The antithesis is as manifest à posteriori as it is necessary à priori. While the minutest organisms multiply asexually in their billions; while the Infusoria thus multiply in their millions; while the small compound types next above them thus multiply in their thousands; while larger and more compound types thus multiply in their hundreds and their tens; the largest types do not thus multiply at all. Conversely, those which do not multiply asexually at all, are a billion or a million times the size of those which thus multiply with greatest rapidity; and are a thousand times, or a hundred times, or ten times the size of those which thus multiply with less and less rapidity. Without saying that this inverse proportion is regular, which, as we shall hereafter see, it cannot be, we may unhesitatingly assert its average truth. That the smallest organisms habitually reproduce asexually with immense rapidity; that the largest organisms never reproduce at all in this manner; and that between these extremes there is a general decrease of asexual reproduction along with an increase of bulk; are propositions which admit of no dispute.
CHAPTER VI.
ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS.
§ 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.