facts tend likewise to show, that there does not exist the profound 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 embryocells 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 appears, 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 unspecialized cells in general, their natures cannot be essentially 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 constructive changes, it is readily overthrown by external destructive forces. Por it uniformly happens that spermcells and germ-cells which are not brought in contact, disappear. In a plant, the embryo-cell, if not fertilized, is absorbed or dissipated, while the ovule aborts; and the unimpregnated ovum eventually decomposes.
Such being the characters of these cells, and such being their fates if kept apart, we have now to observe what happens 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 embryosac. 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 embryocell; 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-multiplication. But the important fact which it chiefly concerns 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 presentlj^ begin to aggregate into the rudiment of a new organism.
Thus, passing over the variable concomitants of gamogenesis, 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 themselves distinct individuals, or are thrown off by distinct individuals; that these portions of organic substance, which are severally distinguished by their low degree of specialization, have arrived at states of structural quiescence or 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 correlations 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 individuals 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, 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 showing 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 themselves. 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 nutrition. 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 flowershoots 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 flowerbuds are formed in the autumn — that structure which determines 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 transformation 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 agamogenetic 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 otherwise 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, contained 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 leafbearing 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 chlorophyll from the parts of fructification, is a fact of like meaning. 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 the cambium-layer, represented by the end of the glovefinger; and supposing growth to continue in those parts of the cambium -layer that are nearer to the supply of nutriment, their further longitudinal extension will lead to the formation of a cavity at the extremity of the shoot, like that which results in a glove-finger when the finger is partially withdrawn and the glove sticks to its end. Whence it seems, with bract, but much smaller than the other; corolla large but cleft along the top; six stamens with anthers, pistil, and seed-vessel. 3rd flower, large; six-cleft calyx, cleft corolla, Avith six stamens, pistil, and seed-vessel, with a second pistil half unfolded at its apex. 4th flower, large; divided along the top, six stamens. 5th flower, large; corolla divided into three parts, six stamens. 6th flower, large; corolla cleft, calyx six -cleft, the rest of the flower normal. 7th, and all succeeding flowers, normal.
both that this introversion of the cambium-layer may be considered as due to failing nutrition, and that the ovules growing from its introverted surface (which would have been its outer surface but for the defective nutrition) are extremely aborted homologues of external appendages — either leaves or lateral axes: the essential organs of fructification thus arising where the defective nutrition has reached its extreme.* To all which let us not forget to add, that the sperm- cells and germ -cells are formed at the very ends of the organs of fructification.
Those kinds of animals which multiply by heterogenesis, present us with a parallel relation between the recurrence- of gamogenesis and the recurrence of conditions unfavourable to growth — at least, this is shown where experiments have thrown light on the connexion of cause and effect; namely, among the Aphides. These creatures, hatched from eggs in the spring, multiply by agamogenesis throughout the summer. When the weather becomes cold, and plants no longer afibrd abundant sap, perfect males and females are produced; and from gamogenesis there result fertilized ova. But now observe that beyond this evidence, we have much more conclusive evidence. For it has been shown, both that the rapidity of the agamogenesis is proportionate to the warmth and nutrition, and that if the temperature and * It appears that botanists do not agree respecting the homologies of the o'v ules: some thinking that they are rudimentary foliar organs, and others that they are rudimentary axial organs. Possibly the dispute will prove a bootless one; since there seems evidence that ovules may be transformed into either one or the other. Mr Salter's paper, lately referred to, shows that they may graduate into stamens, which are foliar organs; and the case of the Foxglove, which I have described above, shows that they may develop into flower-buds, which are axial organs. I would venture to suggest, that the conflicting evidence can be reconciled, only by regarding ovules as the homologues of lateral appendages; and considering a lateral appendage as composed of a leaf, plus a rudimentary axis, either of which may abort. This is the view which seems countenanced by development; since, in its first stage, a lateral bud, whence a lateral appendage arises, shows no division into rudimentary leaf and rudimentary axis; and it is to the lateral bud in this first stage, that the seed-bud or ovule is homologous.
supply of food bo ariidcially iniiiiitaincd, tlie agamogenesis coiitiiiiiGs through the winter. Nay more — it not only, under these conditions, continues through one winter, but it has been known to continue for four successive years: some forty or fifty sexless generations being thus produced. And those who have investigated the matter, see no reason to doubt the indefinite continuance of this agamogenetic multiplication, so long as the external requirements are duly met. Evidence of another kind, which points very distinctly to the same conclusion, is furnished by the heterogenesis of the Daplmia — a small crustacean commonly known as the Water-flea, which inhabits ponds and ditches. From the nature of its habitat, this little creature is exposed to very variable conditions. Besides being frozen up in winter, the small bodies of water in which it lives, are often unduly heated by the summer sun, or dried up by continued drought. The circumstances favourable to the DapJmid's life and growth, being thus liable to interruptions which, in our climate, have a regular irregularity of recurrence; w^e may, in conformity with the hypothesis, expect to find both that the gamogenesis recurs along with evidence of declining nutrition, and that its recurrence is very -variable. This we do find. From Mr Lubbock's paper on the Daphnia in the " Philosophical Transactions " for 1857, and from further information which he -has been good enough to furnish me, the following general facts are deducible: — First, that in each ovarium, along with the rudiments of agamic eggs, or eggs which, if developed, produce young b}^ true parthenogenesis, there usually, if not always, exists the rudiment of an ephippial egg; which, from sundry evidences, is inferred to be a sexual or gamic Q^g. Second, that according to circumstances, either agamogenesis or gamogenesis takes place; but that if the agamic eggs develop, the rudimentary gamic e^g disappears, or becomes absorbed; and conversely, if the gamic e^^ develops, the agamic eggs disappear, or are absorbed by it. Third, that the brood of agamic eggs contained in each ovarium, amounts, under favourable circumstances, to as manj^ as eight or nine; while of the gamic eggs, only one at a time is produced in each ovarium, and occasionally one of the ovaria produces none: whence it follows, that as the gamic egg is not more than twice the bulk of the agamic egg J the quantity of matter contained in an agamic brood, is four times, and occasionally even eight times, as great as that contained in a gamic brood. Thus the quantity of nutriment expended in gamogenesis during a given period (making allowance for that which goes to the formation of the ephippium), is far less than that expended in agamogenesis during a like period. Seeing, then, this constant preparation for either gamic or agamic genesis, in a creature liable to such irregular variations of nutrition; and seeing that the agamogenesis implies by its amount, a large excess of nutrition, while the gamogenesis implies by its amount, a small excess of nutrition; we can scarcely doubt that the one or the other mode of multiplication occurs, according as the external conditions are or are not favourable to nutrition.
Passing now to animals which multiply by homogenesis — animals in which the whole product of a fertilized germ aggregates round a single centre or axis, instead of round many centres or axes; we see, as before, that so long as the conditions allow rapid increase in the mass of this germ-product, the formation of new individuals by gamogenesis does not take place. Speaking generally, we find that only when growth is declining in relative rapidity, do perfect spermcells and germ-cells begin to appear; and that the fullest activity of the reproductive function, arises as growth ceases — speaking generally, we must sa)^ because, though this relation is tolerably definite in the highest orders of animals which multiply by gamogenesis, it is less definite in the lower orders. This admission does not militate against the hypothesis, as it seems to do; for the indefiniteness of the relation occurs where the limit of growth is comparatively indefinite. We saw (§ 46) that among active, hot-blooded creatures.
r.ENESTS. 231 r.ENESTS. 231 sucli US mammals and birds, the inevitable balancing of assimilation by expenditure, establishes, for each species, an almost uniform adult size; and among creatures of these kinds, (birds especially, in which this restrictive effect of expenditure is most conspicuous), the connexion between cessation of growth and commencement of reproduction, is distinct. But we also saw (§ 46) that where, as in the Crocodile and the Pike, the conditions and habits of life are such, that expenditure does not overtake assimilation as the size increases, there is no precise limit of growth; and in creatures thus circumstanced, we may naturally look for a comparatively indeterminate relation between declining growth and commencing reproduction.* There is, indeed, among fishes, at least one case which appears very anomalous. The male parr, or young of the male salmon, a fish of four or five inches in length, is said to produce milt. Having, at this early stage of its growth, not one hundredth of the weight of a full-grown salmon, how does its production of milt consist with the alleged general law? The answer must be in a great measure hypothetical. If the salmon is (as it appears in its young state) a species of fresh -water trout, that has contracted the habit of annually migrating to the sea, where it finds a food on which it thrives — if the original size of this species was not much greater than that of the parr (which is nearly as large as some varieties of lake-trout and river- trout) — and if the limit of growth in the trout tribe is very indefinite, as we know it to be; then we may reasonably infer, that the parr has nearly the adult form and size of this species of. trout, before it acquired its migratory habit; and that this production of milt, is, * I owe to Mr Lubbock an important confirmation of this view. After stating his belief, that between Crustaceans and Insects, there exists a physiological relation analogous to that which exists between water-vertebrata and land-vertebrata; he pointed out to me, that while among Insects, there is a definite limit of growth, and an accompanying definite commencement of reproduction, among Crustaceans, where growth has no definite limit, there is no definite relation between the commencement of reproduction and the decrease or arrest of growth.
in such case, a concomitant of the incipient decline of growth naturally arising in the species, when living under the conditions of its remote ancestors. If this be admitted, the immense subsequent growth of the parr into the salmon, must be regarded as due to a suddenly -increased facility in obtaining food — a facility which removes to a great distance the limit at which assimilation is balanced by expenditure; and which has the effect, analogous to that produced in plants, of arresting the incipient reproductive process, and causing a resumption of growth. A confirmation of this view may be drawn from the fact, that when the parr, after its first migration to the sea, returns to fresh water, having increased in a few months from a couple of ounces to five or six pounds, it no longer shows any fitness for propagation: the grilse, or immature salmon, does not produce milt or spawn. But without citing further illustrations, or attempting to meet further difficulties, it has, I think, been made sufficiently clear, that some such connexion as that alleged, exists. Traversed, as is this relation between commencement of sexual reproduction and declining rate of growth, by various other relations, it is quite as manifest as we can expect it to be.
The general law to which both homogenesis and heterogenesis conform, thus appears to be, that the products of a fertilized germ go on accumulating by simple growth^ so long as the forces whence growth results are greatly in excess of the antagonist forces; but that when diminution of the one set of forces, or increase of the other, causes a considerable decline in this excess, and an approach towards equilibrium, fertilized germs are again produced. Whether the germproduct be organized round one axis, or round the many axes that arise by agamogenesis — whether the development be continuous or discontinuous; matters not. Whether, as in concrete organisms like the higher animals, this approach to equilibrium results from that disproportionate increase of expenditure entailed by increase of size; or whether, as in GENESIS. 23'] partially and wholly discrete organisms, like most plants and many inferior animals, this approach to equilibrium results from absolute or relative decline of nutrition; matters not. In any case, the recurrence of gamogenesis is associated with a more or less marked decrease in the excess of tissue-producing power. We cannot say, indeed, that a decrease in this excess always results in gamogenesis; for we have evidence to the contrary, in the fact that some organisms multiply for an indefinite period by agamogenesis only. Thus, the weeping willow, which has been propagated throughout Europe, does not seed in Europe; and yet, as the weeping willow, by its large size and the multiplication of generation upon generation of lateral axes, presents the same causes of local innutrition as other trees, we cannot ascribe the absence of sexual axes to the continued predominance of nutrition. Among animals, too, the anomalous case of the Tineidce, a group of moths in which parthenogenetic multiplication goes on for generation after generation, shows us that gamogenesis does not necessarily result from an approximate balance of assimilation by expenditure. What we must say, is, that an approach towards equilibrium between the forces which cause growth and the forces which oppose growth, is the chief condition to the recurrence of gamogenesis; but that there are other unknown conditions, in the absence of which this approach to equilibrium is not followed by gamogenesis.
§ 79. The above induction is an approximate answer to the question — When does gamogenesis recur? but not to the question which was propounded — Why does gamogenesis recur? — Why cannot multiplication be carried on in all cases, as it is in many cases, by agamogenesis? As already said, biologic science is not yet advanced enough to reply. Meanwhile, the evidence above brought together, suggests a certain hypothetical answer, which it may be well to set down.
Seeing as we do, on the one hand, that gamogenesis recurs only in individuals that are approaching towards a state of organic equilibrium; and seeing, on the other hand, as we do, that the sperm-cells and germ-cells thrown off by such individuals, are cells in which developmental changes have ended in quiescence, but in which, after their union, there arises a process of active cell- formation; we may suspect that the approach towards a state of general equilibrium in such gamogenetic individuals, is accompanied by an approach towards molecular equilibrium in them; and that the need for this union of sperm-cell and germ-cell, is the need for overthrowing this equilibrium, and re-establishing active molecular change in the detached germ — a result which is probably effected by mixing the slightly different physiological units of slightly different individuals. The several arguments that may be brought in support of this view, cannot be satisfactorily set forth until after the topics of Heredity and Yariation have been dealt with. Leaving it for the present, I propose hereafter to reconsider this question, in connexion with sundry others that are raised by the phenomena of Genesis.
Before ending the chapter, however, it may be well to note the relations between these different modes of multiplication, and the conditions of existence under which they are respectively habitual. While the explanation of the teleologist is untrue, it is often an obverse to the truth; for though, on the hypothesis of Evolution, it is clear that things are not arranged thus or thus for the securing of special ends, it is also clear, that arrangements which do secure these special ends, tend continually to establish themselves — ai*e established by their fulfilment of these ends. Besides insuring a structural fitness between each kind of organism and its circumstances, the working of " natural selection " also insures a fitness between the mode and rate of nudtiplication of each kind of organism and its circumstances. We may, therefore, without any teleological implication, consider the fitness of homogencsls and hctcrogoncsis to the needs of the different classes of organisms which exhibit them.
One of the facts to be observed, is, that hcterogenesis prevails among organisms of which the food, though abundant compared with their expenditure, is dispersed in such a way that it cannot be appropriated in a wholesale manner. Protophyta, subsisting on diffused gases and decaying organic matter in a state of minute subdivision; and Protozoa, to which food comes in the shape of extremely small floating particles; are enabled by their rapid agamogenetic multiplication, to obtain materials for growth, better than they would do did they not thus continually divide and disperse in pursuit of it. The higher plants, having for nutriment the carbonic acid of the air and certain mineral components of the soil, show us modes of multiplication adapted to the fullest utilization of these substances. A herb, with but little power of forming the woody-fibre requisite to make a stem that can support wide-spreading branches, after producing a few sexless axes, produces sexual ones; and maintains its race better by the consequent early dispersion of seeds, than by a further production of sexless axes. But a tree, able to lift its successive generations of sexless axes high into the air, where each axis gets carbonic acid and light almost as freely as if it grew by itself, may with advantage go on budding-out sexless axes year after year; since it thereby increases its subsequent power of budding- out sexual axes. Meanwhile, it may advantageously transform into seed-bearers, those axes