Besides being inconsistent with the leading truths of Embryology and Comparative Morphology, the hypothesis of Goethe and Oken is inconsistent with itself. The facts brought forward to show that there exists an archetypal vertebra, and that the vertebrate animal is composed of archetypal vertebræ arranged in a series, and severally modified to fit their positions—these facts, I say, so far from proving as much, suffice, when impartially considered, to disprove it. No assigned, nor any conceivable, attribute of the supposed archetypal vertebra is uniformly maintained. The parts composing it are constant neither in their number, nor in their relative positions, nor in their modes of ossification, nor in the separateness of their several individualities when present. There is no fixity of any one element, or connexion, or mode of development, which justifies even a suspicion that vertebræ are modelled after an ideal pattern. To substantiate these assertions here would require too much space, and an amount of technical detail wearisome to the general reader. The warrant for them will be found in a criticism on the osteological works of Prof. Owen, originally published in the British and Foreign Medico-Chirurgical Review for Oct. 1858. This criticism I add in the Appendices, for the convenience of those who may wish to study the question more fully. (See Appendix B.)
Everything, then, goes to show that the segmental composition which characterises the apparatus of external relation in most Vertebrata, is not primordial or genetic, but functionally determined or adaptive. Our inference must be that the vertebrate animal is an aggregate of the second order, in which a relatively superficial segmentation has been produced by mechanical intercourse with the environment. We shall hereafter see that this conception leads us to a consistent interpretation of the facts—shows us why there has arisen such unity in variety as exists in every vertebral column, and why this unity in variety is displayed under countless modifications in different skeletons.[31]
§ 211. On glancing back at the facts brought together in these two chapters, we see it to be probable that there has gone on among animals a process like that which we saw reason to think has gone on among plants. Minute aggregates of those physiological units which compose living protoplasm, exist as Protozoa: some of them incoherent, indefinite, and almost homogeneous, and others of them more coherent, definite, and heterogeneous. By union of these nucleated particles of sarcode, are produced various indefinite aggregates of the second order—Sponges, Polycytharia, Foraminifers, &c.; in which the compound individuality is scarcely enough marked to subordinate the primitive individualities. But in other types, as in Hydra, the lives of the morphological units are in a considerable degree, though not wholly, merged in the life of the integrated body they form. As the primary aggregate, when it passes a certain size, undergoes fission or gemmation; so does the secondary aggregate. And as on the lower stage so on the higher, we see cases in which the gemmiparously-produced individuals part as soon as formed, and other cases in which they continue united, though in great measure independent. This massing of secondary aggregates into tertiary aggregates, is variously carried on among the Hydrozoa, the Actinozoa, the Polyzoa, and the Tunicata. In most of the types so produced, the component individualities are very little subordinated to the individuality of the composite mass—there is only physical unity and not physiological unity; but in certain of the oceanic Hydrozoa, the individuals are so far differentiated and combined as very much to mask them. Forms showing us clearly the transition to well-developed individuals of the third order, are not to be found. Nevertheless, in the great sub-kingdom Annulosa, there are traits of structure, development, and mode of multiplication, which go far to show that its members are such individuals of the third order; and in the relations to external conditions involved by the mode of union, we find an adequate cause for that obscuration of the secondary individualities which we must suppose has taken place. The two other great subdivisions, Mollusca and Vertebrata, between the lower members of which there are suggestive points of community, present us only with aggregates of the second order, that have in many cases become very large and very complex. We find in them no trace of the union of gemmiparously-produced individuals. Neither the molluscous nor the vertebrate animal shows the faintest trace of a segmentation affecting the totality of its structure; and we see good grounds for concluding that such segmentation as exceptionally occurs in the one and usually occurs in the other, is superinduced.
[Note:—A critic calls in question the statement on p. 121 respecting the Amphioxus. At the outset, however, he admits that in the Amphioxus “the central nervous system and the notochord are not segmented.” In the Annelid, however, the central nervous system is segmented, and there is segmentation of the part which, as a supporting structure, is analogous to the notochord in respect of function—the outer part which represents the exo-skeleton in contrast to the endo-skeleton. He goes on to say that “the gut is not involved [in the segmentation] and exhibits in Amphioxus just as it does in worms differentiations entirely independent of the segmentation of the mesoblast.” Part of this statement is, I think, not congruous with all the facts. In Protodrilus, one of the lowest of the Archiannelida, “the intestine is moniliform, there being a constriction between each segment” and the next. (Shipley.) Complete segmentation of the intestine is obviously impossible, since, were the canal divided into portions by septa, no food could pass. But the fact that the gut has these successive expansions and constrictions, corresponding to the successive segments, and giving to each segment a partially-separate stomach, shows that segmentation has gone as far as consists with the carrying on of the lives of the segments. No such partial segmentation exists in the Amphioxus. Thus, then, three fundamental structures—the directive structure, the supporting structure, and the alimentary structure—are respectively simple in the lowest vertebrate and segmented, or partially segmented, in the lowest Annelid. Again, while it is said that the gill-clefts exhibit segmentation, it is admitted that this has no relevance to any constitutional segmentation: “they are segmented on a plan of their own” irrespective of other organs. Another allegation is that the ovaries of Amphioxus are segmented. Their segmentation, however, like that of the gills, is isolated, and may be considered as illustrating those repetitions of like parts seen in supernumerary vertebræ in various creatures—a repetition which becomes habitual if the resulting structure is advantageous to the species. On the statement that while the Amphioxus has no rudiments of a renal system the Elasmobranch embryo has such rudiments, which are as distinctly segmented as the nephridia of a worm, two comments may be made. The first is that if in these Vertebrates the nephridia bear a relation to the general structure like that which they do in Annelids, then one would expect to find the segmental arrangement shown in the lowest type, as in Annelids, rather than in a type considerably advanced in development. Should it be replied that in the Amphioxus an excretory system had not yet arisen, though one is required for the higher organization of an Elasmobranch, then the answer may be that since the segmental arrangement in the Elasmobranch corresponds with that of the myotomes, it has no reference to any primordial segmentation, since the myotomes have been functionally generated. The second comment is that whereas the nephridia of the Annelid have independent external openings, the nephridia in the Elasmobranch have not. These discharge their secretions into certain general tubes of exit common to them all; showing that each of them, instead of being a member of a partially independent structure, is united with others in subordination to a general structure. That is to say, the segmentations are far from being parallel in their essential natures. The assertion accompanying these criticisms, that there is “no difference in principle between the segmentation of Amphioxus and Annelid” is difficult to reconcile with the visible contrast between the two. Whatever local segmentations there are in an Amphioxus appear to me quite unlike “in principle” to those which an Annelid exhibits. Could its portion of gut be duly supplied with nutriment, the segment of a low Annelid could carry on its vital functions independently. In the parts of the Amphioxus we see nothing approaching to this. Cut it into transverse sections and no one of them contains anything like the assemblage of structures required for living. The Amphioxus is a physiological whole, and in that respect differs radically from the Annelid, each segment of which is in chief measure a physiological whole. No occurrence of local segmentation in the Amphioxus can obliterate this fundamental contrast.
An accompanying contrast tells the same story. On ascending from the lowest to the highest annulose types we see a progressing integration, morphological and physiological; so that whereas in a low annelid the successive parts are in large measure independent in their structures and in their lives, in a high arthropod, as a crab, most of the parts have lost their individualities and have become merged in a consolidated organism with a single life. Quite otherwise is it in the vertebrate series. Its lowest member is at the very outset a complete morphological and physiological whole, and the formation of those serial parts which some think analogous to the serial parts of an Annelid, begins at a later stage and becomes gradually pronounced. That is to say, the course of transformation is reversed.]
CHAPTER VI.
MORPHOLOGICAL DIFFERENTIATION IN PLANTS.
§ 212. While, in the course of their evolution, plants and animals have displayed progressive integrations, there have at the same time gone on progressive differentiations of the resulting aggregates, both as wholes and in their parts. These differentiations and the interpretations of them, form the second class of morphological problems.
We commence as before with plants. We have to consider, first, the several kinds of modification in shape they have undergone; and, second, the relations between these kinds of modification and their factors. Let us glance at the leading questions that have to be answered.
§ 213. Irrespective of their degrees of composition, plants may, and do, become changed in their general forms. Are their changes capable of being formulated? The inquiry which meets us at the outset is—does a plant’s shape admit of being expressed in any universal terms?—terms that remain the same for all genera, orders, and classes.