§ 338. In so far as it is a process of separation, sexual genesis is like asexual genesis; and is therefore, equally with asexual genesis, opposed to that aggregation which results in growth. Whether deduction is made from one parent or from two, whether it is made from any part of the body indifferently or from a specialized part, or whether it is made directly or indirectly, it remains in any case a deduction; and in proportion as it is great, or frequent, or both, it must restrain the increase of the individual.

Here we have to group together the leading illustrations of this truth. We will take them in the same order as before.

§ 339. The lowest vegetal forms, or rather, we may say, those forms which we cannot class as either distinctly vegetal or distinctly animal, show us a process of sexual multiplication that differs much less from the asexual process than in the higher forms. The common character which distinguishes sexual from asexual genesis, is that the mass of protoplasm whence a new generation is to arise, has been produced by the union of two portions of matter which were before more widely separated. I use this general expression because, among the simplest Algæ, this is not invariably matter supplied by different individuals: certain Diatomaceæ exhibit within a single cell, the formation of a sporangium by a drawing together of the opposite halves of the endochrome into a ball. Mostly, however, sporangia are products of conjugation. The protoplasmic contents of two cells unite to form the germ-mass or zygote; and these conjugating cells may be either entirely independent, as in many Desmidiaceæ and in the gametes of many Confervoideæ; or they may be two of the adjacent cells forming a thread, as in some Conjugateæ and the gametes of Confervoideæ; or they may be cells belonging to adjacent threads, as in other Conjugateæ. But whether it is originated by a single parent-cell, or by two parent-cells, the zygote, after remaining quiescent until there recur the fit conditions for growth, either breaks up into a multitude of spores, each of which produces an individual that usually multiplies asexually, or germinates directly to produce one new individual; and the fact here to be noted is, that as the entire contents of the parent-cells unite to form the zygote, their individualities are lost in the germs of a new generation. In these minute simple types, sexual propagation just as completely sacrifices the life of the parent or parents, as does that form of asexual propagation in which the protoplasm resolves itself directly into zoospores. And in the one case as in the other, this sacrifice is the concomitant of a prodigious fertility. Slightly in advance of this, but still showing us an almost equal loss of parental life in the lives of offspring, is the process seen in such unicellular Algæ as Botrydium, and in minute Fungi of the same degree of composition. These exhibit a relatively-enormous development of the spore-producing part, and an almost entire absorption of the parental substance into it. As evidence of the resulting powers of multiplication, we have but to remember that the spread of mould over stale food, the rapid destruction of crops by mildew, and other kindred occurrences, are made possible by the incalculably numerous spores thus generated and universally dispersed.

Plants a degree higher in composition supply a parallel series of illustrations. We have among the larger Fungi, in which the reproductive apparatus is relatively so enormous as to constitute the ostensible plant, a similar subordination of the individual to the race, and a similarly-immense fertility. Thus, as quoted by Dr. Carpenter, Fries says—“in a single individual of Reticularia maxima, I have counted (calculated?) 10,000,000 sporules.” It needs but to note the clouds of particles, so minute as to look like smoke, which ripe puffballs give off when they are burst, and then to remember that each particle is a potential fungus, to be impressed with the almost inconceivable powers of propagation which these plants possess. The Lichens, too, furnish examples. Though they are nothing like so prolific as the Fungi (the difference yielding, as we shall hereafter see, further support to the general argument), yet there is a great production of germs, and a proportionate sacrifice of the parental individuality. Considerable areas of the thallus develop into the fruit-bodies characteristic of the various fungi which, combined with algæ, form the different lichens (various members of the Ascomycetes and the Basidiomycetes). From these are produced great numbers of ascospores or basidiospores, as the case may be. Very many lichens also reproduce themselves by means of Soredia, i.e., little masses of algal cells closely wrapped in a weft of fungal hyphæ. Some contrasts presented by the higher Algæ may also be named as exemplifying the inverse proportion between the size of the individual and the extent of the generative structures. While in the smaller kinds relatively large portions of the fronds are transformed into reproductive elements, in the larger kinds these portions are relatively small: instance the Macrocystis pyrifera, a gigantic seaweed which sometimes attains a length of 1,500 feet, of which Dr. Carpenter remarks—“This development of the nutritive surface takes place at the expense of the fructifying apparatus, which is here quite subordinate.”

When we turn to vegetal aggregates of the third order of composition, facts having the same meaning are conspicuous. On the average these higher plants are far larger than plants of a lower degree of composition; and on the average their rates of sexual reproduction are far less. Similarly if, among Archegoniates and Phænogams, we compare the smaller types with the larger, we find them proportionately more prolific. This is not manifest if we simply calculate the number of seeds ripened by an individual in a single season; but it becomes manifest if we take into account the further factor which here complicates the result—the age at which sexual genesis commences. The smaller Phænogams are mostly either annuals, or perennials that die down annually; and seeding as they do annually before their deaths, or the deaths of their reproductive parts, it results that in the course of a year each gives origin to a multitude of potential plants, of which every one may the next year, if preserved, give origin to an equal multitude. Supposing but a hundred offspring to be produced the first year, ten thousand may be produced in the second year, a million in the third, a hundred millions in the fourth. Meanwhile, what has been the possible multiplication of a large Phænogam? While its small congener has been seeding and dying, and leaving multitudinous progeny to seed and die, it has simply been growing; and may so continue to grow for ten or a dozen years without bearing fruit. Before a Cocoa-nut tree has ripened its first cluster of nuts, the descendants of a wheat plant, supposing them all to survive and multiply, will have become numerous enough to occupy the whole surface of the Earth. So that though, when it begins to bear, a tree may annually shed as many seeds as an herb, yet in consequence of this delay in bearing, its fertility is incomparably less; and its relatively-small fertility becomes still further reduced where, as in Lodoicea callipyge, the seeds take two years from the date of fertilization to the date of germination.

§ 340. Some observers state that in certain Protozoa there occurs a process of conjugation akin to that which the Protophyta exhibit—a coalescence of the substance of two individuals to form a germ-mass. This has been alleged more especially of Actinophrys. If this statement should be proved true,[58] then of the minute forms that appear to be more animal than vegetal in their characters, some have a mode of sexual multiplication by which the parents are sacrificed bodily in the production of a new generation.

Among small animal aggregates of the second order, the first to be considered are of course the Cœlenterata. A Hydra occasionally devotes a large part of its substance to sexual genesis. In the walls of its body groups of ova, or spermatozoa, or both, take their rise; and develop into masses greatly distorting the creature’s form, and leaving it much diminished when they escape. Here, however, gamogenesis is obviously supplementary to agamogenesis—the immensely rapid multiplication by budding continues as long as food is abundant and warmth sufficient, and is replaced by gamogenesis only at the close of the season. A better example of the relation between small size and active gamogenesis among low types of the Metazoa is supplied by the Rotifera. Microscopic as these are, they have a great rate of sexual increase. According to Ehrenberg, Hydatina senta “is capable of a four-fold propagation every twenty-four or thirty hours, bringing forth in this time four ova, which grow from the embryo to maturity, and exclude their fertile ova in the same period. The same individual, producing in ten days forty eggs, developed with the rapidity above cited, this rate, raised to the tenth power, gives one million of individuals from one parent, on the eleventh day four millions, and on the twelfth day sixteen millions, and so on.” Ehrenberg, however, characterized by Huxley as “the greatest looker and the worst observer,” is not a safe authority, and it is better to state the estimate of Ludwig Plate, who says that Hydatina lays fifty eggs in two to three weeks—a number which, multiplying in the manner described, will yield in the time named a much smaller total though still an enormous total.

The Annulosa, including among them the inferior types, have habits and conditions of life so various that only the broadest contrasts can be instanced in support of the proposition before us. The differences of organization and activity greatly complicate the inverse variation of fertility and bulk. Bearing in mind, however, that the rate of multiplication depends much less on the number of each brood than on the quickness with which maturity is reached and a new generation commenced, it will be obvious that though Annelids, relatively enormous in size, produce great numbers of ova, yet as they do this at comparatively long intervals, their rates of increase fall immensely below that just instanced in the Rotifers. And when at the other extreme we come to the large articulate animals, such as the Crab and the Lobster, the further diminution of fertility is seen in the still longer delay which occurs before each new generation begins to reproduce.

Perhaps the best examples are supplied by vertebrate animals, and especially those that are most familiar to us. Comparisons between Fishes are unsatisfactory, because of our ignorance of their histories. In some cases Fishes equal in bulk produce widely different numbers of eggs; as the Cod which spawns millions at once, and the Salmon by which nothing like so great a number is spawned. But then the eggs are very unlike in size; and if the ovaria of the two fishes be compared, the difference between their masses is comparatively moderate. There are, indeed, contrasts which seem at variance with the alleged relation; as that between the Cod and the Stickleback which, though so much smaller, produces fewer ova. The Stickleback’s ova, however, are relatively large; and their total bulk bears as great a ratio to the bulk of the Stickleback as does the bulk of the Cod’s ova to that of the Cod. Moreover if, as is not improbable, the reproductive age is arrived at earlier by the Stickleback than by the Cod, the fertility of the species may be greater notwithstanding the smaller number produced by each individual. Evidence which admits of being tolerably well disentangled is furnished by Birds. They differ but little in their grades of organization; and the habits of life throughout extensive groups of them are so similar, that comparisons may be fairly made. It is true that, as hereafter to be shown, the differences of expenditure which differences of bulk entail, have doubtless much to do with the differences of fertility. But we may set down under the present head some of those cases in which the activity, being relatively slight, does not greatly interfere with the relation we are considering; and may note that among such birds having similarly slight activities, the small produce more eggs than the large, and eggs that bear in their total mass a greater ratio to the mass of the parent. Consider, for example, the gallinaceous birds; which are like one another and unlike birds of most other groups in flying comparatively little. Taking first the wild members of this order, which rarely breed more than once in a season, we find that the Pheasant has from 10 to 14 eggs, the Black-cock from 6 to 10, the Grouse 8 to 14, the Partridge 12 to 20, the Quail still more, sometimes reaching two broods of 7 to 12 in each. Here the only exception to the relation between decreasing bulk and increasing number of eggs, occurs in the cases of the Pheasant and the Black-cock; and it is to be remembered, in explanation, that the Pheasant is constitutionally adapted to a warmer region, is better fed—often artificially—and leads a less active life. If we pass to domesticated genera of the same order, we meet with parallel differences. From the numbers of eggs laid, little can be inferred; for under the favourable conditions artificially maintained, the laying is carried on indefinitely. But though in the sizes of their broods the Turkey and the Fowl do not greatly differ, the Fowl begins breeding at a much earlier age than the Turkey, and produces broods more frequently: a considerably higher rate of multiplication being the result. Now these contrasts among domestic creatures which are similarly conditioned, and closely-allied by constitution, may be held to show, more clearly than most other contrasts, the inverse variation between bulk and sexual genesis; since here the cost of activity is diminished to a comparatively small amount. There is little expenditure in flight—sometimes almost none; and the expenditure in walking about is not great: there is more of standing than of actual movement. It is true that young Turkeys commence their existence as larger masses than chickens; but it is tolerably manifest that the total weight of the eggs laid by a Turkey during each season, bears a less ratio to the Turkey’s weight, than the total weight of the eggs which a Hen lays during each season, bears to the Hen’s weight; and this is the fairest way of making the comparison. The comparison so made shows a greater difference than appears likely to be due to the different costs of locomotion; considering the inertness of the creatures. Remembering that the assimilating surface increases only as the squares of the dimensions, while the mass of the fabric to be built up by the absorbed nutriment increases as the cubes of the dimensions, it will be seen that the expense of growth becomes relatively greater with each increment of size; and that hence, of two similar creatures commencing life with different sizes, the larger one in reaching its superior adult bulk, will do this at a more than proportionate expense; and so will either be delayed in commencing its reproduction, or will have a diminished reserve for reproduction, or both. Other orders of Birds, active in their habits, show more markedly the connexion between augmenting mass and declining fertility. But in them the increasing cost of locomotion becomes an important, and probably the most important, factor. The evidence they furnish will therefore come better under another head. Contrasts among Mammals, like those which Birds present, have their meanings obscured by inequalities of the expenditures for motion. The smaller fertility which habitually accompanies greater bulk, must in all cases be partly ascribed to this. Still, it may be well if we briefly note, for as much as they are worth, the broader contrasts. While a large Mammal bears but a single young one at a time, is several years before it commences doing this, and then repeats the reproduction at long intervals; we find, as we descend to the smaller members of the class, a very early commencement of breeding, an increasing number at a birth, reaching in small Rodents to 10 or even more, and a much more frequent recurrence of broods: the combined result being a relatively prodigious fertility. If a specific comparison be desired between Mammals that are similar in constitution, in food, in conditions of life, and all other things but size, the Deer-tribe supplies it. While the large Red-deer has but one at a birth, the small Roe-deer has frequently two at a birth.[59]

§ 341. The antagonism between growth and sexual genesis, visible in these general contrasts, may also be traced in the history of each plant and animal. So familiar is the fact that sexual genesis does not occur early in life, and in all organisms which expend much begins only when the limit of size is nearly reached, that we do not sufficiently note its significance. It is a general physiological truth, however, that while the building-up of the individual is going on rapidly, the reproductive organs remain imperfectly developed and inactive; and that the commencement of reproduction at once indicates a declining rate of growth, and becomes a cause of arresting growth. As was shown in § 78, the exceptions to this rule are found where the limit of growth is indefinite; either because the organism expends little or nothing in action, or expends in action so moderate an amount that the supply of nutriment is never equilibrated by its expenditure.

We will pass over the inferior plants and, limiting ourselves to Phænogams, will not dwell on the less conspicuous evidence with the smaller types present. A few cases such as gardens supply will serve. All know that a Pear-tree increases in size for years before it begins to bear; and that, producing but few pears at first, it is long before it fruits abundantly. A young Mulberry-tree, branching out luxuriantly season after season, but covered with nothing but leaves, at length blossoms sparingly and sets some small and imperfect berries, which it drops while they are green; and it makes these futile attempts time after time before it succeeds in ripening any seeds. But these multiaxial plants, or aggregates of individuals some of which continue to grow while others become arrested and transformed into seed-bearers, show us the relation less definitely than certain plants that are substantially, if not literally, uniaxial. Of these the Cocoa-nut may be instanced. For some years it goes on shooting up without making any sign of becoming fertile. About the sixth year it flowers; but the flowers wither without result. In the seventh year it flowers and produces a few nuts; but these prove abortive and drop. In the eighth year it ripens a moderate number of nuts; and afterwards increases the number until, in the tenth year, it comes into full bearing. Meanwhile, from the time of its first flowering its growth begins to diminish, and goes on diminishing till the tenth year, when it ceases. Here we see the antagonism between growth and sexual genesis under both its aspects—see a struggle between self-evolution and race-evolution, in which the first for a time overcomes the last, and the last ultimately overcomes the first. The continued aggrandizement of the parent-individual makes abortive for two seasons the tendency to produce new individuals; and the tendency to produce new individuals, becoming more decided, stops any further aggrandizement of the parent individual.

Parallel illustrations occur in the animal kingdom. The eggs laid by a pullet are relatively small and few. Similarly, it is alleged that, as a general rule, “a bitch has fewer puppies at first, than afterwards.” According to Burdach, as quoted by Dr. Duncan, “the elk, the bear, &c., have at first only a single young one, then they come to have most frequently two, and at last again only one. The young hamster produces only from three to six young ones, while that of a more advanced age produces from eight to sixteen. The same is true of the pig.” It is remarked by Buffon that when a sow of less than a year old has young, the number of the litter is small, and its members are feeble and even imperfect. Here we have evidence that in animals growth checks sexual genesis. And then, on the other hand, we have evidence that sexual genesis checks growth. It is well known to breeders that if a filly is allowed to bear a foal, she is thereby prevented from reaching her proper size. And a like loss of perfection as an individual, is suffered by a cow which breeds too early. It may be added, as a converse fact, that castrated animals, as capons and notably cats, often become larger than their unmutilated associates.

§ 342. Notwithstanding the way in which the inverse variation of growth and sexual genesis is complicated with other relations, its existence is, I think, sufficiently manifest. Individually, many of the foregoing instances are open to criticism, and have to be taken with qualifications; but when looked at in the mass their meaning is beyond doubt. Comparisons between the largest with the smallest types, whether vegetal or animal, yield results which are unmistakable. On the one hand, remembering the fact that during its centuries of life an Oak does not produce as many acorns as a Fungus does spores in a single night, we see that the Fungus has a fertility exceeding that of the Oak in a degree literally beyond our powers of calculation or imagination. On the other hand when, taking a microscopic protophyte which has billions of descendants in a few days, we ask how many such would be required to build up the forest tree which is years before it drops a seed, we are met by a parallel difficulty in conceiving the number, if not in setting it down. Similarly, if from the minute and prodigiously-fertile Rotifer we turn to the Elephant, which approaches thirty years before it bears a solitary young one, we find the connexions between small size and great fertility and between great size and small fertility, too intensely marked to be much disguised by the perturbing relations that have been indicated. Finally, as this induction, reached by a survey of organisms in general, is verified by observations on the relation between decreasing growth and commencing reproduction in individual organisms, we may, I think, consider the alleged antagonism as proved.[60]