peculiarity which it concerns us to remark, is, this co- alescence of a detached portion of one organism, with a more or less detached portion of another.
Such protophytes as the PalmellcB and tlie Desjuidiece, which are sometimes distinguished as unicellular plants, show us a coalescence, not of detached portions of two organisms, but of two entire organisms: in the Palmelke, conjugation is a complete fusion of the individuals; and in the Desmidiece, the entire contents of the individuals unite to form the germ- mass. Where, as among the Confervm, we have aggregated cells whose individualities are scarcely at all subordinate to that of the aggregate, the gamogenetic act is eiFected by the union of the contained granules of two adjacent cells. In Spirogyray it is not adjacent cells in the same thread which thus combine; but cells of one thread with those of another. As we ascend to^ plants of high organization, we find that the two reproductive elements become quite distinct in their characters^ and further, that they arise in different organs set apart for their production: the arrangements being such, that the sperm-cells of one plant combine with the germ-cells of another.
There is reason to think that, among the lowest Protozoa, a fusion of two individualities, analogous to that which occurs in the conjugation of certain Algce, is the process from which results the germ of a new series of individuals. But in animals formed by the aggregation of units that are homolo- gous with Protozoa, the sperm-cells and germ-cells are differ- entiated. And even in these humble forms, where there is no differentiation of sexes, we have good evidence that, as in all higher forms, the union is not between sperm-cells and germ- cells that have arisen in the same individual; but between those that have arisen in different individuals.
The marvellous phenomena initiated by the meeting of sperm-cell and germ-cell, naturally suggest the conception of some quite special and peculiar properties possessed by these 220 THE INDITCTIOXS OF BIOLOGY.
cells. It seems obvious that this mysterious power which they disj^lay, of originating a new and complex organism, distinguishes them in the broadest way from portions of organic substance in general. Nevertheless, the more we study the evidence, the more is this assumption shaken — the more are we led towards the conclusion, that these cells have not been made by some unusual elaboration, fundamentally different from all other cells. The first fact which points to this modified conclusion, is the fact recently dwelt upon (§ 63), that in many plants and inferior animals, a small fragment of tissue that is but little difierentiated, is capable of developing into the form of the organism from which it was taken. Conclusive proof obliged us to admit, that the component units of organisms, have inherent powers of arranging themselves into the forms of the organisms to which they belong. And if to these component units, which we distinguished as physiological, such powers must be con- ceded— if, under fit conditions, and when not much specialized, they manifest such powers in a way as marked as that in which the contents of sperm-cells and germ-cells manifest them; then, it becomes clear that the properties of sperm- cells and germ-cells are not so peculiar as we are apt to assume. Again, the organs for preparing spermcells and germ-cells, have none of the speciality of struc- ture which might be looked for, did sperm-cells and germ- cells need endowing with properties essentially unlike those of all other organic agents. On the contrary, these reproductive centres proceed from tissues that are character- ized by their low organization. In plants, for example, it is not appendages that have acquired considerable structure, which produce the fructifying particles: these arise at the extremities of the axes, where the degree of structure is the least. The embryo-cells are formed in the undifferentiated part of the cambium-layer; the pollen-grains are formed at the little-differentiated extremities of the stamens; and both are homologous with simple epithelium-cells. Among many GENESIS. 221 inferior animals devoid of special reproductive organs, such as the Hydraf the ova and spermatozoa originate in the layer of indifferent tissue that lies between the endoderm and the ectoderm; that is, they consist of portions of the least specialized substance. And in the higher animals, these same generative agents appear to be merely modified epithelium- cells — cells not remarkable for their complexity of structure, but rather for their simplicity. If, by way of demurrer to this view, it is asked why other epithe- lium-cells do not exhibit like properties; there are two replies. The first is, that other epithelium- cells are usually so far changed to fit them to their special functions, that they are unfitted for assuming the reproductive function. The second repl}^ is, that in some cases, where the epithelium- cells are but very little specialized, they do exhibit the like properties: not, indeed, by uniting with other epithelium-cells to produce new germs, but by producing new germs without such union. I learn from Dr Hooker, that the Begonia phyllomaniaca habitually develops young plants from the scales of its stem and leaves — nay, that many young plants are developed by a single scale. The epithelium- cells composing one of these scales, swell, here and there, into large globular cells; form chlorophyll in their interiors; shoot out rudimentary axes; and then, by spontaneous constrictions, cut themselves ofi"; drop to the ground; and grow into Begonias. It appears, too, that in a succulent English plant, the Mcdaxis paludosciy a like process occurs: the self-detached cells being, in this case, produced by the surfaces of the leaves. Thus, there is no warrant for the assumption that sperm-cells and germ- cells possess powers fundamentally unlike those of other cells. The inference to which the facts point, is, that they difier from the rest, mainly in not having undergone modifications such as those by which the rest are adapted to particular functions. They are cells that have departed but little from the original and most general type. Or, in the words suggested by a friend, it is not that they are peculiarly 222 THE INDUCTIONS OF BIOLOGY.
specialized, but rather that they are unspecialized: such specializations as some of them exhibit in the shape of loco- motive appliances, &c., being interpretable not as intrinsic, but as extrinsic, modifications, that have reference to nothing beyond certain mechanical requirements. Sundry facts tend likewise to show, that there does not exist the pro- found distinction which we are apt to assume, between the male and female reproductive elements. In the common polype, sperm-cells and germ-cells are developed in the same layer of indifferent tissue; and in Tethya, one of the sponges, Prof. Huxley has observed that they occur mingled together in the general parenchyma. The pollen -grains and embryo- cells of plants, arise in adjacent parts of the cambium-layer; and from a description of a monstrosity in the Passion-flower, recently given by Mr Salter to the Linnsean Society, it ap- pears, both that ovules may, in their general structure, graduate into anthers, and that they may produce pollen in their interiors. All which evidence is in perfect harmony with the foregoing conclusion; since, if sperm-cells and germ-cells have natures not essentially unlike those of un- specialized cells in general, their natures cannot be essen- tially unlike each other.
The next general fact to be noted, is, that these cells whose union constitutes the essential act of gamogenesis, are cells in which the developmental changes have come to a close — cells which, however favourably circumstanced in respect of nutrition, are incapable of further evolution. Though they are not, as many cells are, unfitted for growth and metamorphosis by being highly specialized; jet they have lost the power of growth and metamorphosis. They have severally reached a state of equilibrium. And while the internal balance of forces prevents a continuance of con- structive changes, it is readily overthrown by external destructive forces. Por it uniformly happens that sperm- cells and germ-cells which are not brought in contact, disap- pear. In a plant, the embryo-cell, if not fertilized, is GENESIS. 223 absorbed or dissipated, while the ovule aborts; and the un- impregnated ovum eventually decomposes.
Such being the characters of these cells, and such being their fates if kept apart, we have now to observe what hap- pens when they are united. For a long time, the immediate sequence of their contact was not ascertained. This is at length, however, decided. It has been shown that in plants, the extremity of the elongated pollen-cell applies itself to the surface of the embryo-sac, but does not enter the embryo- sac. In animals, however, the process is different. Careful observers agree, that the spermatozoon passes through the limiting membrane of the ovum. The result in both cases is presumed to be a mixture of the contents of the two cells. The evidence goes to show that in plants, matter passes by osmose from the pollen- cell into the embryo- cell; and that in animals, the substance contained in the spermatozoon becomes mingled with the substance contained in the ovum, either by simple diffusion or by cell-multiplica- tion. But the important fact which it chiefly con- cerns us to notice, is, that on the union of these reproductive elements, there begins, either at once or on the return of favourable conditions, a new series of developmental changes. The state of equilibrium at which each of them had arrived, is destroyed by their mutual influence; and the constructive changes which had come to a close, recommence: a process of cell-multiplication is set up; and the resulting cells pre- sentlj^ begin to aggregate into the rudiment of a new organism.
Thus, passing over the variable concomitants of gamo- genesis, and confining our attention to what is constant in it, we see: — that there is habitually, if not universally, a fusion of two portions of organic substance, which are either them- selves distinct individuals, or are thrown off by distinct individuals; that these portions of organic substance, which are severally distinguished by their low degree of special- ization, have arrived at states of structural quiescence or 224 THE INDUCTIONS OF BIOLOGY.
equilibrium; that if they are not united, this equilibrium ends in dissolution; but that by the mixture of them, this equilibrium is destroyed, and a new evolution initiated.
§ 78. What are the conditions under which Genesis takes place? How does it happen that some organisms multiply by homogenesis, and others by heterogenesis? Why is it that where agamogenesis prevails, it is usually from time to time interrupted by gamogenesis? These are questions of extreme interest; but questions to which decisive answers cannot yet be given. In the existing state of Biology, we must be content if we can learn the direction in which answers lie. A survey of the facts, discloses certain correla- tions which, if not universal, are too general to be without significance.
Where the multiplication of individuals is carried on by heterogenesis, we find, in numerous cases, that agamogenesis continues as long as the forces which result in growth, are greatly in excess of the antagonistic forces. While conversely, we find that the recurrence of gamogenesis, takes place when the conditions are no longer so favourable to growth. In like manner, where there is homogenetic multiplication, new individuals are usually not formed while the preceding in- dividuals are still rapidly growing — that is, while the forces producing growth exceed the opposing forces to a great extent; but the formation of new individuals begins when nutrition is nearly equalled by expenditure. To specify all the facts that seem to warrant these inductions, would take more space than can be here spared. A few of them must suffice.
The relation between fructification and innutrition, among plants, was long ago asserted by a German biologist — by Wolff*, I am told. When, some years ago, I met with the assertion, I was not acquainted with the evidence on which it rested. Since that time, however, I have, when occasion favoured, examined into the facts for myself. The result has been a conviction, strengthened by every further inquiry, GENESIS. 225 GENESIS. 225 that such a relation exists. Uniaxial plants begin to produce their lateral, flowering axes, only after the main axis has developed the great mass of its leaves, and is show- ing its diminished nutrition by smaller leaves, or shorter internodes, or both. In multiaxial plants, two, three, or more generations of leaf-bearing axes, or sexless individuals, are produced before any seed-bearing individuals show them- selves. When, after this first stage of rapid growth and agamogenetic multiplication, some gamogenetic individuals arise, they do so where the nutrition is least; — not on the main axis, or on the secondary axes, or even on the tertiary axes; but on axes that are the most removed from the channels which supply nutriment. Again, a flowering axis is commonly less bulky than the others: either much shorter, or, if long, much thinner. And further, it is an axis of which the terminal internodes are undeveloped: the foliar organs, which instead of becoming leaves become sepals, and petals, and stamens, follow each other in close succession, instead of being separated by portions of the still-growing axis. Another group of evidences meets us, when we observe the variations of fruit-bearing that accompany variations of nutrition, in the plant regarded as a whole. Besides finding, as above, that gamogenesis commences only when the luxuriance of early growth has been somewhat checked, by the extension of the remoter parts of the plant to some distance from the roots; we find that gamogenesis is induced at an earlier stage than usual, by checking the nutri- tion. Trees are made to fruit while still quite small, by cutting their roots, or putting them in pots; and luxuriant branches which have had the flow of sap into them diminished, by what gardeners call ** ringing," begin to produce flower- shoots instead of leaf-shoots. Moreover, it is to be remarked that trees which, by flowering early in the year, seem to show a direct relation between gamogenesis and increasing nutrition, really do the reverse; for in such trees, the flower- buds are formed in the autumn — that structure which deter- 15 226 THE INDUCTIONS OF BIOLOGY.
mines these buds into sexual individuals, is given when the nutrition is declining. Conversely, very high nutrition in plants, prevents, or arrests, gamogenesis. It is notorious that unusual richness of soil, or too large a quantity of manure, results in a continuous production of leaf-bearing, or sexless, shoots. Besides being prevented from producing sexual individuals, by excessive nutrition, plants are, by excessive nutrition, made to change the sexual individuals they were about to produce, into sexless ones. This arrest of gamogenesis may be seen in various stages. The familiar instance of flowers made barren by the trans- formation of their stamens into petals, shows us the lowest degree of this reversed metamorphosis. Where the petals and stamens are partially changed into green leaves, the return from the gamogenetic structure towards the agamo- genetic structure, is more marked; and it is still more marked when, as occasionally happens in luxuriantly-growing plants, new flowering axes, and even leaf-bearing axes, grow out of the centres of flowers.* The anatomical * Among various examples of this which I have observed, some of the most remarkable were among Foxgloves, growing in great numbers and of large size, in a wood between Whatstandwell Bridge and Crich, in Derbyshire. In one case, the lowest flower on the stem, contained, in place of a pistil, a shoot or spike of flower-buds, similar in structiire to the embryo-buds of the main spike. I counted seventeen buds on it; of which the first had three stamens, but was other- wise normal; the second had three; the third, four; the fourth, four; &c. Another plant, having more varied monstrosities, evinced excess of nutrition with equal clearness. The following are the notes I took of its structure: — 1st, or lowest flower on the stem, very large; calyx containing eight divisions, one partly transformed into a corolla, and another transformed into a small bud with bract (this bud consisted of a five-cleft calyx, four sessile anthers, a pistil, and a rudimentary corolla); the corolla of the main flower, which was complete, con- tained six stamens, three of them bearing anthers, two others being flattened and coloured, and one rudimentary; there was no pistil, but, in place of it, a large bud, consisting of a three-cleft calyx, of which two divisions were tinted at the ends, an imperfect corolla, marked internally with the usual purple spots and hairs,jthree anthers sessile on this mal-formed corolla, a pistil, a seed-vessel with ovules, and, growing to it, another bud of which the structure was indistinct. 2nd flower, large; calyx of seven divisions, one being transformed into a bud cii'.NKsis. 227 structure of the sexual axis, affords corroborative evidence: giving very much the impression, as it does, of an aborted sexless axis. Besides lacking those internodes which the leaf-bearing axis commonly possesses, the flowering axis differs by the absence of rudimentary lateral axes. In a leaf- bearing axis, the axil of every leaf usually contains a small bud, which may or may not develop into a lateral axis; but though the petals of a flower are homologous with leaves, they do not bear homologous buds at their bases. Ordinarily, too, the foliar appendages of sexual axes, are much smaller than those of sexless ones — the stamens and pistils especially, which are the last formed, being extremely dwarfed; and there is even reason for thinking that the absence of chloro- phyll from the parts of fructification, is a fact of like mean- ing. Moreover, the formation of the seed-vessel appears to be a direct consequence of arrested nutrition. If a gloved-finger be taken to represent a growing shoot, (the finger standing for the core of the shoot, and the glove for the cambium-la3^er, in which the process of growth takes place); and if it be supposed that there is a diminished supply of material for growth; then, it seems a fair inference, that growth will first cease at the apex of