Form and Function Part 7

The first memoir deals with the h.o.m.ologies of the opercular bones.

Geoffroy considers that the external opening of the ear corresponds to the external opening of the gill-chamber, which lies between the operculum and the pectoral girdle. The ear communicates with the buccal cavity by the Eustachian tube, so does the branchial chamber by means of the gill-slits. The auditory chamber of higher Vertebrates is, therefore, the h.o.m.ologue of the branchial chamber in fish; the opercular bones in fish and the ossicles of the ear in other Vertebrates stand in close relation to this chamber; therefore the opercular bones are the h.o.m.ologues of the ossicles of the ear, the interoperculum corresponding to the malleus, the suboperculum to the lenticular, the minute lower part of the suboperculum to the incus, the operculum to the stapes, and the pre-operculum to the tympanic ring. In making these particular determinations Geoffroy professes to be led by his principle of connections. The pre-operculum has, he says, the same connections with neighbouring bones as the tympanic bone in other Vertebrates, and the other pieces of the gill-cover are h.o.m.ologised with particular ear-ossicles according to the order in which they stand to one another.

The second memoir in the book deals with the sternum, and affords a very good example of Geoffroy's method of dealing with the facts of structure. We shall omit here any detailed reference to the other three memoirs, which deal with the hyoid, with the branchial arches and the structures which correspond in air-breathing Vertebrates, and with the bones of the shoulder-girdle.

In the memoir on the sternum Geoffroy's first care is to arrive at a definition of what a sternum is. He defines it partly by its functions, partly by its connections, as the system of bones which covers and protects the thorax, and gives attachment to certain groups of muscles.

The most highly developed sternum (according to this definition) is the plastron of the tortoise, whose structure it dominates (p. 103). It is important, therefore, to determine of how many bones the plastron is composed, since the full number of elementary parts of which an organ is composed is best seen when the organ is at the maximum of its development. There are nine bones in the plastron of the tortoise. "The conclusion to be drawn from this is that every sternum, provided that it is not inhibited in its development by some obstacle, is composed of _nine elementary parts_" (p. 105). These nine bones are in Geoffroy's nomenclature, the episternals, the hyosternals, the hyposternals, the xiphisternals, which are all paired bones, and the entosternal, which is unpaired. The arrangement of them is in the tortoise:--

Episternal---------------------------Episternal |__ __/| | __ __/ | | __ __/ | | __ Entosternal __/ | | __/ __ | | __/ __ | | __/ __ | |/ | Hyosternal Hyosternal | | | | | | | | Hyposternal-------------------------Hyposternal | | | | | | | | Xiphisternal------------------------Xiphisternal.

The articulations in the tortoise are indicated by the connecting lines. Geoffroy tries to show that the sternum in other animals is composed of these nine bones, or at least of a certain number of them, always in the same invariable relative positions. Thus in birds the sternum consists of five pieces, of a huge keeled entosternal, and of two "annexes" on either side, which are the hyo-and hyposternals.

These are separate only in young birds. Occasionally, especially in young birds, rudiments of episternals and xiphisternals also occur.

The minuteness of the episternals and the xiphisternals may be attributed to the gigantic size of the entosternal, in accordance with the _Loi de balancement_. In the other air-breathing Vertebrates the nine sternal elements can according to Geoffroy be discovered without great difficulty. But when we come to the determination of the sternum in fishes, difficulties abound, which Geoffroy solves in the following way. He points out that between the clavicles (_cleithra_) and the hyoid bone (_basihyal_) in fishes there is a long median bone (_urohyal_) which is attached in front by two strong tendons to the horns of the hyoid and is free behind (see Fig. 1). Gouan (1720) had seen in this bone the h.o.m.ologue of the sternum. Geoffroy adopts this view, but considers that this bone alone cannot represent the whole sternum. He finds the representatives of other bones of the sternum in the large bones (_epihyal_ and _ceratohyal_, or the two pieces of the _ceratohyal_) which are comprised in the hyoid arch. But he is immediately met by the difficulty that this complex of bones is situated in front of the pectoral girdle, whereas the sternum in higher Vertebrates lies behind the pectoral girdle. He reflects, however, that the gills of fish, situated in front of the clavicles, are merely the lungs under another name. The gills have become shifted forward by a metastasis similar to that which brought the whole thoracic organs far forward in fish. This being so, their supporting elements, the sternum and the ribs, must have moved with them, and are hence to be found in front of the pectoral girdle.

[Ill.u.s.tration: FIG. 1.--Hyoid Arch of the Conger. (Original.)]

Geoffroy's next step is to point out that the only possible h.o.m.ologues of sternal ribs are the branchiostegal rays, which arise from the large bones of the hyoid arch. If these are sternal ribs, the bones to which they are attached must be the hyo- and hyposternals or "annexes," the bones from which in birds the ribs take their origin.

The unpaired sternal bone (_urohyal_) cannot be h.o.m.ologous with the entosternal, for it has no connections with the annexes. He decides that it must represent the episternals, for in some young birds there is a two-headed episternal to which two strong tendons are attached, just in the same way as the unpaired piece in fish is bound to the bones of the hyoid by two tendons. "Thus it is not the sternum as a whole that has shifted in front of the clavicles and covered with its side pieces the gills placed there; it is a piece exclusively piscine, in the sense that it is only in the cla.s.s of fishes that it reaches the _maximum_ of its development" (p. 83).

To sum up, the sternum in all four vertebrate cla.s.ses is composed of the same elements, arranged always in the same way. "One is ... led to the conception of an ideal type of sternum for all Vertebrates, which then, considered from a lower standpoint, resolves itself into several secondary forms according as the whole or the majority of the const.i.tuent materials are employed, or even as these elements come to change their respective dimensions or proportions" (p. 134). As to the elementary const.i.tuents, "they give proof of individuality, and sometimes even, in certain abnormalities, of independence, and rise to the level of primary organisatory materials" (p. 132). What holds good for the sternum holds good for other organs--and accordingly the unity of plan and composition can be demonstrated for all the organs of Vertebrates.

Soon after the publication of the _Philosophie anatomique_ (1818) Geoffroy went further in his search for unity, and maintained that the structure of insects and Crustacea could be reduced to the vertebrate type.

He proposed to replace Cuvier's cla.s.sification of the animal kingdom into the four large groups, Vertebrata, Mollusca, Articulata, and Radiata by the following cla.s.sification:--[90]

Hauts-Vertebres (Vertebrata, Cuv.).

Vertebres / Dermo-Vertebres (Articulata, Cuv.).

Mollusques (Mollusca, Cuv.).

Invertebres / Rayonnes (Radiata, Cuv.).

The idea upon which is based the comparison of Articulates with Vertebrates is that each skeletal segment of Articulates is a vertebra.

In the Hauts-vertebres the vertebrae are internal; in the Dermo-vertebres they are external. "_Every animal lives either outside or inside its vertebral column_."[91] The essence of a vertebra is not its form, nor its function, but its composition from four elementary pieces which unite round a central s.p.a.ce (_Isis, loc. cit._, p. 532).

Serres had shown that in the higher animals every vertebra is formed from four centres of ossification, that the body of the vertebra is at first tubular, and that afterwards it becomes filled up. In lobsters and crabs each segment is composed of four elementary pieces, as may be seen most easily in young ones. "Accordingly each segment corresponds to a true vertebra in composition: there is the same number of 'materials,'

the same order in the course of ossification, the same kind of articulation, the same annular arrangement, the same empty s.p.a.ce in the middle" (p. 534). The only difference is that in Articulates the central s.p.a.ce is very great and contains all the organs of the body, whereas in the higher Vertebrates the body of the vertebra becomes completely filled up. In the thoracic region of Crustacea it is not the whole segment with part of the carapace which corresponds to a vertebra, but merely the part round the ventral nerve-cord (endophragmal skeleton).

If the skeleton of the segment in Articulates corresponds to the body of a vertebra and is here external, then the appendages of the Articulate must correspond to ribs (p. 538). The full development of this thought is found in a Memoir of 1822, "Sur la vertebre."[92] He takes as the typical vertebra that of a Pleuronectid, probably the turbot. His original figure is reproduced (Fig. 2).

[Ill.u.s.tration: FIG. 2.--"Vertebra" of a Pleuronectid. (After Geoffroy.)]

He includes as part of the vertebra not only the neural (e', e'') and haemal (o', o'') arches, but also, above and below these, the radialia (a'', u') and the fin-rays (a', u''). (Neither the radialia nor the fin-rays are, by the way, in the same transverse plane as the body of the vertebra). Every vertebra, he considers, contains these nine pieces--the cycleal (or body), the two perials (e', e'') and the two epials (a', a'') above, the two paraals (o', o'') and the two cataals (u', u'') below. The epials and the cataals are in reality paired bones which in fish mount one on top of the other to support the median fins. In the cranial region--the skull is formed of modified vertebrae--the epials and perials open out so as to form the walls and roof of the brain; in the thoracic region the paraals and cataals reach their maximum of development and perform the same service for the thoracic organs, the paraals becoming vertebral, and the cataals sternal, ribs.

We have seen that in Arthropods the body of the vertebra (cycleal) forms the open ring of the segment, which lies immediately under the skin, the vertebral tube coinciding with the epidermal tube. The h.o.m.ologues of the other eight pieces of the vertebra must accordingly be sought in the external appendages. At first sight there seems here a contradiction of the principle of connections, for the appendages in Arthropods are lateral, whereas the paired bones of the vertebra are dorsal and ventral. But there is in reality no contradiction, for "what our law of connections absolutely requires is that all organs, whether internal or external, should stand to one another in the same relations; but it is all one whether the box (_coffre_) that encloses them lies with this or that side on the ground. What similarities in the organisation of man and the digitate mammals, and yet what differences between their att.i.tudes when standing! The same holds true as regards the normal att.i.tudes of the pleuronectids and the other fishes" (p. 107).

The exact way in which Geoffroy h.o.m.ologised the parts of the appendages in Arthropods with the paired pieces of the typical vertebra is best shown by the reproduction of his figure of an abdominal segment of the lobster (Fig. 3), in which the parts h.o.m.ologous with those represented in the figure of the typical vertebra (Fig. 2) are indicated by the same letters. The ingenuity of the comparison is astonishing.

[Ill.u.s.tration: FIG. 3.--Abdominal Segment of the Lobster. (After Geoffroy.)]

The comparison of the Arthropod with the Vertebrate is extended also to the internal organs. The internal organs of the Arthropod are shown to stand in the same order to one another as in the Vertebrate, only the organs are inverted. Thus the nervous system is dorsal in the Vertebrate, ventral in the Arthropod. Turn the Arthropod on its back and the relative positions of the systems of organs are the same as in the Vertebrate. The relation of the organs to the external tube is of course different in Arthropods and Vertebrates, but this is no contradiction of the principle of connections. "Such a tube, although it is the organs essential to life that it contains, can yet behave in different ways with regard to the ma.s.s of these organs: the principle of connections demands only that all the organs maintain with one another fixed and definite relations; but the principle would be in no way invalidated if the whole ma.s.s had rotated inside the tube" (p. 112).

Geoffroy pushed the a.n.a.logy between Arthropods and Vertebrates very far, for he a.s.serted that every piece in the skeleton of an insect was h.o.m.ologous with some bone in Vertebrates, that it stood always in its proper place, and remained faithful to at least one of its connections.[93] It does not appear that he attempted to prove in detail this very big a.s.sumption, but the beginnings of a detailed comparison are found in the paper of 1820, _Sur l'organisation des insectes_. Six segments are distinguished in an insect--the head, the three divisions of the thorax, the abdomen, and the terminal segment of the abdomen (p.

455).

The skeleton of the insect's head is said to correspond to the bones of the face, to the bones of the cerebrum and to the hyoid of higher Vertebrates, the skeleton of the prothorax to the bones of the cerebellum, of the palate, and the pieces of the larynx, the skeleton of the mesothorax to the parietals, interparietals, and opercular bones, and that of the metathorax to the skeleton of the thorax of Vertebrates.

The pieces of the abdomen and of the terminal segment correspond to the bones of the abdomen and coccyx (p. 458). It does not need the subsequent likening of the hind wings of insects to the air bladder of fish, and of the stigmata to the pores of the lateral line, to convince one finally of the fancifulness of the whole comparison.

In 1830 two young naturalists, Meyranx and Laurencet, presented to the Academie des Sciences a memoir in which they likened a Cephalopod to a Vertebrate bent back at the level of the umbilicus, saying that the Vertebrate in this position had all its organs in the same order as in the Cephalopod. Geoffroy took up this idea with enthusiasm, seeing in it a further application of his master-idea of the unity of plan and composition. By means of this comparison Mollusca definitely took their place in the _ech.e.l.le des etres_, after the Articulata, just as Geoffroy had maintained in 1820, saying that crabs formed a link between the other Crustacea and the molluscs.[94] The comparison brought him nearer to the end he had in view, the reference of all animal structure to one single type.

But in championing the memoir of Meyranx and Laurencet, Geoffroy found himself in direct antagonism with Cuvier, who held that his four "Embranchements" had each a separate and distinct plan of structure. In a paper read to the Academy in February 1830,[95] Cuvier easily demolished the crude comparison of the Cephalopod to the Vertebrate. He gave diagrams of the internal organs of a Cephalopod and of a Vertebrate bent back in the manner indicated by Meyranx and Laurencet, and he showed in detail that the arrangement of the main organs was quite different, that the likeness would have been much greater if the Cephalopod had been likened to a Vertebrate doubled up the other way,[96]

but that even then the arrangement of the organs would not be the same.

The organs, too, of the Cephalopod are differently constructed. He sums up his criticism by saying:--"I give true and summary expression to all these facts when I say that Cephalopods have several organs in common with Vertebrates, which fulfil in either case similar functions, but that these organs are differently arranged with respect to one another, and often constructed in a different way; that they are in Cephalopods accompanied by several other organs which Vertebrates do not possess, whilst the latter on their side have many organs which Cephalopods lack"

(p. 257). Geoffroy could not accept this commonsense view of the matter, but made a fight for his transcendental theories. This was the beginning of the famous controversy between Geoffroy and Cuvier which so excited the interest of Goethe. It was a struggle between "comparative anatomy"

and "morphology," between the commonsense teleological view of structure and the abstract, transcendental. Geoffroy brought forward all his theories on the h.o.m.ology of the skeleton of fish with the skeleton of higher Vertebrates, and tried to prove by them his great principle of the unity of plan and composition; Cuvier took Geoffroy's h.o.m.ologies one by one, and showed how very slight was their foundation. Cuvier was on sure ground in insisting upon the observable diversities of structural type, and his vast knowledge enabled him to score a decisive victory.[97]

The controversy was not, as we are sometimes told, a controversy between a believer in evolution and an upholder of the fixity of species, although it raised a question upon which evolution theory was to throw some light.

In these Darwinian days Geoffroy has reaped a little posthumous glory as an early believer in evolution. That he did believe in evolution to a limited extent is certain; that his theory of evolution was, as it were, a by-product of his life-work, is also certain. Geoffroy was primarily a morphologist and a seeker after the unity hidden under the diversity of organic form. His theory of evolution had as good as no influence upon his morphology, for he did not to any extent interpret unity of plan as being due to community of descent. His morphological, non-evolutionary standpoint comes out quite clearly in several places in the _Philosophie anatomique_. He does not derive the structure of the higher Vertebrates from the simpler structure of the lower, but when he finds in fish a part at the maximum of its development, he speaks of the same part, rudimentary in the higher forms, as being, as it were, held in reserve for use in the fish. Thus, speaking of the episternal in fish which forms the central piece of its sternum, he says, "it is a bone that is rudimentary in birds (one might almost add a bone that is held in reserve in birds for this fate) which is destined to form in the centre the princ.i.p.al keel of this new machine" (p. 84). Again, with reference to the h.o.m.ology of the ossicles of the ear with the opercular bones in fish, "employing other resources equally hidden and rudimentary, Nature makes profitable use of the four tiny ossicles lodged in the auditory pa.s.sage, and, raising them in fish to the greatest possible dimensions, forms from them these broad opercula...." (p. 85). Or you may take it the other way about, and start from the organisation of fishes; opercular bones are of no use to air-breathing animals, so they dwindle away, and are pressed into the service of the ear, although they are of little use in hearing (p. 46).

There is here no thought of evolution; in later years, however, his researches upon fossil crocodilians led him to consider the possibility that the living species were descended from the antediluvian. For the factors of the transformation he refers to Lamarck's hypotheses.[98] In a memoir of 1828,[99] dealing with the possible genetic relation of living to fossil species, he still regards the question as more or less open.

Although fossil species are mostly different from living species are we therefore to conclude, he asks, that they are not the ancestors of the present day forms? "The contrary idea arises more naturally in the mind; for otherwise the six-days' creation would have had to be repeated and new beings produced by a fresh creation. Now this proposition, contrary as it is to the most ancient historical traditions, is inadmissible" (p.

210). It is sufficiently clear from this quotation that Geoffroy was thinking only of a transformation of the antediluvian species created by G.o.d, and by no means of an evolution of all species from one primitive type. In matters of religion Geoffroy was orthodox. He goes on to point out how great a resemblance there is in essential structure between fossil and living species. All find their place in one scheme of cla.s.sification; does it not seem that all are modifications "of one single being, of that abstract being or common type, which it is always possible to denote by the same name?" (p. 211). This type is abstract, not actual, and it is certainly not conceived as an original ancestor of all animals.

The fullest development of Geoffroy's views on evolution is found in his memoir "Le degre d'influence du monde ambiant pour modifier les formes animales."[100] Here the relation of his evolution-theory to his morphology is pointed out. The principle of unity of plan and composition cannot be the final goal of zoology; there must follow on it a philosophical study of the _differences_ between organic forms. The causes of these differences are to be found in the environment (pp.

66-7). Geoffroy seems here to be moving from a pure to a causal morphology. It is probable, he continues, that living species have descended by uninterrupted generation from the antediluvian species (p.

74), and that they have in the process become modified through external influences.

Now of all functions respiration is the most important, and upon respiration everything is regulated. "If it be admitted that the slow progression of the centuries has brought in its train successive changes in the proportion of the different elements of the atmosphere, it follows as a rigorously necessary consequence that the organisation has been proportionately influenced by them" (p. 76). The respiratory milieu changes, the species change with it, or are eliminated (p. 79). We may see, perhaps, in the stress which Geoffroy lays upon respiration and the respiratory milieu a result of his constant obsession with the comparison of fish with air-breathing Vertebrates.

In the first geological period, we read in another Memoir of the same year,[101] when ammonites and _Gryphaea_ flourished, hot-blooded animals with lungs could not exist. "A lung constructed like that of mammals and birds would not have been adapted to the essence of the respiratory element such as in my conception of it the system of the environing air used to be"[102] (p. 58).

Geoffroy does not tell us exactly how the milieu is to act upon the organism; the whole theory is little more than a sketch and a pointing out of the way for future research--and in this prophetic enough. The action of external agents was apparently considered as physical, and no power of active adaptation was ascribed to the organism.

From a pa.s.sage in the memoir "Sur la Vertebre" we may perhaps infer that he believed increasing complexity of structure to be due to a realisation of potentialities, to the development of parts present in the lower animals only in potency--"the organisation ... only awaits favourable conditions to rise, by addition of parts, from the simplicity of the first formations to the complication of the creatures at the head of the scale" (p. 112). Evolution takes place as the environment allows, and in a sense in opposition to the environment.

He believed in saltatory evolution, for he considered that the lower oviparous Vertebrates could not be transformed into birds by slow modification, but only by a sudden transformation of their lungs, which would bring about the other characteristics of birds (p. 80). He considered, too, that transformations could arise by means of monstrous development (p. 86). In this connection the experiments which he made on the hen's egg[103] in order to produce artificial monstrosities are significant, though his purpose was rather to obtain proof of the inadequacy of the preformation hypothesis.[104]

It seems probable enough that if Geoffroy had developed his views on evolution he would finally have been led to interpret unity of plan in terms of genetic relationship. But as it was he remained at his morphological standpoint. He did not interpret rudimentary organs as useless heritages of the past; he preferred to think that Nature had prepared double means for the same function, one or other being predominant according as the animal lived in the water or on the land.

"To the animal that lives exclusively in the air Nature has granted an organisation suited to this mode of respiration, without however suppressing the other corresponding means, that is to say, without depriving it of a second system which is applicable only to the mode of respiration by the intermediary of water, and _vice versa_."[105]

He seems, in one instance at least, to have hit upon the root-idea of the biogenetic law, but he was far from appreciating its significance.

He recognised that an amphibian in its development pa.s.sed through a stage when it was in all essentials similar to a fish, and he saw in this visible transformation a picture of the evolutionary transformation. "An amphibian," he writes,[106] "is at first a fish under the name of tadpole, and then a reptile [_sic_] under that of frog....

In this observed fact is realised what we have above represented as an hypothesis, the transformation of one organic stage into the stage immediately superior." But it is not clear that he considered the development of the amphibian to be a _repet.i.tion_ of its ancestral history.

He went, however, a certain length towards recognising the main principle of a law which was a commonplace of German transcendental thought, and was developed later by his disciple E. Serres, the law that the higher animals repeat during their development the main features of the adult organisation of animals lower in the scale. Thus he compared fish as regards certain parts of their structure with the foetus of mammals. He compared also Articulates with embryonic Vertebrates in respect of their vertebrae, for in the higher Vertebrates the body of the vertebra is tubular at an early stage of development, and in Articulates the body of the vertebra remains tubular permanently (_supra_, p. 61).

As regards their vertebrae, "insects occupy a place in the series of the ages and developments of the vertebrate animals, that is to say, they realise one of the states of their embryo, as fishes do one of the states of their foetal condition."[107]

This idea was destined to exercise a great influence upon the development of morphology. A further development of the thought is that certain abnormalities in the higher animals, resulting from arrest of development, represent states of organisation which are permanent in the lower animals.[108]