The entomologist Latreille also tackled the problem of the h.o.m.ologies of the segments in the different cla.s.ses of Arthropods (Cuvier, _loc.
cit._, p. cclxxii.). He thought he could find fifteen segments in all Arthropods. He made the retrograde step of likening the head of insects to a single segment. But some of his h.o.m.ologies showed morphological insight, _e.g._, his comparison of the "first jaws" of Arachnids to antennae, because they were placed above the upper lip. It was he who first pointed out the resemblance of the leaf-like gills of Ephemerid larvae to wings, and suggested that wings were "a sort of tracheal feet."
He made also a rather hazy and speculative contribution on Okenian lines to the problem of the relation of Arthropods to Vertebrates, likening the carapace of Crustacea to an enormously developed hyoid, the appendages of the tail to the ventral and a.n.a.l fins of fish. The masticatory organs of Arthropods were jaws disjointed at their symphysis; antennae, nostrils turned outside in.
Duges also made a comparison of Articulates with Vertebrates.[139] He did not accept Geoffroy"s vertebral theory of the Arthropod skeleton, though he admitted that in Arthropods the dorsal surface was turned towards the ground, basing this a.s.sumption on the position of the nervous system, and also, curiously enough, on the inverted position of the embryo on the lower surface of the yolk. He considered that the mandibles and first maxillae of Arthropods were the h.o.m.ologues of the upper and lower jaws of Vertebrates, adducing as confirmatory evidence the fact that in snakes the rami are separate. The labium was the equivalent of the hyoid, the l.a.b.i.al palps and maxillipedes the equivalent of the "hyoid"
elements which form the branchial arches.
But Duges" main contribution to morphological method was his conception of the living organism as a colony of lesser units, which were themselves real "organisms." "By _organism_ the author means a complex of organs which taken together suffice to const.i.tute, ideally or actually, a complete animal. An "organism" is, as it were, an elementary or simple animal; several organisms combined form a complex animal" (p.
255). Duges. .h.i.t upon this principle, which was first suggested to him by A. Moquin-Tandon"s work on the leech (1827), as a great aid in demonstrating the unity of plan and composition throughout the animal kingdom.[140] According to his view there are three main types of animals--(1) Biserials, including bilaterally symmetrical animals, composed of two parallel series of "organisms"; (2) Radiates, composed of "organisms" arranged like the spokes of a wheel; and (3) Raceme-animals, in which the separate "organisms" were disposed more or less irregularly, in bunches (p. 257). The unitary "organism" is supposed to be the same in all, only the arrangement differing. Duges of course admitted that the centralisation of the complete organism became greater the higher it stood in the scale, and that this held good also in individual development. The appendages of Articulates and Vertebrates were thought of as the members of as many separate organisms. He went so far as to suggest that the fingers of a man"s hand were the free extremities of as many thoracic members.
Duges" conception of the organism has often been revived since in a saner form, _e.g._, by E. Perrier, and it has a certain validity. It has much affinity with the similar conceptions of Goethe and the German transcendentalists.
[130] _Mem. Acad. Sci._, iv., pp. cclx.x.xiv.-ccci., 1824.
[131] _Ann. Sci. Nat._, xi., xii., 1827; xvi., 1829; xxi., 1830.
[132] See Radl, _loc. cit._, i., pp. 225-6.
[133] _Ann. Sci. nat._ (2), ii., p. 248, 1834.
[134] _Ann. Sci. nat._, iii., pp. 377-80, 1824.
[135] _Memoires sur les Animaux sans Vertebres_, Part I., p. 10, Paris, 1816.
[136] _Ann. Sci. Nat._, (1), i., pp. 97-135, 416-432, 1824.
[137] _Isis_, p. 456, 1820 (2).
[138] Cuvier, _Mem. Acad. Sci._, iv., p. cclxx., 1824.
[139] _Acad. Sci._ 18th Oct. 1831. Extract in _Ann. Sci.
Nat._, xxiv., pp. 254-60, 1831.
[140] His views were more fully elaborated in his _Memoire sur la conformite organique dans l"ech.e.l.le animale_, Montpellier, 1832.
CHAPTER VII
THE GERMAN TRANSCENDENTALISTS
To complete our historical survey of the morphology of the early 19th century we have now to turn back some way and consider the curious development of morphological thought in Germany under the influence of the _Philosophy of Nature_. We have already seen many of these notions foreshadowed by Goethe, who had considerable affinity with the transcendentalists, but the full development of transcendental habits of thought comes a little later than the bulk of Goethe"s scientific work, and owes more to Kielmeyer and Oken than to Goethe himself.
A great wave of transcendentalism seems to have pa.s.sed over biological thought in the early 19th century, arising mainly in Germany, but powerfully affecting, as we have seen, the thought of Geoffroy and his followers. Many ideas were common to the French and German schools of transcendental anatomy, the fundamental conception that there exists a unique plan of structure, the idea of the scale of beings, the notion of the parallelism between the development of the individual and the evolution of the race. It is difficult to disentangle the part played by each school and to determine which should have the credit for particular theories and discoveries. The philosophy seems to have come chiefly from Germany, the science from France. It must be borne in mind that German comparative anatomy was largely derivative from French, that the Paris Museum was the acknowledged anatomical centre, and that Cuvier was its acknowledged head.
It is probably correct to say that the credit mainly belongs to the German transcendental school for the law of the parallelism between the stages of individual development and the stages of the scale of beings, and the theory of the repet.i.tion or multiplication of parts within the individual. The vertebral theory of the skull is a particular application of the second of these generalisations.
The law of parallelism[141] seems to have been expressed first by Kielmeyer (1793),[142] who gave to it a physiological form, saying that the human embryo shows at first a purely vegetative life, then becomes like the lower animals, which move but have no sensation, and finally reaches the level of the animals that both feel and move.
The idea was next taught by Autenrieth in 1797.[143]
Oken (1779-1851) in his early tract _Die Zeugung_ (1805), and in his _Lehrbuch der Naturphilosophie_ (1809-11) elaborated the thought, and taught that every animal in its development pa.s.ses through the cla.s.ses immediately below it. "During its development the animal pa.s.ses through all stages of the animal kingdom. The foetus is a representation of all animal cla.s.ses in time."[144] The Insect, for example, is at first Worm, next Crab, then a perfect volant animal with limbs, a Fly (_ibid._, p.
542).
As Nature is "the representation of the individual activities of the spirit," so the animal kingdom is the representation of the activities or organs of man. The animal kingdom is therefore "a dismemberment of the highest animal, _i.e._, of Man" (p. 494). Now "animals are gradually perfected, entirely like the single animal body, by adding organ unto organ"--the way of evolution is the way of development. Hence "animals are only the persistent foetal stages or conditions of Man," who is the microcosm, and contains within himself all the animal kingdom.
Oken was himself a careful student of embryology; von Baer[145] speaks of his work (published in Oken and Kieser, _Beitrage zur vergleichenden Zoologie, Anatomie und Physiologie_, 2 pts., 1806-7) as forming the turning-point in our understanding of the mammalian ovum. He had accordingly actually observed a resemblance in certain details of structure between the human foetus and the lower animals; but the peculiar form which the law took in his hands was a consequence of his hazy philosophy. He saw the relation of teratological to foetal structure, for he affirmed that "malformations are only persistent foetal conditions" (p. 492).
The idea of comparing the embryo of higher animals with the adult of lower was widely spread at this time among German zoologists. We find, for example, in Tiedemann"s brilliant little textbook[146] the statement that "Every animal, before reaching its full development, pa.s.ses through the stage of organisation of one or more cla.s.ses lower in the scale, or, every animal begins its metamorphosis with the simplest organisation"
(p. 57).
Thus the higher animals begin life as a kind of fluid animal jelly which resembles the substance of a polyp; the young mammal, like the lower Vertebrates, has only a simple circulation, and, like them, lives in water (the amniotic fluid); the frog is first like a worm, then develops gills and becomes like a fish (p. 57). In his work on the anatomy of the brain,[147] Tiedemann established the h.o.m.ology of the optic lobes in birds by comparing them with foetal corpora quadrigemina in man (see Serres, _Ann. Sci. nat._, xii., p. 112).
J. F. Meckel, in 1811, devoted a long essay to a detailed proof of the parallelism between the embryonic states of the higher animals and the permanent states of the lower animals. In a previous memoir in the same collection[148] (i., 1, 1808) he had made some comparisons of this kind in dealing with the development of the human foetus; in this memoir (ii., 1, 1811) he brings together all the facts which seem to prove the parallelism.
His collection of facts is a very heterogeneous one; he mingles morphological with physiological a.n.a.logies, and makes the most far-fetched comparisons between organs belonging to animals of the most diverse groups. He compares, for instance, the placenta with the gills of fish, of molluscs and of worms, h.o.m.ologising the cotyledons with the separate tufts of gills in _Tethys, Scyllaea_ and _Arenicola_(p. 26).
This is purely a physiological a.n.a.logy. He compares the closed a.n.u.s of the early human embryo with the permanent absence of an a.n.u.s in Coelentera, and the embryo"s lack of teeth with the absence of teeth in many reptiles and fish, in birds, and in many Cetacea (p. 46).[149] These are merely chance resemblances of no morphological importance. He considers bladderworms as animals which have never escaped from their amnion, and _Volvox_ as not having developed beyond the level of an egg (p. 7). He lays much stress upon likeness of shape and of relative size, comparing, for instance, the large multilobate liver of the human foetus with the many-lobed liver of lower Vertebrates and of Invertebrates. In general he shows himself, in his comparisons, lacking in morphological insight.
His treatment of the vascular system affords perhaps the best example of his method (pp. 8-25). The simplest form of heart is the simple tubular organ in insects, and it is under this form that the heart first appears in the developing chick. The bent form of the embryonic heart recalls the heart of spiders; it lies at first free, as in the mollusc _Anomia_.
The heart consists at first of one chamber only, recalling the one-chambered heart of Crustacea. A little later three chambers are developed, the auricle, ventricle, and aortic bulb; at this stage there is a resemblance to the heart of fish and amphibia. At the end of the fourth day the auricle becomes divided into two, affording a parallel with the adult heart of many reptiles.
In his large text-book of a somewhat later date, the _System der vergleichenden Anatomie_ (i., 1821), he works out the idea again and gives to it a much wider theoretic sweep, hinting that the development of the individual is a repet.i.tion of the evolutionary history of the race. Meckel was a timid believer in evolution. He thought it quite possible that much of the variety of animal form was due to a process of evolution caused by forces inherent in the organism. "The transformations," he writes, "which have determined the most remarkable changes in the number and development of the instruments of organisation are incontestably much more the consequence of the tendency, inherent in organic matter, which leads it insensibly to rise to higher states of organisation, pa.s.sing through a series of intermediate states."[150]
His final enunciation of the law of parallelism in this same volume shows that he considered the development of the individual to be due to the same forces that rule evolution. "The development of the individual organism obeys the same laws as the development of the whole animal series; that is to say, the higher animal, in its gradual evolution, essentially pa.s.ses through the permanent organic stages which lie below it; a circ.u.mstance which allows us to a.s.sume a close a.n.a.logy between the differences which exist between the diverse stages of development, and between each of the animal cla.s.ses" (p. 514).
He was not, of course, able fully to prove his contention that the lower animals are the embryos of the higher, and we gather from the following pa.s.sage that he could maintain it only in a somewhat modified form. "It is certain," he writes, "that if a given organ shows in the embryo of a higher animal a given form, identical with that shown throughout life by an animal belonging to a lower cla.s.s, the embryo, in respect of this portion of its economy, belongs to the cla.s.s in question" (p. 535). The embryo of a Vertebrate might at a certain stage of development, be called a mollusc, if for instance, it had the heart of a mollusc.
He admits, too, that the highest animal of all does not pa.s.s through in his development the entire animal series. But the embryo of man always and necessarily pa.s.ses through many animal stages, at least as regards its single organs and organ-systems, and this is enough in Meckel"s eyes to justify the law of parallelism (p. 535).
In his excellent discussion of teratology Meckel points out how the idea of parallelism throws light upon certain abnormalities which are found to be normal in other (lower) forms (p. 556).[151]
We may refer to one other statement of the law of parallelism--by K. G.
Carus in his _Lehrbuch der vergleichenden Anatomie_ (Leipzig, 1834). The standpoint is again that of _Naturphilosophie_. It is a general law of Nature, Carus thinks, that the higher formations include the lower; thus the animal includes the vegetable, for it possesses the "vegetative" as well as the "animal" organs. So it is, too, by a rational necessity that the development of a perfect animal repeats the series of antecedent formations.
As we have said, the main credit for the enunciation of the law of parallelism belongs to the German transcendental school; but the law owes much also to Serres, who, with Meckel, worked out its implications.
It might for convenience, and in order to distinguish it from the laws later enunciated by von Baer and Haeckel, be called the law of Meckel-Serres.
Under the "theory of the repet.i.tion or multiplication of parts within the organism" may be included, first, generalisations on the serial h.o.m.ology of parts, and second, more or less confused attempts to demonstrate that the whole organisation is repeated in certain of the parts. The recognition of serial h.o.m.ologies const.i.tuted a real advance in morphology; the "philosophical" idea of the repet.i.tion of the whole in the parts led to many absurdities. It led Oken to a.s.sert that in the head the whole trunk is repeated, that the upper jaw corresponds to the arms, the lower to the legs, that in each jaw the same bony divisions exist as in the limbs, the teeth, for instance, corresponding to the claws (_loc. cit._, p. 408). It led him to distinguish "two animals" in every body--the cephalic and the s.e.xual animal. Each of these has its own organs; thus "in the perfect animal there are two intestinal systems thoroughly distinct from each other, two intestines which belong to two different animals, the s.e.xual and cephalic animal, or the plant and the animal" (p. 382). The intestine of the s.e.xual animal is the large intestine; the lungs of the s.e.xual animal are the kidneys, its glottis is the urethra, its mouth the a.n.u.s. So, too, the mouth is the stomach of the head. On another line of thought the sternum is a ventral vertebral column. Limbs are connate ribs, the digits indicating the number of ribs included (_cf._ Duges, _supra_, p. 88).
J. F. Meckel[152] discusses "h.o.m.ologies" of this kind in the thorough and pedestrian way so characteristic of him. Not only, he says, are the right and left halves of the body comparable with one another, but also the upper and the lower, the dividing line being drawn at the level of the diaphragm. The lumbar complex corresponds to the skull, the a.n.u.s to the mouth, the urino-genital opening to the nasal opening; in general, the urino-genital system corresponds to the respiratory, the kidneys to the lungs, the ureters to bronchi, the testes and ovaries to the thymus (he had observed the physiological relation between the development of the thymus and the state of the genital organs), the prostate and the uterus to the thyroid gland, and the p.e.n.i.s and c.l.i.toris to the tongue.
The fore-limbs and girdle correspond in detail with the hind limbs and the pelvis--a point already worked out by Vicq d"Azyr; the dorsal and ventral halves of the body are likewise comparable in some respects, the sternum, for example, answering in the arrangement of its bones, muscles and arteries to the vertebral column. The skeleton of each member is in some respects a repet.i.tion of the vertebral column.
His brother, D. A. Meckel,[153] worked out an elaborate comparison between the alimentary ca.n.a.l and the genital organs, basing the legitimacy of the comparison upon early embryological relations and upon the state of things in Coelentera, where genital and digestive organs occupy the same cavity. In his view the uterus corresponded to the stomach, the v.a.g.i.n.a to the oesophagus, the fallopian tubes to the intestine, and so on.
The vertebral theory of the skull took its origin from the same habit of thought. As part of the wider idea of the metameric repet.i.tion of parts it had some scientific worth, but the theory was pushed too far, and the facts were twisted to suit it. Among annulate animals the theory of repet.i.tion found ample scope; Oken was able to compare with justice the jaws of crabs and insects with their other limbs, as Savigny did later in a more scientific way. Among Vertebrates the application of the theory of serial repet.i.tion was not so obvious, except in the case of the vertebrae. Goethe seems to have been the first to hit upon the idea that the skull is composed of a number of vertebrae, serially h.o.m.ologous with those of the vertebral column. He tells us that the idea flashed into his mind when contemplating in the Jewish cemetery at Venice a dried sheep"s skull. The discovery was made in 1790, but not published till 1820.[154]