§ 6. Whether or not we regard reproduction as resulting from a repulsive force (and, as seen above, both Owen and Carpenter lean to some such view), and whether or not we consider the formation of the individual as due to the reverse of this—an attractive force—we cannot, on studying the phenomena, help admitting that two opposite activities thus generalized are at work; we cannot help admitting that the aggregative and separative tendencies do in each case determine the respective developments of the individual and the race. On ascending one degree in the scale of organic life, we shall find this truth clearly exemplified.
For if these single-celled organisms which multiply so rapidly be supposed to lose some of their separative tendency, what must be the result? They now not only divide frequently, but the divided portions fly apart. How, then, will a diminution of this separative tendency first show itself? May we not expect that it will show itself in the divided portions not flying apart, but remaining near each other, and forming a group? This we find in nature to be the first step in advance. The lowest compound organisms are "simple aggregations of vesicles without any definite arrangement, sometimes united, but capable of existing separately."[[73]] In these cases, "every component cell of the aggregate mass that springs from a single germ, being capable of existing independently of the rest, may be regarded as a distinct individual."[[74]] The several stages of this aggregation are very clearly seen in both the animal and vegetable kingdoms. In the Hæmatococcus binalis, the plant producing the reddish slime seen on damp surfaces, not only does each of the cells retain its original sphericity, but each is separated from its neighbour by a wide interval filled with mucus; so that it is only as being diffused through a mass of mucus common to them all, that these cells can be held to constitute one individual. We find, too, that "the component cells, even in the highest Algæ, are generally separated from each other by a large quantity of mucilaginous intercellular substance."[[75]] And, again, the tissue of the simpler Lichens, "in consequence of the very slight adhesion of its component cells, is said to be pulverulent."[[76]] Similarly the Protozoa, by their feeble union, constitute the organisms next above them. Amongst the Polygastrica there are many cases "in which the individuals produced by fission or gemmation do not become completely detached from each other."[[77]] The Ophrydium, for instance, "exists under the form of a motionless jelly-like mass ... made up of millions of distinct and similar individuals imbedded in a gelatinous connecting substance;"[[78]] and again, the Uvella, or "grape monad," consists of a cluster "which strongly resembles a transparent mulberry rolling itself across the field of view by the ciliary action of its component individuals."[[79]] The parenchyma of the Sponge, too, is made up of cells "each of which has the character of a distinct animalcule, having a certain power of spontaneous motion, obtaining and assimilating its own food, and altogether living by and for itself;" and so small is the cohesion of these individual cells, that the tissue they constitute "drains away when the mass is removed from the water, like white of egg."[[80]]
Of course in proportion as the aggregate tendency leading to the formation of these groups of monads is strong, we may expect that, other things equal, the groups will be large. Proceeding upwards from the yeast fungus, whose cells hold together in groups of four, five, and six,[[81]] there must be found in each species of these composite organisms a size of group determined by the strength of the aggregative tendency in that species. Hence we may expect that, when this limit is passed, the group no longer remains united, but divides. Such we find to be the fact. These groups of cells undergo the same process that the cells themselves do. They increase up to a certain point, and then multiply either by simple spontaneous fission or by that modification of it called gemmation. The Volvox globator, which is made up of a number of monads associated together in the form of a hollow sphere, develops within itself a number of smaller spheres similarly constituted; and after these, swimming freely in its interior, have reached a certain size, the parent group of animalcules bursts and sets the interior groups free. And here we may observe how this compound individuality of the Volvox is destroyed in the act of reproduction as the simple individuality of the monad is. Again, in the higher forms of grouped cells, where something like organisation begins to show itself, the aggregations are not only larger, but the separative process, now carried on by the method of gemmation, no longer wholly destroys the individual. And in fact, this gemmation may be regarded as the form which spontaneous fission must assume in ceasing to be fatal; seeing that gemmation essentially consists in the separation, not into halves, but into a larger part and a smaller part; the larger part continuing to represent the original individual. Thus in the common Hydra or fresh-water polype, "little bud-like processes are developed from the external surface, which are soon observed to resemble the parent in character, possessing a digestive sac, mouth, and tentacula; for a long time, however, their cavity is connected with that of the parent; but at last the communication is cut off, and the young polype quits its attachment, and goes in quest of its own maintenance."[[82]]
§ 7. Progress from these forms of organisation to still higher forms is similarly characterized by increase of the aggregative tendency or diminution of the separative, and similarly exhibits the necessary antagonism between the development of the individual and the increase of the race. That process of grouping which constitutes the first step towards the production of complex organisms, we shall now find repeated in the formation of series of groups. Just as a diminution of the separative tendency is shown in the aggregation of divided monads, so is a further diminution of it shown in the aggregation of the divided groups of monads. The first instance that occurs is afforded by the compound polypes. "Some of the simpler forms of the composite Hydroida," says Carpenter, "may be likened to a Hydra, whose gemmæ, instead of becoming detached, remain permanently connected with the parent; and as these in their turn may develop gemmæ from their own bodies, a structure of more or less arborescent character may be produced."[[83]] A similar species of combination is observable amongst the Bryozoa, and the compound Tunicata. Every degree of union may be found amongst these associated organisms; from the one extreme in which the individuals can exist as well apart as together, to the other extreme in which the individuals are lost in the general mass. Whilst each Bryozoon is tolerably independent of its neighbour, "in the compound Hydroida, the lives of the polypes are subordinate to that of the polypdom."[[84]] Of the Salpidæ and Pyrosomidæ, Carpenter says:—"Although closely attached to one another, these associated animals are capable of being separated by a smart shock applied to the sides of the vessel in which they are swimming.... In other species, however, the separate animals are imbedded in a gelatinous mass," and in one kind "there is an absolute union between the vascular systems of the different individuals."[[85]]
In the same manner that with a given aggregative tendency there is a limit to the size of groups, so is there a similarly-determined limit to the size of series of groups; and that spontaneous fission which we have seen in cells and groups of cells we here find repeated. In the lower Annelida, for example, "after the number of segments in the body has been greatly multiplied by gemmation, a separation of those of the posterior portion begins to take place; a constriction forms itself about the beginning of the posterior third of the body, in front of which the alimentary canal undergoes a dilatation, whilst on the segment behind it a proboscis and eyes are developed, so as to form the head of the young animal which is to be budded off; and in due time, by the narrowing of the constriction, a complete separation is effected."[[86]] Not unfrequently in the Nais this process is repeated in the young one before it becomes independent of the parent. The higher Annelida are distinguished by the greater number of segments held in continuity; an obvious result of comparatively infrequent fission. In the class Myriapoda, which stands next above, "there is no known instance of multiplication by fission."[[87]] Yet even here the law may be traced both in the number and structure of the segments. The length of the body is still increased after birth "by gemmation from (or partial fission of) the penultimate segment." The lower members of the class are distinguished from the higher by the greater extent to which this gemmation is carried. Moreover, the growing aggregative tendency is seen in the fact, that each segment of the Julus "is formed by the coalescence of two original segments,"[[88]] whilst in the Scolopendridæ, which are the highest of this class, "the head, according to Mr. Newport, is composed of eight segments, which are often consolidated into one piece;"[[89]] both of which phenomena may be understood as arrests of that process of fission, which, if allowed to go a little further, would have produced distinct segments; and, if allowed to go further still, would have separated these segments into groups.
§ 8. Remarking, first, how gradually this mode of multiplication disappears—how there are some creatures that spontaneously divide or not according to circumstances; others that divide when in danger (the several parts being capable of growing into complete individuals); others which, though not self-dividing, can live on in each half if artificially divided; and others in which only one of the divided halves can live—how, again, in the Crustaceans the power is limited to the reproduction of lost limbs; how there are certain reptiles that can re-supply a lost tail, but only imperfectly; and how amongst the higher Vertebrata the ability to repair small injuries is all that remains—remarking thus much, let us now, by way of preparation for what is to follow, consider the significance of the foregoing facts taken in connection with the definition of Life awhile since given.
This spontaneous fission, which we have seen to be, in all cases, more or less destructive of individual life, is simply a cessation in the co-ordination of actions. From the single cell, the halves of whose nucleus, instead of continuing to act together, begin to repel each other, fly apart, establish distinct centres of assimilation, and finally cause the cell to divide; up to the Annelidan, whose string of segments separates, after reaching a certain length; we everywhere see the phenomenon to be fundamentally this. The tendency to separate is the tendency not to act together, probably arising from inability to act together any longer; and the process of separation is the process of ceasing to act together. How truly non-co-ordination is the essence of the matter will be seen on observing that fission takes place more or less rapidly, according as the co-ordinating apparatus is less or more developed. Thus, "the capability of spontaneous division is one of the most distinctive attributes of the acrite type of structure;"[[90]] the acrite type of structure being that in which the neurine or nervous matter is supposed to be diffused through the tissues in a molecular state, and in which, therefore, there exists no distinct nervous or co-ordinating system. From this point upwards the gradual disappearance of spontaneous fission is clearly related to the gradual appearance of nerves and ganglia—a fact well exemplified by the several grades of Annelida and Myriapoda. And when we remember that in the embryotic development of these classes, the nervous system does not make its appearance until after the rest of the organism has made great progress, we may even suspect that that coalescence of segments characteristic of the Myriapoda, exhibits the co-ordinating power of the rapidly-growing nervous system overtaking and arresting the separative tendency; and doing this most where it (the nervous system) is most developed, namely, in the head.
And here let us remark, in passing, how, from this point of view, we still more clearly discern the antagonism of individuation and reproduction. We before saw that the propagation of the race is at the expense of the individual: in the above facts we may contemplate the obverse of this—may see that the formation of the individual is at the expense of the race. This combination of parts that are tending to separate and become distinct beings—this union of many incipient minor individualities into one large individuality—is an arrest of reproduction—a diminution in the number produced. Either these units may part and lead independent lives, or they may remain together and have their actions co-ordinated. Either they may, by their diffusion, form a small, simple, and prolific race, or, by their aggregation, a large, complex, and infertile one. But manifestly the aggregation involves the infertility; and the fertility involves the smallness.
§ 9. The ability to multiply by spontaneous fission, and the ability to maintain individual life, are opposed in yet another mode. It is not in respect of size only, but still more in respect of structure, that the antagonism exists.
Higher organisms are distinguished from lower ones partly by bulk, and partly by complexity. This complexity essentially consists in the mutual dependence of numerous different organs, each subserving the lives of the rest, and each living by the help of the rest. Instead of being made up of many like parts, performing like functions, as the Crinoid, the Star-fish, or the Millipede, a vertebrate animal is made up of many unlike parts, performing unlike functions. From that initial form of a compound organism, in which a number of minor individuals are simply grouped together, we may, more or less distinctly, trace not only the increasing closeness of their union, and the gradual disappearance of their individualities in that of the mass, but the gradual assumption by them of special duties. And this "physiological division of labour," as it has been termed, has the same effect as the division of labour amongst men. As the preservation of a number of persons is better secured when, uniting into a society, they severally undertake different kinds of work, than when they are separate and each performs for himself every kind of work; so the preservation of a congeries of parts, which, combining into one organism, respectively assume nutrition, respiration, circulation, locomotion, as separate functions, is better secured than when those parts are independent, and each fulfils for itself all these functions.