A further complication of the analogy is at hand. From the aggregation of units into organized groups, we pass to the multiplication of such groups, and their coalescence into compound groups. The Hydra, when it has reached a certain bulk, puts forth from its surface a bud which, growing and gradually assuming the form of the parent, finally becomes detached; and by this process of gemmation the creature peoples the adjacent water with others like itself. A parallel process is seen in the multiplication of those lowly-organized tribes above described. When one of them has increased to a size that is either too great for co-ordination under so rude a structure, or else that is greater than the surrounding country can supply with game and other wild food, there arises a tendency to divide; and as in such communities there often occur quarrels, jealousies, and other causes of division, there soon comes an occasion on which a part of the tribe separates under the leadership of some subordinate chief and migrates. This process being from time to time repeated, an extensive region is at length occupied by numerous tribes descended from a common ancestry. The analogy by no means ends here. Though in the common Hydra the young ones that bud out from the parent soon become detached and independent; yet throughout the rest of the class Hydrozoa, to which this creature belongs, the like does not generally happen. The successive individuals thus developed continue attached; give origin to other such individuals which also continue attached; and so there results a compound animal. As in the Hydra itself we find an aggregation of units which, considered separately, are akin to the lowest Protozoa; so here, in a Zoophyte, we find an aggregation of such aggregations. The like is also seen throughout the extensive family of Polyzoa or Molluscoida. The Ascidian Mollusks, too, in their many forms, show us the same thing: exhibiting, at the same time, various degrees of union among the component individuals. For while in the Salpæ the component individuals adhere so slightly that a blow on the vessel of water in which they are floating will separate them; in the Botryllidæ there exist vascular connexions among them, and a common circulation. Now in these different stages of aggregation, may we not see paralleled the union of groups of connate tribes into nations? Though, in regions where circumstances permit, the tribes descended from some original tribe migrate in all directions, and become far removed and quite separate; yet, where the territory presents barriers to distant migration, this does not happen: the small kindred communities are held in closer contact, and eventually become more or less united into a nation. The contrast between the tribes of American Indians and the Scottish clans, illustrates this. And a glance at our own early history, or the early histories of continental nations, shows this fusion of small simple communities taking place in various ways and to various extents. As says M. Guizot, in his History of the Origin of Representative Government—
"By degrees, in the midst of the chaos of the rising society, small aggregations are formed which feel the want of alliance and union with each other. … Soon inequality of strength is displayed among neighbouring aggregations. The strong tend to subjugate the weak, and usurp at first the rights of taxation and military service. Thus political authority leaves the aggregations which first instituted it, to take a wider range."
That is to say, the small tribes, clans, or feudal groups, sprung mostly from a common stock, and long held in contact as occupants of adjacent lands, gradually get united in other ways than by kinship and proximity.
A further series of changes begins now to take place, to which, as before, we find analogies in individual organisms. Returning to the Hydrozoa, we observe that in the simplest of the compound forms the connected individuals are alike in structure, and perform like functions; with the exception that here and there a bud, instead of developing into a stomach, mouth, and tentacles, becomes an egg-sac. But with the oceanic Hydrozoa this is by no means the case. In the Calycophoridæ some of the polypes growing from the common germ, become developed and modified into large, long, sack-like bodies, which, by their rhythmical contractions, move through the water, dragging the community of polypes after them. In the Physophoridæ a variety of organs similarly arise by transformation of the budding polypes; so that in creatures like the Physalia, commonly known as the "Portuguese Man-of-war," instead of that tree-like group of similar individuals forming the original type, we have a complex mass of unlike parts fulfilling unlike duties. As an individual Hydra may be regarded as a group of Protozoa which have become partially metamorphosed into different organs; so a Physalia is, morphologically considered, a group of Hydræ of which the individuals have been variously transformed to fit them for various functions.
This differentiation upon differentiation is just what takes place during the evolution of a civilized society. We observed how, in the small communities first formed, there arises a simple political organization: there is a partial separation of classes having different duties. And now we have to observe how, in a nation formed by the fusion of such small communities, the several sections, at first alike in structures and modes of activity, grow unlike in both—gradually become mutually-dependent parts, diverse in their natures and functions.
The doctrine of the progressive division of labour, to which we are here introduced, is familiar to all readers. And further, the analogy between the economical division of labour and the "physiological division of labour," is so striking as long since to have drawn the attention of scientific naturalists: so striking, indeed, that the expression "physiological division of labour," has been suggested by it. It is not needful, therefore, to treat this part of the subject in great detail. We shall content ourselves with noting a few general and significant facts, not manifest on a first inspection.
Throughout the whole animal kingdom, from the Cœlenterata upwards, the first stage of evolution is the same. Equally in the germ of a polype and in the human ovum, the aggregated mass of cells out of which the creature is to arise, gives origin to a peripheral layer of cells, slightly differing from the rest which they include; and this layer subsequently divides into two—the inner, lying in contact with the included yelk, being called the mucous layer, and the outer, exposed to surrounding agencies, being called the serous layer: or, in the terms used by Prof. Huxley, in describing the development of the Hydrozoa—the endoderm and ectoderm. This primary division marks out a fundamental contrast of parts in the future organism. From the mucous layer, or endoderm, is developed the apparatus of nutrition; while from the serous layer, or ectoderm, is developed the apparatus of external action. Out of the one arise the organs by which food is prepared and absorbed, oxygen imbibed, and blood purified; while out of the other arise the nervous, muscular, and osseous systems, by the combined actions of which the movements of the body as a whole are effected. Though this is not a rigorously-correct distinction, seeing that some organs involve both of these primitive membranes, yet high authorities agree in stating it as a broad general distinction. Well, in the evolution of a society, we see a primary differentiation of analogous kind, which similarly underlies the whole future structure. As already pointed out, the only manifest contrast of parts in primitive societies, is that between the governing and the governed. In the least organized tribes, the council of chiefs may be a body of men distinguished simply by greater courage or experience. In more organized tribes, the chief-class is definitely separated from the lower class, and often regarded as different in nature—sometimes as god-descended. And later, we find these two becoming respectively freemen and slaves, or nobles and serfs. A glance at their respective functions, makes it obvious that the great divisions thus early formed, stand to each other in a relation similar to that in which the primary divisions of the embryo stand to each other. For, from its first appearance, the warrior-class, headed by chiefs, is that by which the external acts of the society are carried on: alike in war, in negotiation, and in migration. Afterwards, while this upper class grows distinct from the lower, and at the same time becomes more and more