Читать книгу Form and Function - E. S. Russell - Страница 9

For two thousand years after Aristotle little advance was made upon his comparative anatomy. Knowledge of the human body was increased not long after his death by Herophilus and Erasistratus, but not even Galen more than four centuries later made any essential additions to Aristotle's anatomy.

During the Middle Ages, particularly after the introduction to Europe in the 13th century of the Arab texts and commentaries, Aristotle dominated men's thoughts of Nature. The commentary of Albertus Magnus, based upon that of Avicenna, did much to impose Aristotle upon the learned world. Albertus seems to have contented himself with following closely in the footsteps of his master. There are noted, however, by Bonnier certain improvements made by Albertus on Aristotle's view of the seriation of living things. "He is the first," writes Bonnier, "to take the correct view that fungi are lower plants allied to the most lowly organised animals. From this point there start, for Albertus Magnus, two series of living creatures, and he regards the plant series as culminating in the trees which have well-developed flowers."[9]

Aristotle's influence is predominant also in the work of Edward Wotton (1492–1555), who in his book De differentiis animalium adopted a classification similar to that proposed by Aristotle. He too laid stress upon the gradation shown from the lower to the higher forms.

In the 16th century, two groups of men helped to lay foundations for a future science of comparative anatomy—the great Italian anatomists Vesalius, Fallopius and Fabricius, and the first systematists (though their "systems" were little more than catalogues) Rondeletius, Aldrovandus and Gesner.

The anatomists, however, took little interest in problems of pure morphology; the anatomy of the human body was for them simply the necessary preliminary of the discovery of the functions of the parts—they were quite as much physiologists as anatomists.

One of them, Fabricius, made observations on the development of the chick (1615). Harvey, who was a pupil of Fabricius, likewise published an account of the embryology of the chick.[10] In his philosophy and habit of thought Harvey was a follower of Aristotle. It is worth noting that in his Exercitationes anatomicae de motu cordis (1628) there is a passage which dimly foreshadows the law of recapitulation in development which later had so much vogue.[11]

A stimulating contribution to comparative anatomy was made by Belon,[12] who published in 1555 a Histoire de la nature des Oyseaux, in which he showed opposite one another a skeleton of a bird and of a mammal, giving the same names to homologous bones. The anatomy of animals other than man was indeed not altogether neglected at this time. Coiter (1535–1600) studied the anatomy of Vertebrates, discovering among other things the fibrous structure of the brain. Carlo Ruini of Bologna wrote in 1598 a book on the anatomy of the horse.[13] Somewhat later Severino, professor at Naples, dissected many animals and came to the conclusion that they were built upon the same plan as man.[14] Willis, of Oxford and London, in his Cerebri Anatome (1659) recognised the necessity for comparative study of the structure of the brain. He found out that the brain of man is very like that of other mammals, the brain of birds, on the contrary, like that of fishes![15] He described the anatomy of the oyster and the crayfish. He had, however, not much feeling for morphology.

The foundation of the Jardin des Plantes at Paris in 1626 and the subsequent addition to it of a Museum of Natural History and a menagerie gave a great impulse to the study of comparative anatomy by supplying a rich material for dissection. Advantage was taken of these facilities, particularly by Claude Perrault and Duverney.[16] In a volume entitled De la Mécanique des Animaux, Perrault recognises clearly the idea of unity of type, and even pushes it too far, seeking to prove that in plants there exists an arterial system and veins provided with valves.[17]

The beginning of the 17th century saw the invention of the microscope, which was to have such an enormous influence upon the development of biological studies. It did not come into scientific use until well on in the middle of the century. Just before it came into use Francis Glisson (1597–1677), an Englishman, gave in the introduction to his treatise on the liver an account of the notions then current on the structure of organic bodies. He classifies the parts as "similar" and "organic," the former determined by their material, the latter by the form which they assume. The similar parts are divided into the sanguineous or rich in blood and the spermatic. Both sets are further subdivided according to their physical characters,[18] the latter, for instance, into the hard, soft, and tensile tissues. The classification resembles greatly that propounded by Aristotle, though it is notably inferior in the details of its working out.

For Aristotle, as for all anatomists before the days of the microscope, the tissues were not much more than inorganic substances, differing from one another in texture, in hardness, and other physical properties. They possessed indeed properties, such as contractility, which were not inorganic, but as far as their visible structure was concerned there was little to raise them above the inorganic level. The application of the microscope changed all that, for it revealed in the tissues an organic structure as complex in its grade as the gross and visible structure of the whole organism. Of the four men who first made adequate use of the new aid, Malpighi, Hooke, Leeuenhoek, and Swammerdam, the first-named contributed the most to make current the new conceptions of organic structure. He studied in some detail the development of the chick. He described the minute structure of the lungs (1661), demonstrating for the first time, by his discovery of the capillaries, the connection of the arteries with the veins. In his work, De viscerum structura (1666), he describes the histology of the spleen, the kidney, the liver, and the cortex of the brain, establishing among other things the fact that the liver was really a conglomerate gland, and discovering the Malpighian bodies in the kidney. This work was done on a broad comparative basis. "Since in the higher, more perfect, red-blooded animals, the simplicity of their structure is wont to be involved by many obscurities, it is necessary that we should approach the subject by the observation of the lower, imperfect animals."[19] So he wrote in the De viscerum structura, and accordingly he studied the liver first in the snail, then in fishes, reptiles, mammals, and finally man. In the introduction to his Anatome plantarum (1675), in which he laid the foundations of plant histology, he vindicates the comparative method in the following words:—"In the enthusiasm of youth I applied myself to Anatomy, and although I was interested in particular problems, yet I dared to pry into them in the higher animals. But since these matters enveloped in peculiar mystery still lie in obscurity, they require the comparison of simpler conditions, and so the investigation of insects[20] at once attracted me; finally, since this also has its own difficulties I applied my mind to the study of plants, intending after prolonged occupation with this domain, to retrace my steps by way of the vegetable kingdom, and get back to my former studies. But perhaps not even this will be sufficient; since the simpler world of minerals and the elements should have been taken first. In this case, however, the undertaking becomes enormous and far beyond my powers."[21] There is something fine in this life of broad outlines, devoted whole-heartedly to an idea, to a plan of research, which required a lifetime to carry out.

An important histological discovery dating from this time is that of the finer structure of muscle, made by Stensen (or Steno) in 1664. He described the structure of muscle-fibres, resolving them into their constituent fibrils.

To the microscope we owe not only histology but the comparative anatomy of the lower animals. Throughout the 17th and 18th centuries the discovery of structure in the lower animals went on continuously, as may be read in any history of Zoology.[22] We content ourselves here with mentioning only some representative names.

In the 17th century Leeuenhoek, applying the microscope almost at random, discovered fact after fact, his most famous discovery being that of the "spermatic animalcules."

Swammerdam studied the metamorphoses of insects and made wonderfully minute dissections of all sorts of animals, snails and insects particularly. He described also the development of the frog. It is curious to see what a grip his conception of metamorphosis had upon him when he homologises the stages of the frog's development with the Egg, the Worm, and the Nymph of insects (Book of Nature, p. 104, Eng. trans., 1785). He even speaks of the human embryo as being at a certain stage a Man-Vermicle.

In the 18th century, Réaumur and Bonnet continued the minute study of insects, laying more stress, however, on their habits and physiology than upon their anatomy. Lyonnet made a most laborious investigation of the anatomy of the willow-caterpillar (1762). John Hunter (1728–93) dissected all kinds of animals, from holothurians to whales. His interest was, however, that of the physiologist, and he was not specially interested in problems of form. It is interesting to note a formulation in somewhat confused language of the recapitulation theory. The passage occurs in his description of the drawings he made to illustrate the development of the chick. It is quoted in full by Owen (J. Hunter, Observations on certain Parts of the Animal Œconomy, with Notes by Richard Owen. London, 1837. Preface, p. xxvi). We give here the last and clearest sentence—"If we were to take a series of animals from the more imperfect to the perfect, we should probably find an imperfect animal corresponding with some stage of the most perfect."

The tendency of the time was not towards morphology, but rather to general natural history and to systematics, the latter under the powerful influence of Linnæus (1707–1778). The former tendency is well represented by Réaumur (1683–1757) with his observations on insects, the digestion of birds, the regeneration of the crayfish's legs, and a hundred other matters. To this tendency belong also Trembley's famous experiments on Hydra (1744), and Rösel von Rosenhof's Insektenbelustigungen (1746–1761).

Bonnet (1720–1793) deserves special mention here, since in his Traité d'Insectologie (1745), and more fully in his Contemplation de la Nature (1764), he gives the most complete expression to the idea of the Échelle des êtres.

This idea seems to have taken complete possession of his imagination. He extends it to the universe. Every world has its own scale of beings, and all the scales when joined together form but one, which then contains all the possible orders of perfection. At the end of the Preface to his Traité d'Insectologie (Œuvres, i., 1779) he gives a long table, headed "Idée d'une Échelle des êtres naturels," and rather resembling a ladder, on the rungs of which the following names appear:—

Man.	Shell Fish.	Stones.
Orang-utan.	Tube-worms.	Figured stones.
Ape.	Clothes-worms.	Crystals.
Quadrupeds.	Insectes.	Salts.
Flying squirrel.	Gall insectes.	Vitriols.
Bat.	Taenia.
Ostrich.	Polyps.	Metals.
	Sea Nettles.
Birds.	Sensitive plant.	Half-metals.
Aquatic birds.
Amphibious birds.	Plants.	Sulphurs.
Flying Fish.	Lichens.	Bitumens.
	Moulds.
Fish.	Fungi, Agarics.	Earths.
Creeping fish.	Truffles.	Pure earth.
Eels.	Corals, and Coralloids.
Water sepents.	Lithophytes.	Water.
	Asbestos.
Serpents.	Talc, Gypsums.	Air.
Slugs.	Selenites, Slates.
Snails.		Fire.
		More subtile matter.

The nature of the transitional forms which he inserts between his principal classes show very clearly his entire lack of morphological insight—the transitions are functional. The positions assigned to clothes-moths and corals are very curious! The whole scheme, so fantastic in its details, was largely influenced by Leibniz's continuity philosophy, and is in no way an improvement on the older and saner Aristotelian scheme.

Robinet, in the fifth volume of his book De la nature (1761–6), foreshadows the somewhat similar views of the German transcendentalists. "All beings," he writes, "have been conceived and formed on one single plan, of which they are the endlessly graduated variations: this prototype is the human form, the metamorphoses of which are to be considered as so many steps towards the most excellent form of being."[23]

The idea of a gradation of beings appears also in Buffon (1707–1788), but here it takes more definitely its true character as a functional gradation.[24] "Since everything in Nature shades into everything else," he says, "it is possible to establish a scale for judging of the degrees of the intrinsic qualities of every animal."[25]

He is quite well aware that the groups of Invertebrates are different in structural plan from the Vertebrates—"The animal kingdom includes various animated beings, whose organisation is very different from our own and from that of the animals whose body is similarly constructed to ours."[26]

He limits himself to a consideration of the Vertebrates, deeming that the economy of an oyster ought not to form part of his subject matter! He has a clear perception of the unity of plan which reigns throughout the vertebrate series.[27] What is new in Buffon is his interpretation of the unity of plan. For the first time we find clearly expressed the thought that unity of plan is to be explained by community of origin.

Buffon's utterances on this point are, as is well known, somewhat vacillating. The famous passage, however, which occurs in his account of the Ass shows pretty clearly that Buffon saw no theoretical objection to the descent of all the varied species of animals from one single form. Once admit, he argues, that within the bounds of a single family one species may originate from the type species by "degeneration," then one might reasonably suppose that from a single being Nature could in time produce all the other organised beings.[28] Elsewhere, e.g., in the discourse De la Dégéneration des Animaux,[29] Buffon expresses himself with more caution. He finds that it is possible to reduce the two hundred species of quadrupeds which he has described to quite a small number of families "from which it is not impossible that all the rest are derived."[30] Within each of the families the species branch off from a parent or type species. This we may note is a great advance on the linear arrangement implied in the idea of an Échelle des êtres.[31]

It is a mistake to suppose that Buffon was par excellence a maker of hypotheses. On the contrary he saw things very sanely and with a very open mind. He expressly mentions the great difficulties which one encounters in supposing that one species may arise from another by "degeneration." How does it happen that two individuals "degenerate" just in the right direction and to the right stage so as to be capable of breeding together? How is it that one does not find intermediate links between species? One is reminded of the objections, not altogether without validity, which were made to the Darwinian theory in its early days. I cannot agree with those who think that Buffon was an out-and-out evolutionist, who concealed his opinions for fear of the Church. No doubt he did trim his sails—the palpably insincere "Mais non, il est certain, par la révélation, que tous les animaux ont également participé à la grace de la création,"[32] following hard upon the too bold hypothesis of the origin of all species from a single one, is proof of it. But he was too sane and matter-of-fact a thinker to go much beyond his facts, and his evolution doctrine remained always tentative. One thing, however, he was sure of, that evolution would give a rational foundation to the classification which, almost in spite of himself, he recognised in Nature. If, and only if, the species of one family originated from a single type species, could families, be founded rationally, avec raison.

Buffon was, curiously enough, rather unwilling to recognise any systematic unit higher than the species. Strictly speaking there are only individuals in Nature; but there are also groups of individuals which resemble one another from generation to generation and are able to breed together. These are species—Buffon adheres to the genetic definition of species—and the species is a much more definite unit than the genus, the order, the class, which are not divisions imposed by us upon Nature. Species are definitely discontinuous,[33] and this is the only discontinuity which Nature shows us. Buffon put his views into practice in his Histoire Naturelle, where he describes species after species, never uniting them into larger groups. We have seen, however, how the facts forced upon him the conception of the "family."

Buffon was no morphologist. He left to Daubenton what one might call the "dirty work" of his book, the dissection and minute description of the animals treated.

But Buffon was a man of genius, and accordingly his ideas on morphology are fresh and illuminating. Few naturalists have been so free from the prejudices and traditions of their trade. He makes in the Discours sur la Nature des Animaux[34] a distinction, which Bichat and Cuvier later developed with much profit, between the "animal" and the "vegetative" part of animals.[35] The vegetative or organic functions go on continuously, even in sleep, and are performed by the internal organs, of which the heart is the central one. The active waking life of the animal, that part of its life which distinguishes it from the plant, involves the external parts—the sense-organs and the extremities. An animal is, as it were, made up of a complex of organs performing the vegetative functions, assimilation, growth, and reproduction, surrounded by an envelope formed by the limbs, the sense-organs, the nerves and the brain, which is the centre of this "envelope."[36] Animals may differ from one another enormously in the external parts, particularly in the appendicular skeleton, without showing any great difference in the plan and arrangement of their internal organs. Quadrupeds, Cetacea, birds, amphibians and fish are as unlike as possible in external form and in the shape of their limbs; but they all resemble one another in their internal organs. Let the internal organs change, however—the external parts will change infinitely more, and you will get another animal, an animal of a totally different nature. Thus an insect has a most singular internal economy, and, in consequence, you find it is in every point different from any vertebrate animal.

In this contrast, on the whole justified, between the importance of variations in the "vegetative" and variations in the "animal" parts, one may see without doing violence to Buffon's thought, an indication of the difference between homology and analogy. It is usually in the external parts, in the organs by which the animal adapts itself to its environment, that one meets with the greatest number of analogical resemblances. This contrast of vegetative and animal parts and their relative importance for the discovery of affinities was at any rate a considerable step towards an analysis of the concept of unity of plan.

To Xavier Bichat (1771–1802) belongs the credit of working out in detail the distinction drawn by Aristotle and Buffon between the animal and the vegetative functions. Bichat was not a comparative anatomist; his interest lay in human anatomy, normal and pathological. So his views are drawn chiefly from the consideration of human structure.

He classifies functions into those relating to the individual and those relating to the species. The functions pertaining to the individual may be divided into those of the animal and those of the organic life.[37] "I call animal life that order of functions which connects us with surrounding bodies; signifying thereby that this order belongs only to animals" (p. lxxviii.). Its organs are the afferent and efferent nerves, the brain, the sense-organs and the voluntary muscles; the brain is its central organ. "Digestion, circulation, respiration, exhalation, absorption, secretion, nutrition, calorification, or production of animal heat, compose organic life, whose principal and central organ is the heart" (p. lxxix.).

The contrast of the animal and the organic life runs through all Bichat's work; it receives classical expression in his Recherches Physiologiques sur la Vie et la Mort (1800). The plant and the animal stand for two different modes of living. The plant lives within itself, and has with the external world only relations of nutrition; the animal adds to this organic life a life of active relation with surrounding things (3rd ed., 1805, p. 2). "One might almost say that the plant is the framework, the foundation of the animal, and that to form the animal it sufficed to cover this foundation with a system of organs fitted to establish relations with the world outside. It follows that the functions of the animal form two quite distinct classes. One class consists in a continual succession of assimilation and excretion; through these functions the animal incessantly transforms into its own substance the molecules of surrounding bodies, later to reject these molecules when they have become heterogeneous to it. Through this first class of functions the animal exists only within itself; through the other class it exists outside; it is an inhabitant of the world, and not, like the plant, of the place which saw its birth. The animal feels and perceives its surroundings, reflects its sensations, moves of its own will under their influence, and, as a rule, can communicate by its voice its desires and its fears, its pleasures or its pains. I call organic life the sum of the functions of the former class, for all organised creatures, plants or animals, possess them to a more or less marked degree, and organised structure is the sole condition necessary to their exercise. The combined functions of the second class form the 'animal' life, so named because it is the exclusive attribute of the animal kingdom" (pp. 2–3).

In both lives there is a double movement, in the animal life from the periphery to the centre and from the centre to the periphery, in the organic life also from the exterior to the interior and back again, but here a movement of composition and decomposition. As the brain mediates between sensation and motion, so the vascular system is the go-between of the organs of assimilation and the organs of dissimilation.

The most essential structural difference between the organs of animal life and the organs of organic life is, in man and the higher animals at least, the symmetry of the one set and the irregularity of the other—compare the symmetry of the nerves and muscles of the animal life with the asymmetrical disposition of the visceral muscles and the sympathetic nerves, which belong to the organic life.

Noteworthy differences exist between the two lives with respect to the influence of habit. Everything in the animal life is under the dominion of habit. Habit dulls sensation, habit strengthens the judgment. In the organic life, on the contrary, habit exercises no influence. The difference comes out clearly in the development of the individual. The organs of the organic life attain their full perfection independently of use; the organs of the animal life require an education, and without education they do not reach perfection (loc. cit., p. 127).

Bichat was the founder of what was known for a time as General Anatomy—the study of the constituent tissues of the body in health and disease. His classification of tissues was macroscopical and physiological; he relied upon texture and function in distinguishing them rather than upon microscopical structure. The tissues he distinguished are as follows:—[38]

1.	The cellular membrane.	12.	Fibro-cartilage.
2.	Nerves of animal life.	13.	Muscles of organic life.
3.	Nerves of organic life.	14.	Muscles of animal life.
4.	Arteries.	15.	Mucous membrane.
5.	Veins.	16.	Serous membrane.
6.	Exhalants.	17.	Synovial membrane.
7.	Absorbents and glands.	18.	The Glands.
8.	Bones.	19.	The Dermis.
9.	Medulla.	20.	Epidermis.
10.	Cartilage.	21.	Cutis.
11.	Fibrous tissue.

The "cellular membrane" seems to mean undifferentiated connective tissue; "exhalants" are imperceptible tubes arising from the capillaries and secreting fat, serum, marrow, etc.; the "absorbents and glands" are the lymphatics and the lymphatic glands.

In Bichat's eyes this resolution of the organism into tissues had a deeper significance than any separation into organs, for to each tissue must be attributed a vie propre, an individual and peculiar life. "When we study a function we must consider the complicated organ which performs it in a general way; but if we would be instructed in the properties and life of that organ we must absolutely resolve it into its constituent parts."[39] The tissues have, too, a great importance for pathology, for diseases are often diseases of tissues rather than of organs.[40]

[9] Le Monde végétal, p. 41, Paris, 1907.

[10] Exercitationes de generatione animalium, 1651. For an account of Harvey's work on generation and development, see Em. Rádl's masterly Geschichte der biologischen Theorien, i., pp. 31–8, Leipzig, 1905.

[11] The passage runs:—"Sic natura perfecta et divina nihil faciens frustra, nec quipiam animali cor addidit, ubi non erat opus, neque priusquam esset ejus usus, fecit; sed iisdem gradibus in formatione cujuscumque animalis, transiens per omnium animalium constitutiones (ut ita dicam) ovum, vermem, fœtum, perfectionem in singulis acquirit."

[12] See I. Geoffroy St. Hilaire, Essais de Zoologie générale, p. 71, Paris, 1841.

[13] M. Foster, Lectures on the History of Physiology, Cambridge, p. 53, 1901.

[14] Zootomia democritea, Nuremberg, 1645; Antiperipatias, seu de respiratione piscium, Amsterdam, 1661.

[15] Rádl, loc. cit., i., p. 50.

[16] Perrault et Duverney, Mémoires pour servir à l'histoire des Animaux, Paris, 1699.

[17] F. Houssay, Nature et Sciences naturelles, Paris, p. 76, n.d.

[18] Foster, loc. cit., p. 85.

[19] Trans. by Foster, loc. cit., p. 113.

[20] He made a careful study of the silkworm.

[21] "Etenim, ferventi actatis calore, Anatomica aggressus, licet circa peculiaria fuerim solicitus, in perfectioribus tamen haec rimari sum ausus. Verum, cum haec propriis tenebris obscura jaceant, simplicium analogismo egent; inde insectorum indago illico arrisit; quae cum et ipsa suas habeat difficultates ad Plantarum perquisitionem animum postremo adjeci, ut diu hoc lustrato mundo gressu retroacto Vegetantis Naturae gradu, ad prima studia iter mihi aperirem. Sed nec forte hoc ipsum sufficiet cum simplicior Mineralium Elementorumque mundus praeire debeat. At in immensum excrescit opus, et meis viribus omnino impar," Opera Omnia, i., p. 1, London, 1686.

[22] See particularly E. Rádl, loc. cit.. 1 Teil. J. V.. Carus, Geschichte der Zoologie, München, 1872.

[23] For a good historical account of the gradation theories see Thienemann's paper in the Zoologische Annalen (Würzburg) iii., pp. 185–274, 1910, from which the quotation from Robinet is taken.

[24] Histoire naturelle, i., p. 13; ii, p. 9; iv., p. 101; and xiv., pp. 28–9, 1749 and later.

[25] No translation can render the beauty of the original—"Comme tout se fait et que tout est par nuance dans la Nature … " (iv., p. 101).