Part 4

I have not been able to discover whether the _Zoonomia_, with its revolutionary ideas, attracted much attention at the time when it appeared, but it would seem not. In any case, it was afterwards so absolutely forgotten, that in an otherwise very complete _History of Zoology_, published in 1872 by Victor Carus, it was not even mentioned. About a year after the appearance of _Zoonomia_, Isidore Geoffrey St.-Hilaire in Paris expounded the view that what are called species are really only 'degenerations,' deteriorations from one and the same type, which shows that he too had begun to have doubts as to the fixity of species. Yet it was not till the third decade of the nineteenth century that he clearly and definitely took up the position of the doctrine of transformation, and to this we shall have to return later on.

But as early as the first decade of the century this position was taken up by two noteworthy naturalists, a German and a Frenchman, Treviranus and Lamarck.

Gottfried Reinhold Treviranus, born at Bremen in 1776, an excellent observer and an ingenious investigator, published, in 1802, a book entitled _Biologie, oder Philosophie der lebenden Natur_ [_Biology, or Philosophy of Animate Nature_], in which he expresses and elaborates the idea of the Evolution theory with perfect clearness. We read there, for instance: 'In every living being there exists a capacity for endless diversity of form; each possesses the power of adapting its organization to the variations of the external world, and it is this power, called into activity by cosmic changes, which has enabled the simple zoophytes of the primitive world to climb to higher and higher stages of organization, and has brought endless variety into nature.' But where the motive power lies, which brings about these transformations from the lowliest to ever higher forms of life, was a question which Treviranus apparently did not venture to discuss. To do this, and thus to take the first step towards a causal explanation of the assumed transformations, was left for his successor.

Jean Baptiste de Lamarck, born in 1744 in a village of Picardy, was first a soldier, then a botanist, and finally a zoologist. He won his scientific spurs first by his _Flora of France_, and zoology holds him in honour as the founder of the category of 'vertebrates.' Not that he occupied himself in particular detail with these, but he recognized the close alliance of the classes of animals in question--an alliance which was subsequently expressed by Cuvier by the systematic term 'type' or 'embranchement.'

In his _Philosophie zoologique_, published in 1809, Lamarck set forth a theory of evolution whose truth he attempted to vindicate by showing--as Treviranus had done before him--that the conception of species, on the immutability of which the whole hypothesis of creation had been based, was an artificial one, read into nature by us; that sharply circumscribed groups do not exist in nature at all; and that it is often very difficult, and not infrequently quite impossible, to define one species precisely from allied forms, because it is connected with these on all sides by transition stages. Groups of forms which thus melted into one another indicated that the doctrine of the fixity of species could not be correct, any more than that of their absolute nature. Species, he maintained, are not immutable, and are not so old as nature; they are fixed only for a certain time. The shortness of our life prevents our directly recognizing this. 'If we lived a much shorter time, say about a second, the hour-hand of the clock would appear to us to stand still, and even the combined observations of thirty generations would afford no decisive evidence as to the hand's movement, and yet it had been moving.'

The causes on which, according to Lamarck, the transformation of species, their modification into new species, depends, lie in the changes in the conditions of life which must have occurred ceaselessly from the earliest period of the earth's history till our own day, now here, now there, due in part to changes in climate and in food-supply, in part to changes in the earth's crust by the rising or sinking of land-masses, and so forth. These external changes have sometimes been the _direct_ cause of changes in bodily structure, as in the case of heat or cold; but they have sometimes and much more effectively operated _indirectly_. Thus changed conditions may have prompted an animal of a given species to use certain parts of its body in a new way, more vigorously, or less actively, or even not at all, and the more vigorous use, or, conversely, the disuse, has brought about variations in the organ in question.

Thus the whales lost their teeth when they abandoned their fish diet, and acquired the habit of feeding on minute and delicate molluscs, which they swallowed whole without seizure or mastication. Thus, too, the eyes of the mole degenerated through its life in the dark, and a still greater degeneration of the eyes has taken place in animals, like the proteus-salamander, which always inhabit lightless caves. In mussels both head and eyes degenerated because the animals could no longer use them after they became enclosed in opaque mantles and shells. In the same way snakes lost their legs _pari passu_ with the acquisition of the habit of moving along by wriggling their long bodies, and of creeping through narrow fissures and holes. On the other hand, Lamarck interpreted the evolution of the web-feet of swimming birds by supposing that some land-bird or other had formed the habit of going into the water to seek for food, and consequently of spreading out its toes as widely as possible so as to strike the water more vigorously. In this way the fold of skin between the toes was stretched, and as the extension of the toes was very frequent and was continued through many generations, the web expanded and grew larger, and thus formed the web-foot.

In the same way the long legs of the wading birds have been, according to Lamarck, gradually evolved by the continual stretching of the limbs by wading in deeper and deeper water, and similarly for the long necks and bills of the waders, the herons and the storks. Finally we may mention the case of the giraffe, whose enormously long neck and tall forelegs are interpreted as due to the fact that the animal feeds on the foliage of trees, and was always stretching as far as possible, in order to reach the higher leaves.

We shall see later in what a different way Charles Darwin explained this case of the giraffe. Lamarck's idea is at once clear; it is true that exercising an organ strengthens it, that disuse makes it weaker. Through much gymnastic exercise the muscles of the arm become thicker and more capable, and memory too may be improved, that is to say, even a definite part of the brain may be considerably strengthened by use. Indeed, we may now go so far as to admit that every organ is strengthened by use and weakened by disuse, and so far the foundations of Lamarck's interpretations are sound. But he presupposes something that cannot be admitted so readily, namely, that such 'functional' improvement or diminution in the strength of an organ can be transmitted by inheritance to the succeeding generation. We shall have to discuss this question in detail at a later stage, and I shall only say now that opinions as to whether this is possible or not are very much divided. I myself doubt this possibility, and therefore cannot admit the validity of the Lamarckian evolutionary principle in so far as it implies the directly transforming effect of the functioning of an organ. But even if we recognize the Lamarckian factor as a _vera causa_, it is easy to show that there are a great many characters which it is not in a position to interpret. Many insects which live upon green leaves are green, and not a few of them possess exactly the shade of green which marks the plant on which they feed; they are thus protected in a certain measure from injuries. But how could this green colour of the skin have been brought about by the activity of the skin, since the colour of the surroundings does not usually stimulate the skin to activity at all? Or how should a grasshopper, which is in the habit of sitting on dry branches of herbs, have thereby been incited to an activity which imparts to it the colour and shape of a dry twig? Just as little, or perhaps still less, can the protective green colour of a bird's or insect's eggs be explained through the direct influence of their usually green surroundings, even if we disregard the fact that the eggs are green when they are laid--that is, before the environment can have had any influence on them.

The Lamarckian principle of modification through use does not, in any case, nearly suffice as an interpretation of the transformations of the organic world. It must be allowed that Lamarck's theory of transformation was well founded at the time when it was advanced; it not only attacked the doctrine of the immutability of species, but sought for the first time to indicate the forces and influences which must be operative in the transformations of species; it was therefore well worth careful testing. Nevertheless it did not divert science from its chosen path; very little notice was taken of it, and in the great Cuvier's chronicle of scientific publications for 1809, not a syllable is devoted to Lamarck's book, so strong was the power of prejudice.

But, although the new doctrine was thus ignored, it did not altogether fall to the ground; it glimmered for a while in Germany, where it found its champions in the 'Naturphilosophie' of the time, and especially in Lorenz Oken, a peasant's son, born at Ortenau, near Offenburg, in 1783.

Oken professed views similar to those of Erasmus Darwin, Treviranus, and Lamarck, though they were not clothed in such purely scientific garb, being, in fact, bound up with the general philosophical speculations which came increasingly into favour at that time, chiefly through the writings of Schelling. In the same year, 1809, in which Lamarck published his _Philosophie zoologique_, Oken's _Lehrbuch der Naturphilosophie_ appeared.

This book is by no means simply a theory of descent; its scope is much wider, including the phenomena of the whole cosmos; on the other hand, it goes too little into details and is too indefinite to deserve its title. Its way of playing with ideas, its conjectures and inferences from a fanciful basis, make it difficult for us now to think ourselves into its mode of speculation, but I should like to give some indication of it, for it was just these speculative encroachments of the 'categories' of the so-called 'Naturphilosophie' which played a fatal part in causing the temporary disappearance of the Evolution-theory from science, so that, later on, it had to be established anew.

Oken defines natural science as 'the science of the everlasting transmutations of God (the Spirit) in the world': Every thing, considered in the light of the genetic process of the whole, includes, besides the idea of being, that of not-being, in that it is involved in a higher form. 'In these antitheses the category of polarity is included. The simpler elementary bodies unite into higher forms, which are thus merely repetitions at a potential higher than that of their causes. Thus the different genera of bodies form parallel and corresponding series, the reasonable arrangement of which results as an intrinsic necessity from their genetic connexion. In individuals these lowlier series make their appearance again during development. The contrasts in the solar system between planets and sun are repeated in plants and animals, and, as light is the principle of movement, animals have the power of independent movement in advance of the plants which belong to the earth.'

Obviously enough, this is no longer the study of nature; it is nature-construction from a basis of guesses and analogies rather than of knowledge and facts. Light is the principle of motion, and as animals move, they correspond to the sun, and plants to the planets! Here there is not even a hint of a deepening of knowledge, and all these deductions now seem to us quite worthless.

On the other hand, it must be allowed that good ideas are by no means absent from this 'philosophy,' nor can we deny to this restlessly industrious man a great mind always bent on discovering what was general and essential. Much of what we now _know_ he even then guessed at and taught, as, for instance, that the basis of all forms of life in this infinitely diverse world of organisms was one and the same substance--'primitive slime,' 'Urschleim' as he called it, or, as we should now say, 'protoplasm.' We can therefore, _mutatis mutandis_, agree with Oken when he says,'Everything organic has come from slime, and is nothing but diversely organized slime.' Many naturalists of the present day would go further, and agree with Oken when he suggests that 'this primitive slime has arisen in the sea, in the course of the planet's (the earth's) evolution out of inorganic material.'

Thus Oken postulated, as the specific vehicle of life, a primitive substance, in essence at least homogeneous. But he went further, and maintained that his 'Urschleim' assumed _the form of vesicles_, of which the various organisms were composed. 'The organic world has as its basis an infinitude of such vesicles.' Who is not at once reminded of the now dominant _Cell-theory_? And, in fact, thirty years later, when the cell was discovered, Oken did claim priority for himself. In so doing, he obviously confused the formulating of a problem with the solving of it; he had, quite rightly, divined that organisms must be built up of very minute concentrations of the primitive substance, but he had never seen a cell, or proved the necessity for its existence, or even attempted to prove it. His vesicle-theory was a pure divination, a prevision of genius, but one which could not directly deepen knowledge; it did not prompt, or even hasten, the discovery of the cell. Here, as throughout in his natural philosophy, Oken built, not from beneath upwards, by first establishing facts and then drawing conclusions from them, but, inversely, he invented ideas and principles, and out of them reconstructed the world. In this he differs essentially from his predecessors Erasmus Darwin, Treviranus, and Lamarck, who all reasoned inductively, that is, from observed data.

Thus the whole evolutionary movement was lost in indefiniteness; because men wanted to find a reason for everything, they missed even what might then have been explained. Moreover, the theory of evolution still lacked a sufficiently broad basis of facts; the 'Naturphilosophie,' by its want of moderation, robbed it of all credit; and it is not to be wondered at that men soon ceased to occupy themselves with the problem of the evolution of the living world. A few indeed held fast to the doctrine of evolution during the first third of the century, but then it disappeared completely from the realm of science.

Its last flicker of life was seen in France, in 1830, at the time of the July revolution, when the legitimate sovereignty of Charles X was overthrown. It is interesting to note the lively interest that Goethe, the first forerunner of the theory, and then aged eighty-one, had in the intellectual combat that took place in the French Academy between Cuvier and Isidore Geoffroy St.-Hilaire. A friend of Goethe's, Soret, relates that on August 2, 1830, he went into the poet's room, and was greeted with the words: 'Well, what do you think of this great event? The volcano is in eruption, and all is in flames. There can no longer be discussion with closed doors.' Soret replied: 'It is a terrible business! But what else was to be expected with things as they are, and with such a ministry, than that it should end in the expulsion of the reigning family?' To which Goethe answered: 'We don't seem to understand each other, my dear friend. I am not talking of these people at all; I am thinking of quite different affairs. I refer to the open rupture in the Academy between Cuvier and Geoffroy St.-Hilaire; it is of the utmost importance to science.'

In this conflict of opinions, Cuvier opposed Geoffroy's conception of the unity of the plan of structure in all animals, confronting him with the four Cuvierian types, in each of which the plan of structure was altogether different, and strongly insisting on the doctrine of the fixity of species, which he maintained to be the necessary postulate of a scientific natural history.

The victory fell to Cuvier, and it cannot be denied that there was much justification for his opinions at the time, for the knowledge of facts at that stage was not nearly comprehensive enough to give security to the Evolution theory, and moreover the quiet progress of science might have been hindered rather than furthered by premature generalization and theorizing. It had now been seen how far the interpretation of general biological problems could be carried with the available material; the 'Naturphilosophie' had not merely exploited it as far as possible, but had burdened it much beyond its carrying power, and the world was weary of insecure speculations. The 'Naturphilosophie' was for the time quite worked out, and a long period set in, during which all energies were devoted to detailed research.

LECTURE II

THE DARWINIAN THEORY

Period of detailed research--Appearance of Darwin's _Origin of Species_--Darwin's life--Voyage round the world--His teaching--Domesticated animals, dog, horse--Pigeons--Artificial selection--Unconscious selection--Correlated variations.

THE period of wholly unphilosophical, purely detailed research may be reckoned as from about 1830 to 1860, though, of course, many of the labours of the earlier part of the century must be counted among the investigations which were carried out without any reference to general questions, and even after 1860 numerous such works appeared. Nor could it be otherwise, for the basis of all science must be found in facts, and the thorough working up of the fact-material will always remain the first and most indispensable condition of our scientific progress. During the period referred to, however, it had become the sole end to be striven for; and all energies were concentrated exclusively on the accumulation of facts.

The previous century had added much to the knowledge of the inner structure of animals, the so-called 'comparative anatomy,' and in the nineteenth century this line of investigation was pursued even more extensively and energetically, so that the knowledge increased enormously. Up till this time it was chiefly the structure of the backboned animals and of a few 'backboneless' animals, so called, that had been studied, but now all the lower groups of the animal kingdom were also investigated, and became known better and in more detail as the methods of research improved.

Not content, however, with a knowledge of the adult animal, naturalists began to investigate its development. In the year 1814 the first great work on development appeared, on the development of the chick, by Pander and Von Baer. It was there shown for the first time, how the chick begins as a little disk-shaped membrane on the surface of the yolk of the egg, at first simply as a pale streak, the 'primitive streak,' then as a groove, the 'primitive groove,' by the side of which arise two folds, the 'medullary folds,' and further how a system of blood-vessels is developed around this primitive rudiment on the upper surface of the yolk, how a heart arises before the rest of the body is complete, and how the blood begins to circulate; in short, there was disclosed all the marvel of development to which we are now so much accustomed, that we can hardly understand the sensation it made at that time.

Later on, attention was turned to the development of Fishes and Amphibians (Agassiz and Vogt, later Remak), then to that of the Worms (Bagge), of Insects (Kölliker), and gradually the development of all the groups of the animal-kingdom--from Sponges to Man--was so thoroughly investigated that it almost seems to-day as if there could not be much that is new to discover in this department. This impression may indeed be true as far as the less complex processes and the more obvious questions are concerned, but it is impossible to predict what new problems may confront us, whose solution will depend on a still more detailed study of development.

As embryology is a science of the nineteenth century, so also is histology, the science of tissues. Its pioneer was Bichat, but its real foundations were not laid till Schwann and Schleiden formulated the conception of the 'cell,' and proved that all animals and plants were composed of cells. What Oken had only guessed at they now proved, that there are very minute form-elements of life which build up all the parts of animals and plants or produce them by processes of secretion. New light was thus shed on embryonic development, and this gradually led to the recognition of the fact that the egg, too, is a cell, and that development depends on a cell-division process in this egg-cell. This led further to the conception of many-celled and single-celled organisms, and so on to many items of knowledge to speak of which here would carry us too far.

For it is not my intention to attempt a complete review of the development of biology in the nineteenth century, or even in the period which we have mentioned as devoted to detailed research; it is rather my desire to convey a general impression of the enormous extent and many-sidedness of the progress that was made in this time. Let us therefore briefly recall the entirely new facts which were brought to light in this period with regard to the reproduction of animals. Asexual reproduction by budding and division was already known, but parthenogenesis is a discovery of this period, and so also is alternation of generations, so far-reaching in its bearing on general problems. It was first observed (1819) by Chamisso in Salpa, then by Steenstrup in Medusæ and trematodes, and was later made fully clear in its most diverse forms and relations by the researches of Leuckart, Vogt, Kölliker, Gegenbaur, Agassiz, and other illustrious investigators. Reproduction by heterogony, too, which occurs in many crustaceans, and in aphides and certain worms, was recognized at that time, and in the sixties Carl Ernst von Baer added to the list precocious reproduction, or pædogenesis, which is illustrated in certain insects which reproduce in the larval state.

This may suffice to convey some idea of the great mass of new, and in some cases startling facts previously unguessed at, which were then brought to light in the department of animal biology alone. To this must be added the vast increase in the number of known species and varieties, their distribution on the earth, and all this, _mutatis mutandis_, for plants also. Nor can we omit to mention the rapidly growing number of fossil species of animals and plants.