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

by E. S. Russell

How is the fact of multiple formation to be reconciled with the principle of repetition, according to which organs are simplest in the early embryo and in the lower animals? But observation shows that, as a rule, the further down the scale you go the more divided organs become—the more numerous the bones of the skull, for example. There is thus a parallel between multiple formation of organs in the embryos of the higher Vertebrates and their subdivided state in the lower. Take, for example, the kidney. In the genus Felis, and in birds, each kidney has two lobes, in the elephant four, in the otter ten, in the ox twelve to fourteen. The human kidney in its development starts with about a dozen lobes, and the number diminishes as the kidney grows. Thus the permanent state of the kidney in the animals mentioned is reproduced by the stages of its development in man (xii., p. 126).

So, too, at the second or third month the uterus of the human embryo is bicornuate, and afterwards passes through stages comparable to the adult and permanent uterus of rodents, ruminants, and carnivores. There is indeed a time in the development of the human embryo when it resembles in many of its organs the adult stage of various lower animals. It is about this time that it possesses a tail.

We note that Serres' theory of parallelism applies, strictly speaking, only to organs, not to organisms, although he, too, readily fell into the error of supposing that the organisation of an embryo could be compared as a whole with the adult organisation of an animal lower in the scale. Thus he wrote in one of his later papers[133]—"As our researches have made clear, an animal high in the organic scale only reaches this rank by passing through all the intermediate states which separate it from the animals placed below it. Man only becomes man after traversing transitional organisatory states which assimilate him first to fish, then to reptiles, then to birds and mammals." Serres was not altogether free from the besetting sin of the transcendentalists—hasty generalisation.

The law of parallelism applied not only to Vertebrates but also to Invertebrates. In a short paper[134] of 1824 Serres attempted an explanation of the nervous system of Invertebrates. Invertebrates, he considered, lacked the cerebrospinal axis of Vertebrates, and their nervous system was the homologue of the sympathetic system of Vertebrates. The relation of the invertebrate to the vertebrate nervous system being thus fixed, can the nervous system of Invertebrates be reduced to one plan? It does not seem possible to establish a common plan for the adult nervous systems. But apply the principle of parallelism, which has proved so valuable within the limits of the vertebrate series. Taking insects as the highest class, we find that there are three stages in the development of their nervous system; in the first the nervous system is composed of two separate strands, in the second the strands unite round the œsophagus, in the third they unite also behind. Now in Bulla aperta, stage (1) is permanent; in Clio, Doris, Aplysia, Tritonia, Sepia, Helix, stage (2) is permanent, and in Unio stage (3). In fact, all the varieties of the nervous system of molluscs fall into one or other of these three classes. "It follows, then, that as regards their nervous system, the Mollusca are more or less advanced larvæ of insects" (p. 380). The law of parallelism is here applied to single organ-systems, but in later years Serres applied it to whole organisations also, saying that the lower Invertebrates were permanent embryos of the higher.

In the paper of 1834, already referred to, Serres pushed his speculations further and attempted to establish the unity of type of all animals, Vertebrates and Invertebrates alike—a favourite pastime of the transcendentalists. It is incontestable, he admits, that adult Invertebrates are quite different in structure from adult Vertebrates, "but if one regards them as what I take them to be, namely, permanent embryos, and if one compares their organisation with the embryogeny of Vertebrates, one sees the differences disappear, and from their analogies arise a crowd of unsuspected resemblances" (loc. cit., p. 247).

The last point of Serres' doctrine which calls for remark is his interpretation of abnormalities as being often comparable to grades of structure permanent in the lower animals. Thus the double aorta which may occur as an abnormality in man is the normal and permanent state in reptiles. This idea, of course, he got from Etienne Geoffroy St Hilaire. It is further developed in his "Théorie des formations et des déformations organiques appliquée à l'anatomie comparée des monstruosités (1832), and in his final large memoir of 1860 (see below, p. [205]).

In 1816 appeared a fine piece of work by J. C. Savigny on the homologies of the appendages in Articulates. The standpoint was that of pure morphology. "I am convinced," he wrote, "that when a more complete examination has been made of the mouth of insects, properly so called, that is to say, having six legs and two antennæ, it will be found that whatever form it affects it is always essentially composed of the same elements.... The organ remains the same, only the function is modified or changed—such is Nature's constant plan."[135] In this the influence of Geoffroy can be traced; but the work was very free from the exaggerations of the transcendentalists, and many of Savigny's homologies are accepted even to-day. The first memoir dealt with the mouth-parts of insects; the second with the anterior appendages of Articulates generally. Savigny shows that the mouth-parts of insects can be reduced to the type shown in Orthoptera, where there are clearly two mandibles, two maxillæ, and a lower lip formed by the fusion of two second maxillæ. All other insects have these same mouth-parts, disposed in the same order, however much their form may have been modified in response to new functions. He goes on to compare the anterior set of appendages in a long series of Articulates, in Julus, Scolopendra, Cancer, Gammarus, Cyamus, Nymphon, Phalangium, Apus, Caligus, Limulus, and a few others. For Crustacea he established the homologies now accepted, of the mandibles with the mandibles of insects, of the first and second pairs of maxillæ with the parts so named in insects, and so on. He is quite clear that the maxillipedes of Crustacea are the homologues of the feet of Hexapoda. "Their disposition must lead one to think that the six anterior feet of Julus, that is to say, all the feet of the Hexapoda, are here transformed into jaws" (loc. cit., p. 48). In Scolopendra also there is a similar transformation of two pairs of legs into auxiliary jaws. In Gammarus, where there is only the first pair of maxillipedes, the other two pairs have become "retransformed" into feet. We find him supporting his comparison of the three anterior pairs of legs in Julus to the three pairs of legs in insects by an argument drawn from embryology; for only the first three pairs of feet are present in Julus at birth (Degeer), "an observation, which, together with their position, should cause them to be considered as the representatives of the six thoracic feet of Hexapoda" (p. 44).

His comparison of the Arachnid appendages with those of insects and Crustacea is very curious. As his starting-point he takes Cyamus, which has antennæ (two pairs) and mouth parts (four pairs) as in many Crustacea, and then seven pairs of legs; he compares with it Nymphon, which has in all seven pairs of appendages. These appendages he homologises with the seven pairs of legs of Cyamus, so that the first appendage in Nymphon corresponds to the seventh appendage of Cyamus. This homology is extended to all Arachnids; their first two pairs of appendages, however they may be modified as "false" mandibles and "false" maxillæ, really correspond to the second and third maxillipedes in Crustacea, and to the second and third pairs of feet in insects. It is interesting to note that he treats Limulus as an Arachnid, pointing out that there is as much difference between Apus and Limulus as between Cancer and Phalangium. He describes the "gnathobases" in Phalangium and Limulus. We may note that he had just an inkling of the modern doctrine that all the appendages of Articulates consist of a basal joint bearing an inner and an outer terminal piece, for he observes that the "cirri" of the maxillipedes of Crustacea give the appendage the same bifid appearance as the appendages of the abdomen and the thoracic legs of Mysis (p. 50).

V. Audouin, in his memoir, Recherches anatomiques sur le thorax des animaux articulés,[136] applied the principle of the unity of plan and composition to the exoskeleton of insects, Crustaceans, and Arachnids. His guiding ideas were, "(1) that the skeleton of articulated animals is formed of a definite number of pieces, which are either distinct or intimately fused with one another; (2) that in many cases, some pieces diminish or altogether disappear, while others reach an excessive development; (3) that the increase of one piece seems to exert on the neighbouring pieces a kind of influence which explains all the differences one finds between the individuals of each order, family and genus" (Sep. copy, p. 16p. Geoffroy had already stated, without proof, that the parts of the Arthropod's skeleton, however they might change in shape and size, remained faithful to the principle of connections, at least at their points of insertion.[137] Audouin gave the detailed demonstration of this by his accurate and minute determination of the pieces of the arthropod skeleton. He recognised that the body of Arthropods was made up of a series of similar rings, and that even the compact head of insects consisted of fused segments. In each segment Audouin distinguished a fixed number of hard chitinous parts, the dorsal tergum, the ventral sternum, the lateral "flanc" of three pieces, all to be recognised by their positions relative to one another. Many of the names which he proposed are still in use; it was he who introduced the terms prothorax, mesothorax, and metathorax, for the three segments of the insect's thorax. He used Geoffroy's Loi de balancement to explain cases of correlative development, such as the relation between the size of the front wings and the development of the mesothorax. In another paper Audouin compared the three pieces of the dorsal skeleton of Trilobites to the tergum and the upper part of the "flanc."[138] In a third paper of about the same time he tried to establish the homologies of the segments throughout the Articulate series—with less success than Savigny.

Later on, in conjunction with Milne-Edwards, he demonstrated the unity of composition of the nervous system in Crustacea, showing how the concentrated system of the crab was formed by the same series of ganglia as in the Macrura.