In setting down some examples we may conveniently adopt the same arrangement as before. We will look at the facts as they are presented by vegetal aggregates of the first order, of the second order, and of the third order; and then as they are presented by animal aggregates of the same three orders.
§ 335. The ordinary unicellular plants are at once microscopic and enormously prolific. The often cited Sphærella nivalis, which shows its immense powers of multiplication by reddening wide tracts of snow in a single night, does this by developing in its cavity a brood of young cells, which, being presently set free by the bursting of the parent-cell, severally grow and quickly repeat the process. The like occurs among sundry of those kindred forms of minute Algæ which, by their enormous numbers, sometimes suddenly change pools to an opaque green. So, too, the Desmidiaceæ often multiply so greatly as to colour the water; and among the Diatomaceæ the rate of genesis by self-division, “is something really extraordinary. So soon as a frustule is divided into two, each of the latter at once proceeds with the act of self-division; so that, to use Professor Smith’s approximative calculation of the possible rapidity of multiplication, supposing the process to occupy, in any single instance, twenty-four hours, ‘we should have, as the progeny of a single frustule, the amazing number of one thousand millions in a single month.’” In these cases the multiplication is so carried on that the parent is lost in the offspring—the old individuality disappears either in the swarms of zoospores it dissolves into, or in the two or four new individualities simultaneously produced by fission. Vegetal aggregates of the first order, have, however, a form of agamogenesis in which the parent individuality is not lost: the young cells arise from the old cells by external gemmation. This process, too, repeated as it is at short intervals, results in immense fertility. The Yeast-fungus, which in a few hours thus propagates itself throughout a large vat of wort, offers a familiar example.
In certain compound forms that must be classed as plants of the second order of aggregation, though very minute ones, self-division similarly increases the numbers at high rates. The Sarcina ventriculi, a parasitic plant which infests the stomach and swarms afresh as fast as previous swarms are vomited, shows us a spontaneous fission of clusters of cells. An allied mode of increase occurs in Gonium pectorale: each cell of the cluster resolving itself into a secondary cluster, and the secondary clusters then separating. “Supposing, which is very probable, that a young Gonium after twenty-four hours is capable of development by fission, it follows that under favourable conditions a single colony may on the second day develop 16, on the third 256, on the fourth 4,096, and at the end of a week 268,435,456 other organisms like itself.” In the Volvocineæ this continual dissolution of a primary compound individual into secondary compound individuals, is carried on endogenously, and on a modified system: some only of the component cells giving origin to young colonies, and the parent bursting to liberate them. The numbers arising by this method also, are sometimes so great as to tint large bodies of water. More fully established and organized aggregates of the second order, such as the higher Thallophytes and the lower Archegoniates, do not sacrifice their individualities by fission; but nevertheless, by the kindred process of gemmation, are continually hindered in the increase of their individualities. The gemmæ called tetraspores are cast off in great numbers by the marine Algæ. Among those simple Jungermanniaceæ which consist of single fronds, the young ones that bud out grow for a time in connexion with their parents, send rootlets from their under sides into the soil, and presently separate themselves—a habit which augments the number of individuals in proportion as it checks their growths.
Plants of the third order of composition, arising by arrest of this separation, exhibit a further corresponding decrease in the abundance of the aggregates formed. Archegoniates of inferior types, in which the axes produced by integration of fronds are but small and feeble, are characterized by the habit of throwing off bulbils—bud-shaped axes which, falling and taking root, add to the number of distinct individuals. This agamic multiplication, very general among the Mosses and their kindred, and not uncommon under a modified form in such higher types as the Ferns, many of which produce young ones from the surfaces of their fronds, becomes very unusual among Phænogams. The detachment of bulbils, though not unknown among them, is exceptional. And while it is true that some flowering plants, as the Strawberry, multiply by a process allied to gemmation, yet this is not characteristic of the class. A leading trait of these highest groups, to which the largest members of the vegetal kingdom belong, is that agamogenesis has so far ceased that it does not usually originate independent plants. Though the axes which, budding one out of another, compose a tree, are the equivalents of asexually-produced individuals; yet the asexual production of them stops short of separation. These vast integrations arise where spontaneous disintegration, and the multiplication effected by it, have come to an end.
Thus, not forgetting that certain Phænogams, as Begonia phyllomaniaca, revert to quite primitive modes of increase, we may hold it as beyond question that while among the most minute plants asexual multiplication is universal, and produces enormous numbers in short periods, it becomes step by step more restricted in range and frequency as we advance to large and compound plants; and disappears so generally from the highest and largest, that its occurrence is regarded as anomalous.
§ 336. Parallel examples furnished by animals make clear the purely quantitative nature of this relation under its original form. Among the Protozoa, as among the Protophyta, there occurs that process by which the individuality of the parent is wholly lost in producing offspring—the breaking up of the parental mass into a number of germs. Some of the Infusoria, as for instance those of the genus Kolpoda and several allied genera, become encysted and subsequently break up into young ones. The more familiar mode of increase among these animal-aggregates of the first order, by fission, though it sacrifices the parent individuality by merging it in the individualities of the two produced, sacrifices it less completely than does the dissolution into a great number of germs. Occurring, however, as this fission does, very frequently, and being completed, in some cases that have been observed, in the course of half-an-hour, it results in immensely-rapid multiplication. If all its offspring survive, and continue dividing themselves, a single Paramœcium is said to be capable of thus originating 268 millions in the course of a month.[55] Nor is this the greatest known rate of increase. Another animalcule, visible only under a high magnifying power, “is calculated to generate 170 billions in four days.”[56] And these enormous powers of propagation are accompanied by a minuteness so extreme, that of some species one drop of water would contain as many individuals as there are human beings on the Earth! Even if we allow a large margin for exaggeration in these estimates, it is beyond question that among these smallest of animals the rate of asexual multiplication is immensely the greatest; and this suffices for the purposes of argument.
Of animal aggregates belonging to the second order, that multiply asexually with rapidity, the familiar Polypes furnish conspicuous examples. By gemmation in most cases, in other cases by fission, and in some cases by both, the agamogenesis is carried on among these tribes. As shown in Fig. [148], the budding of young ones from the parent Hydra is carried on so actively, that before the oldest of them is cast off half-a-dozen or more others have reached various stages of growth; and even while still attached, the first-formed of the group have commenced budding out from their sides a second generation of young ones. In the Hydra tuba this gemmiparous multiplication is from time to time interrupted by a transverse splitting-up of the body into segments, which successively separate and swim away: the result of the two processes being that, in the course of a season, there are produced from a single germ great numbers of young Medusæ, which are the adult or sexual forms of the species. Respecting cœlenterate animals of this degree of composition, it may be added that when we ascend to the larger kinds we find asexual genesis far less active. Though comparisons are interfered with by differences of structure and mode of life, yet the contrasts are too striking to have their meanings much obscured. If, for instance, we take a solitary Actinozoon and a solitary Hydrozoon, we see that the relatively-great bulk of the first, goes along with a relatively-slow agamogenesis. The common Sea-anemones are but occasionally observed to undergo self-division: multiplication by budding being in some cases largely followed, but their numbers are not rapidly increased by either process. A higher class of secondary aggregates exemplifies the same general truth with a difference. In the smaller members the agamogenesis is incomplete, and in the larger it disappears. The gemmation of the minute Polyzoa, though it does not end in the separation of the young individuals, habitually goes to the extent of producing families of partially independent individuals; but their near ally, the Phoronis, which immensely exceeds them in size, is solitary and not gemmiparous. So, too, is it with the Ascidioida. And then among the true Mollusca, which are relatively large, no such thing is known as fission or gemmation.
Take next the Annulosa, including under this title the Annelida and Arthropoda. When treating of morphological composition, reasons were given for the belief that the annulose animal is an aggregate of the third order, the segments of which, produced one from another by gemmation, originally became separate; but by progressive integration, or arrested disintegration, there resulted a type in which many such segments were permanently united ([§§ 205–7] and note to [§ 207]). Part of the evidence there assigned, is evidence to be here repeated in illustration of the direct antagonism of Growth and Asexual Genesis. We saw how, among the lower Annelids, the string of segments produced by gemmation presently divides transversely into two strings; and how, in some cases, this resolution of the elongating string of segments into groups that are to form separate individuals, goes on so actively that as many as six groups are found in different stages of progress to ultimate independence—a fact implying a high rate of fissiparous multiplication.[57] Then we saw that, in the superior annulose types, distinguished in the mass by including the larger species, fission does not occur. The higher Annelids do not propagate in this way; there is no known case of new individuals being so formed among the Myriapoda; nor do the Crustaceans afford us a single instance of this primordial mode of increase. It is, indeed, true that while articulate animals never multiply asexually after this simplest method, and while they are characterized in the mass by the cessation of agamogenesis of every kind, there nevertheless occur in a few of their small species, those higher forms of agamogenesis known as parthenogenesis and pseudo-parthenogenesis; and that by these some of them multiply very rapidly. Hereafter we shall find, in the interpretation of these anomalies, further support for the general doctrine.
To the above evidence has to be added that which the Vertebrata present. This may be very briefly summed up. On the one hand this class, whether looked at in the aggregate or in its particular species, immensely exceeds all other classes in the sizes of its individuals; and on the other hand, agamogenesis under any form is absolutely unknown in it. If it be said that budding occurs among the Tunicata which, under the common title of Chordata, are included in the same phylum with the Vertebrata, then it may firstly be replied that those types which have no vertebræ cannot properly be called Vertebrata, and secondly that if, as being Chordata, they must be recognized, then the exception which they present further illustrates the truth that agamogenetic multiplication occurs only in creatures small in size, or low in structure, or both.
§ 337. Such are a few leading facts serving to show how deduction is inductively verified, in so far as the antagonism between Growth and Asexual Genesis is concerned. In whatever way we explain this opposition of the integrative and disintegrative processes, the facts and their implications remain the same. Indeed we need not commit ourselves to any hypothesis respecting the physical causation. It suffices to recognize the results under their most general aspects. We cannot help admitting there are at work these two antagonist tendencies to aggregation and separation; and we cannot help admitting that the proportion between the aggregative and separative tendencies, must in each case determine the relation between increase in bulk of the individual and increase of the race in number.