Form and Function: A Contribution to the History of Animal Morphology

by E. S. Russell

Dohrn conceived that the transformation of the Annelid into the Vertebrate took place mainly by reason of an important transforming principle, which he calls the principle of function-change. Each organ, Dohrn thinks, has besides its principal function a number of subsidiary functions which only await an opportunity to become active. "The transformation of an organ takes place by reason of the succession of the functions which one and the same organ possesses. Each function is a resultant of several components, of which one is the principal or primary function, while the others are the subsidiary or secondary functions. The weakening of the principal function and the strengthening of a subsidiary function alters the total function; the subsidiary function gradually becomes the chief function, the total function becomes quite different, and the consequence of the whole process is the transformation of the organ" (p. 60). Examples of function-change are not difficult to find. Thus the stomach in most Vertebrates performs both a chemical and a mechanical function, but in some forms a part of it specialises in the mechanical side of the work and becomes a gizzard, while the remaining part confines its energies to the secretion of the gastric juice. So, too, it is through function-change that certain of the ambulatory appendages of Arthropods have become transformed into jaws—their function as graspers of food has gradually prevailed over their main function as walking limbs. In the evolution of Vertebrates from Annelids the principle came into action in many connections—in the formation of a new mouth from gill-slits, in the transformation of gills into fins and limbs, of segmental organs into gill-slits, and so on. Dohrn tells us that the principle of function-change was suggested to him by Mivart's Genesis of Species (1870), and he points out how it enables a partial reply to be made to the dangerous objection raised against the theory of natural selection that the first beginnings of new organs are necessarily useless in the struggle for existence.

We may note in passing that a somewhat similar idea was later applied by Kleinenberg to the explanation of some of the ancestral features of development. He pointed out in his classical memoir on the embryology of the Annelid Lopadorhynchus[400] that many embryonic organs seem to be formed for the sole purpose of providing the necessary stimulus for the development of the definitive organs. Thus the notochord is the necessary forerunner of the vertebral column, cartilage the precursor of bone. "From this point of view," he writes, "many rudimentary organs appear in a different light. Their obstinate reappearance throughout long phylogenetic series would be hard to understand were they really no more than reminiscences of bygone and forgotten stages. Their significance in the processes of individual development may in truth be far greater than is generally recognised. When in the course of the phylogeny they have played their part as intermediary organs (Vermittelungsorgane) they assume the same function in the ontogeny. Through the stimulus or by the aid of these organs, now become rudimentary, the permanent parts of the embryo appear and are guided in their development; when these have attained a certain degree of independence, the intermediary organ, having played its part, may be placed upon the retired list."[401]

Dohrn was well aware of the functional, or as he calls it, the physiological, orientation of his principle, and he rightly regarded this as one of its chief merits. He held that morphology became too abstract and one-sided if it disregarded physiology completely; he saw clearly that the evolution of function was quite as important a problem as the evolution of form, and that neither could be solved in isolation from the other. "The concept of function-change is purely physiological;" he writes, "it contains the elements out of which perhaps a history of the evolution of function may gradually arise, and for this very reason it will be of great utility in morphology, for the evolutionary history of structure is only the concrete projection of the content and course of the evolution of function, and cannot be comprehended apart from it" (p. 70).[402]

It is very instructive in this connection to note that Dohrn was not, like so many of his contemporaries, a dogmatic materialist, but upheld the commonsense view that vital phenomena must, in the first instance at least, be accepted as they are. "It is for the time being irrelevant," he writes, "to squabble over the question as to whether life is a result of physico-chemical processes or an original property (Urqualität) of all being.... Let us take it as given" (p. 75).

Semper's speculations on the genetic affinity of Articulates and Vertebrates are contained in two papers[403] which appeared about the same time as Dohrn's. He openly acknowledges that his work is essentially a continuation of Geoffroy's transcendental speculations, and gives in his second paper a good historical account of the views of his great predecessor. It is a significant fact that evolutionary morphologists very generally held that Geoffroy was right in maintaining against Cuvier[404] the unity of plan of the whole animal kingdom, for they saw in this a strong argument for the monophyletic descent of all animals from one common ancestral form.

In his first paper Semper does little more than break ground; he insists on the fact that both Annelids and Vertebrates are segmented animals, and he points out how close is the analogy between the nephridia or "segmental organs" of the former and the excretory (mesonephric) tubules of the latter, upon which he published in the same volume an extensive memoir. At this time he considered Balanoglossus—by reason of its gill-slits (its notochord he did not know)—to be the nearest living representative of the ancestral form of Vertebrates and Annelida.

His second paper is a more exhaustive piece of work and deals with every aspect of the problem, both from an anatomical and from an embryological standpoint. It is consciously and admittedly an attempt to apply Geoffroy's principle of the unity of plan and composition to the three great metameric groups, the Annelida, Arthropoda, and Vertebrata. Semper follows Geoffroy's lead very closely in maintaining that it is not the position of the organs relative to the ground that must be taken into account in establishing their homologies, but solely their spatial relations one to another. He holds that dorsum and venter are terms of purely physiological import, and he proposes to substitute for them the terms neural and cardial (better, hæmal) surfaces, either of which may be either dorsal or ventral in position.

Having established this primary principle, Semper has little difficulty in showing that the main organs of the body lie to one another in the same relative positions in Annelida, Arthropoda, and Vertebrata; and this, together with the metameric segmentation common to them all, constitutes his first great argument in favour of their genetic relationship. But he has still to show that Annelids possess at least the rudiments of certain organs which seem to be peculiar to Vertebrates, as the gill-slits, the notochord, and a nervous system developed from the ectoderm of the "dorsal" surface. He takes particular cognisance also of the old distinction drawn by von Baer, that Vertebrates show a "double-symmetrical" mode of development (evolutio bigemina), the dorsal muscle-plates forming a tube above the notochord, the ventral plates a tube below the notochord, whereas Articulates do not possess this axis, and form only one tube, namely, that round the "vegetative" organs (evolutio gemina). Semper is at pains to prove that evolutio bigemina is characteristic also of Annelidan development.

Fig. 14.—Transverse Section (Inverted) of the Worm Nais. (After Semper.)