A familiar fact supports this conclusion. Everyone has at hand, not figuratively but literally, an illustration. Let him compare the veins on the backs of his two hands, either with one another or with the veins on another person's hands, and he will see that the branchings and inosculations do not correspond: there is no fixed pattern. But on progressing inwards from the extremities, the distribution of the veins becomes settled—there is a pattern-arrangement common to all persons. These facts imply a predominating control by adjacent parts where control by the aggregate is less easy. A constant combination of forces which, towards the centre, produces a typical structure, fails to do this at the periphery where, during development, the play of forces is less settled. This peripheral vascular structure, not having become fixed because one arrangement is as good as another, is in each determined by the immediately surrounding influences.
§ 97e. And now let us contemplate the verifications which recent experiments have furnished—experiments made by Prof. G. Born of Breslau, confirming results earlier reached by Vulpian and adding more striking results of kindred nature. They leave no longer doubtful the large share taken by local organizing power as distinguished from central organizing power.
The independent vitality shown by separated portions of ventral skin from frog-larvæ may be named as the first illustration. With their attached yolk-cells these lived for days, and underwent such transformations as proved some structural proclivity, though of course the product was amorphous. Detached portions of tails of larvæ went on developing their component parts in much the same ways as they would have done if remaining attached. More striking still was the evidence furnished by experiments in grafting. These proved that the undifferentiated rudiment of an organ will, when cut off and joined to a non-homologous place in another individual, develop itself as it would have done if left in its original place. In brief, then, we may say that each part is in chief measure autogenous.
These strange facts presented by small aggregates of organic matter, which are the seats of extremely complex forces, will seem less incomprehensible if we observe what has taken place in a vast aggregate of inorganic matter which is the seat of very simple forces—the Solar System. Transcendently different as this is in all other respects, it is analogous in the respect that, as factors of local structures, local influences predominate over the influences of the aggregate. For while the members of the Solar System, considered as a whole, are subordinate to the totality of its forces, the arrangements in each part of it are produced almost wholly by the play of forces in that part. Though the Sun affects the motions of the Moon, and though during the evolution of the Earth-and-Moon system the Sun exercised an influence, yet the relations of our world and its satellite in respect of masses and motions were in the main locally determined. Still more clearly was it thus with Jupiter and his satellites or Saturn with his rings and satellites. Remembering that the ultimate units of matter of which the Solar System is composed are of the same kinds, and that they act on one another in conformity with the same laws, we see that, remote as the case is from the one we are considering in all other respects, it is similar in the respect that during organization the energies in each locality work effects which are almost independent of the effects worked by the general energies. In this vast aggregate, as in the minute aggregates now in question, the parts are practically autogenous.
Having thus seen that in a way we have not hitherto recognized the same general principles pervade inorganic and organic evolution, let us revert to the case of super-organic evolution from which a parallel was drawn above. As analogous to the germinal mass of units out of which a new organism is to evolve, let us take an assemblage of colonists not yet socially organized but placed in a fertile region—men derived from a society (or rather a succession of societies) of long-established type, who have in their adapted natures the proclivity towards that type. In passing from its wholly unorganized state to an organized state, what will be the first step? Clearly this assemblage, though it may have within the constitutions of its units the potentialities of a specific structure, will not develop forthwith the details of that structure. The inherited natures of its units will first show themselves by separating into large groups devoted to strongly-distinguished occupations. The great mass, dispersing over promising lands, will make preparations for farming. Another considerable portion, prompted by the general needs, will begin to form a cluster of habitations and a trading centre. Yet a third group, recognizing the demand for wood, alike for agricultural and building purposes, will betake themselves to the adjacent forests. But in no case will the primary assemblage, before these separations, settle the arrangements and actions of each group: it will leave each group to settle them for itself. So, too, after these divisions have arisen. The agricultural division will not as a whole prescribe the doings of its members. Spontaneous segregation will occur: some going to a pastoral region and some to a tract which promises good crops. Nor within each of these bodies will the organization be dictated by the whole. The pastoral group will separate itself into clusters who tend sheep on the hills and clusters who feed cattle on the plains. Meanwhile those who have gravitated towards urban occupations will some of them make bricks or quarry stone, while others fall into classes who build walls, classes who prepare fittings, classes who supply furniture. Then along with completion of the houses will go occupation of them by men who bake bread, who make clothing, who sell liquors, and so on. Thus each great group will go on organizing itself irrespective of the rest; the sub-groups within each will do the same; and so will the sub-sub-groups. Quite independently of the people on the hills and the plains and in the town, those in the forest will divide spontaneously into parties who cut down trees, parties who trim and saw them, parties who carry away the timbers; while every party forms for itself an organization of "butty" or "boss," and those who work under him. Similarly with the ultimate divisions—the separate families: the arrangements and apportionments of duties in each are internally determined. Mark the fact which here chiefly concerns us. This formation of a heterogeneous aggregate with its variously adapted parts, which while influenced by the whole are mainly self-formed, goes on among units of essentially the same natures, inherited from units who belonged to similar societies. And now, carrying this conception with us, we may dimly perceive how, in a developing embryo, there may take place the formation, first of the great divisions—the primary layers—then of the outlines of systems, then of component organs, and so on continually with the minor structures contained in major structures; and how each of these progressively smaller divisions develops its own organization, irrespective of the changes going on throughout the rest of the embryo. So that though all parts are composed of physiological units of the same nature, yet everywhere, in virtue of local conditions and the influence of its neighbours, each unit joins in forming the particular structure appropriate to the place. Thus conceiving the matter, we may in a vague way understand the strange facts of autogenous development disclosed by the above named experiments.
§ 97f. "But how immeasurably complex must be the physiological units which can behave thus!" will be remarked by the reader. "To be able to play all parts, alike as members of the whole and as members of this or that organ, they must have an unimaginable variety of potentialities in their natures. Each must, indeed, be almost a microcosm within a microcosm."
Doubtless this is true. Still we have a consensus of proofs that the component units of organisms have constitutions of extremely involved kinds. Contemplate the facts and their implications. (1) Here is some large division of the animal kingdom—say the Vertebrata. The component units of all its members have certain fundamental traits in common: all of them have proclivities towards formation of a vertebral column. Leaving behind the great assemblage of Fishes with its multitudinous types, each having special units of composition, we pass to the Amphibia, in the units of which there exist certain traits superposed upon the traits they have in common with those of Fishes. Through unknown links we ascend to incipient Mammalian types and then to developed Mammalian types, the units of which must have further superposed traits. Additional traits distinguish the units of each Mammalian order; and, again, those of every genus included in it; while others severally characterize the units of each species. Similarly with the varieties in each species, and the stirps in each variety. Now the ability of any component unit to carry within itself the traits of the sub-kingdom, class, order, genus, species, variety, and at the same time to bear the traits of immediate ancestors, can exist only in a something having multitudinous proximate elements arranged in innumerable ways. (2) Again, these units must be at once in some respects fixed and in other respects plastic. While their fundamental traits, expressing the structure of the type, must be unchangeable, their superficial traits must admit of modification without much difficulty; and the modified traits, expressing variations in the parents and immediate ancestors, though unstable, must be considered as capable of becoming stable in course of time. (3) Once more we have to think of these physiological units (or constitutional units as I would now re-name them) as having such natures that while a minute modification, representing some small change of local structure, is inoperative on the proclivities of the units throughout the rest of the system, it becomes operative in the units which fall into the locality where that change occurs.
But unimaginable as all this is, the facts may nevertheless in some way answer to it. As before remarked, progressing science reveals complexity within complexity—tissues made up of cells, cells containing nuclei and cytoplasm, cytoplasm formed of a protoplasmic matrix containing granules; and if now we conclude that the unit of protoplasm is itself an inconceivably elaborate structure, we do but recognize the complexity as going still deeper. Further, if we must assume that these component units are in every part of the body acting on one another by extremely complicated sets of forces (ethereal undulations emanating from each of the constituent molecules) determining their relative positions and actions, we are warranted by the discoveries which every day disclose more of the marvellous properties of matter. When to such examples as were given in [§ 36e] we add the example yielded by recent experiments, showing that even a piece of bread, after subjection to pressure, exhibits diamagnetic properties unlike those it previously exhibited, we cannot doubt that these complex units composing living bodies are all of them seats of energies diffused around, enabling them to act and re-act so as to modify one another's states and positions. We are shown, too, that whatever be the natures of the complex forces emanating from each, it will, as a matter of course, happen that the power of each will be relatively great in its own neighbourhood and become gradually smaller in parts increasingly remote: making more comprehensible the autogenous character of each local structure.
Whatever be their supposed natures we are compelled to ascribe extreme complexity to these unknown somethings which have the power of organizing themselves into a structure of this or that species. If gemmules be alleged, then the ability of every organ and part of an organ to vary, implies that the gemmules it gives off are severally capable of receiving minute modifications of their ordinary structures: they must have many parts admitting of innumerable relations. Supposing determinants be assumed, then in addition to the complexity which each must have to express in itself the structure of the part evolved from it, it must have the further complexity implied by every superposed modification which causes a variation of that part. And, as we have just seen, the hypothesis of physiological units does not relieve us from the need for kindred suppositions.
One more assumption seems necessary if we are to imagine how changes of structure caused by changes of function can be transmitted. Reverting to [§ 54d], where an unceasing circulation of protoplasm throughout an organism was inferred, we must conceive that the complex forces of which each constitutional unit is the centre, and by which it acts on other units while it is acted on by them, tend continually to remould each unit into congruity with the structures around: superposing on it modifications answering to the modifications which have arisen in those structures. Whence is to be drawn the corollary that in course of time all the circulating units,—physiological, or constitutional if we prefer so to call them—visiting all parts of the organism, are severally made bearers of traits expressing local modifications; and that those units which are eventually gathered into sperm-cells and germ-cells also bear these superposed traits.