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 pa.s.sage, however, which occurs in his account of the a.s.s 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 Degeneration 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 _ech.e.l.le des etres_.[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 revelation, que tous les animaux ont egalement participe a la grace de la creation,"[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 cla.s.sification 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 cla.s.s, 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, a.s.similation, 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 h.o.m.ology and a.n.a.logy. 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 a.n.a.logical 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 a.n.a.lysis 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 cla.s.sifies 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 princ.i.p.al 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 cla.s.sical 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 cla.s.ses. One cla.s.s consists in a continual succession of a.s.similation 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 cla.s.s of functions the animal exists only within itself; through the other cla.s.s 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 cla.s.s, 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 cla.s.s 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 a.s.similation 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 const.i.tuent tissues of the body in health and disease. His cla.s.sification 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.

2. Nerves of animal life.

3. Nerves of organic life.

4. Arteries.

5. Veins.

6. Exhalants.

7. Absorbents and glands.

8. Bones.

9. Medulla.

10. Cartilage.

11. Fibrous tissue.

12. Fibro-cartilage.

13. Muscles of organic life.

14. Muscles of animal life.

15. Mucous membrane.

16. Serous membrane.

17. Synovial membrane.

18. The Glands.

19. The Dermis.

20. Epidermis.

21. Cutis.

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 const.i.tuent 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 vegetal_, p. 41, Paris, 1907.

[10] _Exercitationes de generatione animalium_,1651. For an account of Harvey's work on generation and development, see Em. Radl's masterly _Geschichte der biologischen Theorien_, i., pp. 31-8, Leipzig, 1905.

[11] The pa.s.sage 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 cujusc.u.mque animalis, transiens per omnium animalium const.i.tutiones (ut ita dicam) ovum, vermem, foetum, perfectionem in singulis acquirit."

[12] See I. Geoffroy St Hilaire, _Essais de Zoologie generale_, 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] Radl, _loc. cit._, i., p. 50.

[16] Perrault et Duverney, _Memoires pour servir a 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 aetatis calore, Anatomica aggressus, licet circa peculiaria fuerim solicitus, in _perfectioribus_ tamen haec rimari sum ausus. Verum, c.u.m haec propriis tenebris obscura jaceant, simplicium a.n.a.logismo egent; inde _insectorum_ indago illico arrisit; quae c.u.m et ipsa suas habeat difficultates ad Plantarum perquisitionem animum _postremo_ adjeci, ut diu hoc l.u.s.trato mundo gressu retroacto Vegetantis Naturae gradu, ad prima studia iter mihi aperirem. Sed nec forte hoc ipsum sufficiet c.u.m 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. Radl, _loc. cit._. I Teil. J. V.

Carus, _Geschichte der Zoologie_, Munchen, 1872.

[23] For a good historical account of the gradation theories see Thienemann's paper in the _Zoologische Annalen_(Wurzburg) 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).

[26] _Hist. nat._, iv., p. 5.

[27] See particularly his comparison of the skeleton of the horse with that of man. _Hist. Nat._, iv., p. 381, also p. 13.