This integration of aggregates of the second order, is carried on among the Polyzoa in divers ways, and with different degrees of completeness. The little patches of minute cells, shown as magnified in Fig. [153], so common on the fronds of sea-weeds and the surfaces of rocks at low-water mark, display little beyond mechanical combination. The adjacent individuals, though severally originated by gemmation from the same germ, have but little physiological dependence. In kindred kinds, however, as shown in Figs. [154 and 155], one of which is a magnified portion of the other, the integration is somewhat greater: the co-operation of the united individuals being shown in the production of those tubular branches which form their common support, and establish among them a more decided community of nutrition.
Figs. 156–159.
Among the Ascidians this general law of morphological composition is once more displayed. Each of these creatures subsists on the nutritive particles contained in the water which it draws in through one orifice and sends out through another; and it may thus subsist either alone, or in connexion with others that are in some cases loosely aggregated and in other cases closely aggregated. Fig. [156], Phallusia mentula, is one of the solitary forms. A type in which the individuals are united by a stolon that gives origin to them by successive buds, is shown in Perophora, Fig. [157]. Among the Botryllidæ, of which one kind is drawn on a small scale in Fig. [159], and a portion of the same on a larger scale in Fig. [158], there is a combination of the individuals into annular clusters, which are themselves imbedded in a common gelatinous matrix. And in this group there are integrations even a stage higher, in which several such clusters of clusters grow from a single base. Here the compounding and recompounding appears to be carried further than anywhere else in the animal kingdom.
Thus far, however, among these aggregates of the third order, we see what we before saw among the simpler aggregates of the second order—we see that the component individualities are but to a very small extent subordinated to the individuality made up of them. In nearly all the forms indicated, the mutual dependence of the united animals is so slight, that they are more fitly comparable to societies, of which the members co-operate in securing certain common benefits. There is scarcely any specialization of functions among them. Only in the last type described do we see a number of individuals so completely combined as to simulate a single individual. And even here, though there appears to be an intimate community of nutrition, there is no physiological integration beyond that implied in several mouths and stomachs having a common vent.[20]
§ 204. We come now to an extremely interesting question. Does there exist in other sub-kingdoms composition of the third degree, analogous to that which we have found so prevalent among the Cœlenterata and the Polyzoa and Tunicata? The question is not whether elsewhere there are tertiary aggregates produced by the branching or clustering of secondary aggregates, in ways like those above traced; but whether elsewhere there are aggregates which, though otherwise unlike in the arrangement of their parts, nevertheless consist of parts so similar to one another that we may suspect them to be united secondary aggregates. The various compound types above described, in which the united animals maintain their individualities so distinctly that the individuality of the aggregate remains vague, are constructed in such ways that the united animals carry on their several activities with scarcely any mutual hindrance. The members of a branched Hydrozoon, such as is shown in Fig. [149] or Fig. [150], are so placed that they can all spread their tentacles and catch their prey as well as though separately attached to stones or weeds. Packed side by side on a flat surface or forming a tree-like assemblage, the associated individuals among the Polyzoa are not unequally conditioned: or if one has some advantage over another in a particular case, the mode of growth and the relations to surrounding objects are so irregular as to prevent this advantage re-appearing with constancy in successive generations. Similarly with the Ascidians growing from a stolon or those forming an annular cluster: each of them is as well placed as every other for drawing in the currents of sea-water from which it selects its food. In these cases the mode of aggregation does not expose the united individuals to multiform circumstances; and therefore is not calculated to produce among them any structural multiformity. For the same reason no marked physiological division of labour arises among them; and consequently no combination close enough to disguise their several individualities. But under converse conditions we may expect converse results. If there is a mode of integration which necessarily subjects the united individuals to unlike sets of incident forces, and does this with complete uniformity from generation to generation, it is to be inferred that the united individuals will become unlike. They will severally assume such different functions as their different positions enable them respectively to carry on with the greatest advantage to the assemblage. This heterogeneity of function arising among them, will be followed by heterogeneity of structure; as also by that closer combination which the better enables them to utilize one another’s functions. And hence, while the originally-like individuals are rendered unlike, they will have their homologies further obscured by their progressing fusion into an aggregate individual of a higher order.
These converse conditions are in nearly all cases fulfilled where the successive individuals arising by continuous development are so budded-off as to form a linear series. I say in nearly all cases, because there are some types in which the associated individuals, though joined in single file, are not thereby rendered very unlike in their relations to the environment; and therefore do not become differentiated and integrated to any considerable extent. I refer to such Ascidians as the Salpidæ. These creatures float passively in the sea, attached together in strings. Being placed side by side and having mouths and vents that open laterally, each of them is as well circumstanced as its neighbours for absorbing and emitting the surrounding water; nor have the individuals at the two extremities any marked advantages over the rest in these respects. Hence in this type, and in the allied type Pyrosoma, which has its component individuals built into a hollow cylinder, linear aggregation may exist without the minor individualities becoming obscured and the major individuality marked: the conditions under which a differentiation and integration of the component individuals may be expected, are not fulfilled. But where the chain of individuals produced by gemmation, is either habitually fixed to some solid body by one of its extremities or moves actively through the water or over submerged stones and weeds, the several members of the chain become differently conditioned in the way above described; and may therefore be expected to become unlike while they become united. A clear idea of the contrast between these two linear arrangements and their two diverse results, will be obtained by considering what happens to a row of soldiers, when changed from the ordinary position of a single rank to the position of Indian file. So long as the men stand shoulder to shoulder, they are severally able to use their weapons in like ways with like efficiency; and could, if called on, similarly perform various manual processes directly or indirectly conducive to their welfare. But when, on the word of command “right face,” they so place themselves that each has one of his neighbours before him and another behind him, nearly all of them become incapacitated for fighting and for many other actions. They can walk or run one after another, so as to produce movement of the file in the direction of its length; but if the file has to oppose an enemy or remove an obstacle lying in the line of its march, the front man is the only one able to use his weapons or hands to much purpose. And manifestly such an arrangement could become advantageous only if the front man possessed powers peculiarly adapted to his position, while those behind him facilitated his actions by carrying supplies, &c. This simile, grotesque as it seems, serves to convey better perhaps than any other could do, a clear idea of the relations that must arise in a chain of individuals arising by gemmation, and continuing permanently united end to end. Such a chain can arise only on condition that combination is more advantageous than separation; and for it to be more advantageous, the anterior members of the series must become adapted to functions facilitated by their positions, while the posterior members become adapted to functions which their positions permit. Hence, direct or indirect equilibration or both, must tend continually to establish types in which the connected individuals are more and more unlike one another, at the same time that their several individualities are more and more disguised by the integration consequent on their mutual dependence.
Such being the anticipations warranted by the general laws of evolution, we have now to inquire whether there are any animals which fulfil them. Very little search suffices; for structures of the kind to be expected are abundant. In that great division of the animal kingdom at one time called Annulosa, but now grouped into Annelida and Arthropoda, we find a variety of types having the looked-for characters. Let us contemplate some of them.
§ 205. An adult Chætopod is composed of segments which repeat one another in their details as well as in their general shapes. Dissecting one of the lower orders, such as is shown in Fig. [160], proves that the successive segments, besides having like locomotive appendages, like branchiæ, and sometimes even like pairs of eyes, also have like internal organs. Each has its enlargement of the alimentary canal; each its contractile dilatation of the great blood-vessel; each its portion of the double nervous cord, with ganglia when these exist; each its branches from the nervous and vascular trunks answering to those of its neighbours; each its similarly answering set of muscles; each its pair of openings through the body-wall; and so on throughout, even to the organs of reproduction. That is to say, every segment is in great measure a physiological whole—every segment contains most of the organs essential to individual life and multiplication: such essential organs as it does not contain, being those which its position as one in the midst of a chain, prevents it from having or needing. If we ask what is the meaning of these homologies, no adequate answer is supplied by any current hypothesis. That this “vegetative repetition” is carried out to fulfil a predetermined plan, was shown to be quite an untenable notion (§§ [133], [134]). On the one hand, we found nothing satisfactory in the conception of a Creator who prescribed to himself a certain unit of composition for all creatures of a particular class, and then displayed his ingenuity in building up a great variety of forms without departing from the “archetypal idea.” On the other hand, examination made it manifest that even were such a conception worthy of being entertained, it would have to be relinquished; since in each class there are numerous deviations from the supposed “archetypal idea.” Still less can these traits of structure be accounted for teleologically. That certain organs of nutrition and respiration and locomotion are repeated in each segment of a dorsibranchiate annelid, may be regarded as functionally advantageous for a creature following its mode of life. But why should there be a hundred or even two hundred pairs of ovaries? This is an arrangement at variance with that physiological division of labour which every organism profits by—is a less advantageous arrangement than might have been adopted. That is to say, the hypothesis of a designed adaptation fails to explain the facts. Contrariwise, these structural traits are just such as might naturally be looked for, if these annulose forms have arisen by the integration of simpler forms. Among the various compound animals already glanced at, it is very general for the united individuals to repeat one another in all their parts—reproductive organs included. Hence if, instead of a clustered or branched integration, such as the Cœlenterata, Polyzoa and Tunicata exhibit, there occurs a longitudinal integration; we may expect that the united individuals will habitually indicate their original independence by severally bearing germ-producing or sperm-producing organs.
