Part 5

Thus there gradually accumulated a new mass of material; investigation became more and more specialized, and the danger became imminent that workers in the various departments would be unable to understand each other, so completely were they independent of one another in their specialist researches. There was lack of any unifying bond, for workers had lost sight of the general problem in which all branches of the science meet, and through which alone they can be united into a general science of biology. The time had come for again combining and correlating the details, lest they should grow into an unconnected chaos, through which it would be impossible to find one's way, because no one could overlook it and grasp it as a whole. In a word, it was high time to return to general questions.

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Though I have called the period from 1830 to 1860 that of purely detailed research, I do not mean to ignore the fact that, during that time, there were a few feeble attempts to return to the great questions which had been raised at the beginning of the century. But the point is, that all such attempts remained unnoticed. Thus there appeared, in 1844, a book entitled _Vestiges of the Natural History of Creation_, the anonymous author of which revealed himself much later as Robert Chambers, an Edinburgh publisher. In this book the evolution of species was ascribed to two powers, a power of transformation and a power of adaptation. Two Frenchmen, Naudin and Lecoq, also published a work in which the theory of evolution was set forth, and from 1852 to 1854 the well-known German anthropologist Schaafhausen was writing on similar lines. But all these calls sounded unheard, so deeply were naturalists plunged in detailed investigations, and it required a much mightier voice to command the ear of the scientific world.

It is impossible to estimate the effect of Darwin's book on _The Origin of Species_, published in English in 1858, in German in 1859 unless we fully realize how completely the biologists of that time had turned away from general problems. I can only say that we, who were then the younger men, studying in the fifties, had no idea that a theory of evolution had ever been put forward, for no one spoke of it to us, and it was never mentioned in a lecture. It seemed as if all the teachers in our universities had drunk of the waters of Lethe, and had utterly forgotten that such a theory had ever been discussed, or as if they were ashamed of these philosophical flights on the part of natural science, and wished to guard their students from similar deviations. The over-speculation of the 'Naturphilosophie' had left in their minds a deep antipathy to all far-reaching deductions, and, in their legitimate striving after purely inductive investigation, they forgot that the mere gathering of facts is not enough, that the drawing of conclusions is an essential part of the induction, and that a mass of bare facts, however enormous, does not constitute a science.

One of my most stimulating teachers at that time, the gifted anatomist, Jacob Henle, had written as a motto under his picture, 'There is a virtue of renunciation, not in the province of morality alone, but in that of intellect as well,' a sentence which expressly indicated the desirability of refraining from all attempts to probe into the more general problems of life. Thus the young students of that time were nourished only on the results of detailed research, in part indeed interesting enough, but in part dry and, because uncorrelated, unintelligible in the higher sense, and only here and there awakening a deeper interest, when, as in physiology and in embryology, they formed a connected system in themselves. Without being fully clear as to what was lacking, we certainly missed the deeper correlation of the many separate disciplines.

It is therefore not to be wondered that Darwin's book fell like a bolt from the blue; it was eagerly devoured, and while it excited in the minds of the younger students delight and enthusiasm, it aroused among the older naturalists anything from cool aversion to violent opposition. The world was as though thunderstruck, as we can readily see from the preface with which the excellent zoologist of Heidelberg, Bronn, introduced his translation of Darwin's book, where he asks this question among others, 'How will it be with you, dear reader, after you have read this book?' and so forth.

But before I enter on a detailed examination of the contents of this epoch-making book, I should like to say a few words about the man himself, who thus revolutionized our thinking.

Charles Darwin was born in 1809, the year of the publication of Lamarck's _Philosophie zoologique_, and of Oken's _Lehrbuch der Naturphilosophie_. There was thus a whole generation between the first emergence of the Evolution theory and its later revival. Darwin's father was a physician, and his education was not a regular one. In his youth he seems to have devoted much time and enthusiasm to hunting, and only very slowly to have taken up regular studies towards a definite end. In accordance with his father's wishes, he studied medicine for a time, but soon abandoned it to devote himself to botany and zoology. Before he had had time to distinguish himself in any special way in these subjects, he was offered, in his twenty-first year, the post of naturalist on an English war-ship which was to make a voyage round the world, and that at a leisurely rate.

This was decisive not only for Darwin's immediate studies, but for the work of his life, for, as he tells us himself, it was during this voyage on the _Beagle_ that the idea of the Evolution theory first came to him. While the vessel made a stay at the Galapagos Islands, west of South America, he noticed that quite a number of little land-birds occurred there which closely resembled those of the neighbouring mainland, but yet were different from them. Almost every little island had its own species, and so he concluded that all these might be descended from representatives of a few species which had long before drifted over from the mainland to these volcanic islands, become established there, and in the course of time taken on the character of new species. The problem of the transformation of species opened up before him, and he made up his mind to follow up the idea after his return, in the hope that by a patient collecting of facts, he would by and by arrive at some security with regard to this great question.

I need not linger over any detailed account of his travels; one can readily understand how a voyage round the world, lasting for five years, would offer to the inquiring mind of a Darwin rich opportunities for the most varied observations. That he did not fail to make use of these is evidenced not only by his book on _The Origin of Species_, but by several more special works, published shortly after his return--his natural history of those remarkable sessile crustaceans, the barnacles or Cirripedia, and his studies on the origin of coral reefs. The first-named book still holds its own as a classic monograph on this animal group, with its wealth of forms; and the theory of the origin of coral reefs which Darwin elaborated has still many adherents, in spite of various rival interpretations.

But Darwin would hardly have achieved what he did if he had been compelled to secure for himself a professional position in order to obtain bread and butter. Such great problems demand not only the whole of a man's mental energy, they monopolize his time. Studies of detail may well be taken up in leisure hours, but big problems absorb all the thoughts and must always be present to the mind, lest the connexion between the many individual inquiries, which make up the whole task, be lost sight of. Darwin had the good fortune to be a free investigator, and to be able to retire, on his return from his travels, to a small property at Down in Kent, there to live for his family and his work. Here he followed up the idea of evolution which he had already formulated, and it has always seemed to me the most remarkable thing about him, that he was able to keep in mind and work up the hundreds of isolated inquiries that were eventually to be brought together to form the main fabric of his theory. When one studies his many later writings, one cannot but be surprised afresh by the number of different sets of facts he collected at the same time, partly from others, partly from personal observation, and continually also from his own experiments. He made experiments on plants and on animals, and the number of people with whom he carried on a scientific correspondence is simply astounding. In this way he brought together, in the course of twenty years, an extraordinarily rich material of facts, from the fullness of which he was able later to write his book on _The Origin of Species_. Never before had a theory of evolution been so thoroughly prepared for, and it is undoubtedly to this that it owed a great part of its success; not to this alone, however, but still more, if not mainly, to the fact that it presented a principle of interpretation that had never before been thought of, but whose importance was apparent as soon as attention was called to it--the principle of selection.

Charles Darwin championed, in the main, the same fundamental ideas as had been promulgated by his grandfather, Erasmus Darwin, by Treviranus, and by Lamarck: species only seem to us immutable; in reality they can vary, and become transformed into other species, and the living world of our day has arisen through such transformations, through a sublime process of evolution which began with the lowest forms of life, but by degrees, in the course of unthinkably long ages, progressed to organisms more and more complex in structure, more and more effective in function.

It is interesting to note at what point Darwin first put in his lever to attempt the solution of the problem of evolution. He started from quite a different point from the investigators of the early part of the century, for he began with forms of life which had previously been markedly neglected by science, the varieties of our domesticated animals and cultivated plants.

Previously these had been in a sense mere step-children of biology, inconvenient existences which would not fit properly into the system, which were therefore as far as possible ignored or dismissed as outside the scope of 'the natural,' because it was difficult to know what else to do with them. I can quite well remember that, even as a boy, I was struck by the fact that one could find nothing in the systematic books about the many well-established garden forms of plants, or about our domestic animals, which seemed to be regarded as in a sense artificial products, and as such not worthy of scientific consideration. But it was in these that Darwin particularly interested himself, making them virtually the basis of his theory, for he led up from them to the very principle of transformation, which was his most important addition to the earlier presentations of the Evolution theory.

He started from the existence of varieties which may be observed in so many wild species. His line of thought was somewhat as follows: If species have really arisen through a gradual process of transformation, then varieties must be regarded as possible first steps towards new species; if, therefore, we can only succeed in finding out the causes which underlie the formation of any varieties whatever, we shall have discovered the causes of the transformation of species. Now we find by far the greatest number of varieties, and the most marked ones, among our domesticated animals and plants, and unless we are to assume that each of these is descended from a special wild species, the reason why there has been such a wealth of variety-formation among them must lie in the conditions which influence the relevant species in the course of domestication; and it remains for us to analyse these conditions till we come upon the track of the operative factors. With this conviction, Darwin devoted himself to the study of domesticated animals and plants.

The first essential was to prove that every variety had not a separate wild species as ancestor, but that the whole wealth of our domesticated breeds originated, in each case, from one, or at least from a few wild species. Of course I cannot here recapitulate the multitudinous facts which were marshalled by Darwin, especially in his later works, notably his _Animals and Plants under Domestication_, but this is not necessary to an understanding of his conclusions, and I shall therefore restrict myself to a few examples.

Let us take first the domestic dog, _Canis familiaris_, Linné. We have at the present day no fewer than seven main breeds, each of which has its sub-breeds, often numerous. Thus there are forty-eight sub-breeds which are used as guardians of our houses, 'house-dogs' in the restricted sense, thirty sub-breeds of dogs with silk-like hair (King Charles dogs, Newfoundland dogs, &c.), twelve of terriers, and thirty-five of sporting dogs, among them such different forms as the deerhound and the pointer. We have further nineteen sub-breeds of bulldogs, thirty-five of greyhounds, and six of naked or hairless dogs. Not only the main breeds, but even the sub-breeds often differ as markedly from one another as wild species do, and the question must first be decided whether each of the very distinct breeds has not a special wild species as ancestor.

Obviously, however, this cannot be maintained, for so many species of wild dog have never existed on the earth at any time. We know, too, that 4,000 or 5,000 years ago a large number of breeds of dogs were in existence in India and Egypt. There were Pariah dogs, coursers, greyhounds, mastiffs, house-dogs, lapdogs and terriers. It is not possible that the products of all lands could, at that time, have been gathered into one, and it is inconceivable that so many wild species could have existed in the one country of India.

On the other hand, however, it cannot be maintained that all our present breeds have descended from _a single_ wild species; it is much more probable that several wild species were domesticated in different countries.

It has often been supposed that the manifold diversity of our present breeds has been brought about by crossing the various tamed species. That cannot be the case, however, because crossing gives rise only to hybrid mongrel forms, not to distinct breeds with quite new characters. It is true that all breeds of dogs can be very readily crossed with each other, but the result is not new breeds, but those numberless and transient intermediate forms which the dog-breeder despises as worthless for his purpose. It must therefore have been through the influence of domestication, combined with crossing, that a few wild species gave rise to the various breeds of dogs.

The pedigree of the horse is rather more clear than that of the dog. Even in this case, indeed, one cannot definitely name the ancestral wild form, but it is very probable that it was of a grey-brown colour, and similar to the wild horses of our own day. Darwin supposes that it must also have had the black stripe on the back which is exhibited by the domestic ass, and by several wild species of ass, basing his opinion on the fact that the spinal stripe often occurs in foals, especially in those of a grey-brown colour.

But though there can be no doubt that this is to be interpreted as a reversion to a character of a remote ancestor, it by no means follows that the _direct_ ancestral form must have had this stripe. I am more inclined to believe that the ancestor which bore this mark was considerably more remote, and lived before the differentiation of the horse from the ass. Darwin himself noted the remarkable fact that in rare cases, especially in foals, not only may the stripe on the back be present, but there may be more or less distinct zebra-striping on the legs and withers: this, however, must be interpreted as a reversion to the character of a very much more remote ancestor, to a common ancestor of all our present-day horses and asses, which must have been striped over its whole body, like the zebra living in Africa now.

It cannot be proved of any of the wild horses of to-day that they are not descended from domesticated ancestors; indeed, we can say with certainty that the thousands of wild horses which roam the plains of North and South America are descended from domestic horses, for there was no horse in America at the time it was discovered by the Europeans. In all probability our horse originated in Middle Asia, was there first domesticated, and has thence been gradually introduced into other countries. In Egypt it appears first on the monuments in the seventeenth century B.C., and it seems to have been introduced by the conquering Hyksos. On the ancient Assyrian monuments the chase after wild horses is depicted, and they were not caught, but killed with arrow and lance, like the lion and the gazelle.

But even if two wild species of horse had been tamed in different parts of the great continent of Asia, these two domesticated animals would have varied much and in the most diverse manner, as we may infer from our different breeds of horses at the present day. There are a great many of these, and many of them differ very considerably from each other. If we think of the lightly built Arab horse, and place beside it the little pony, or the enormous Percheron, the powerful cart-horse from the old French province of La Perche, which easily draws a load of fifty kilograms, we are face to face with differences as great as those between natural species. And we may realize how many breeds of horses there are now upon the earth if we remember that nearly every oceanic island has its special breed of ponies. Not only in the cold Shetland Islands, England, Sardinia and Corsica, but in almost every one of the larger islands of the extensive Indian Archipelago there is one, and Borneo and Sumatra have several.

But the most conclusive proof of descent from a single wild species is afforded by the pigeons, and as the production of new breeds among them has been, and will continue to be, carried on with particular enthusiasm and deliberateness, I propose to deal with them somewhat more in detail.

Darwin's work proves beyond a doubt that all our present-day breeds of pigeons are descended from a single wild species, the rock-dove, _Columba livia_. In appearance, this form, which still lives in a wild state, differs little from our half-wild blue-grey field-pigeon. It has the same metallic shimmer on the feathers of the neck, the same two black cross-bars on the wings as well as the band over the tail, and it has also the same slate-blue general colour. Now, all breeds of pigeons are without restriction fertile _inter se_, so that any breed can be crossed with any other, and it often happens that, in the products of such crossing, characters appear which the parents, that is, the two or more crossed breeds, did not possess, but which are among the characters of the rock-dove. Thus Darwin obtained, by crossing a pure white fantail with a black barb, hybrids which were partly blackish brown, partly mixed with white, but when he crossed these hybrids with others from two breeds which were likewise not blue, and had no bars, he obtained a slate-blue rock-pigeon, with bars on the wings and tail. We shall inquire later on how far it is correct to regard such cases as reversions to remote ancestors, but if we take it for granted in the meantime, we have here a proof of the descent of our breeds from a single wild species. This is corroborated, too, by everything that we know about the distribution of the rock-pigeon and the place and time of its domestication. It still lives on the cliff-guarded shores of England, Brittany, Portugal, and Spain, and both in India and in Egypt there were tame pigeons at a very early period. Pigeons appear on the menu of a Pharaoh of the fourth dynasty (3000 B.C.), and of India we know at least that in 1600 A.D. there were 20,000 pigeons belonging to the court of one of the princes.

The beauty of this bird, and the ease with which it can be tamed, obviously called man's attention to it at a very early date, and it has been one of man's domestic companions for several thousands of years. Now we can distinguish at least twenty main races (Fig. 1), which differ from each other as markedly as, if not more markedly than, the most nearly allied of the 288 wild species of pigeons which inhabit the earth. We have carriers and tumblers, runts and barbs, pouters, turbits and Jacobins, trumpeters and laughers, fantails, swallows, Indian pigeons, &c.

Each of these races falls into sub-races; thus there is a German, an English, and a Dutch pouter-pigeon. The books on pigeons mention over 150 kinds which are quite distinct from one another, and breed true, that is, always produce young similar to themselves.

Without entering upon a detailed description of any of these, I should like to call attention to the way in which certain characters have varied among them. Colour is a subordinate race-character, in so far that colour alone does not constitute a race, yet the colouring within a particular sub-race is usually very sharply defined, and in every breed there are sub-races of different colours. Thus there are white, black, and blue fantails, there are white turbits with red-brown wings, but also red ones with white heads, and white tumblers with black heads, &c. Very unusual colours and colour-markings sometimes occur. Thus one sub-race of tumblers exhibits a peculiar clayey-yellow colour splashed with black markings, otherwise rare among pigeons, and almost suggestive of a prairie-hen; there is also a copper-red spot-pigeon, a cherry-red 'Gimpel'-pigeon, lark-coloured pigeons, &c. Then we find all possible juxtapositions of colours, limited to quite definite regions of the body; thus we have white tumblers with a red head, red tail, and red wing-tips, or white tumblers with a black head, red turbits with white head, Indian pigeons quite black except for white wing-tips, and so on. The distribution of colour is often very complicated, but nevertheless, all the individuals of the breed show it in exactly the same manner. Thus there are the so-called blondinettes in which almost the whole body is copper-red, but the wings white, save that each quill bears at the rounded end of its vane a black and red fringe. I should never come to an end, if I were to try to give anything like a complete idea of the diversity of colouring among the various breeds of pigeons.

Even such an important and, among wild species, unusually constant organ as the bill has varied among pigeons to an astonishing degree. Carrier-pigeons (Fig. 1, No. 6) have an enormously long and strong bill, which is moreover covered with a thick red growth of the cere, while in the turbits and owls (Fig. 1, Nos. 8 and 10) the bill is shorter than any we find among wild birds. In many breeds even the _form_ of the bill deviates far from the normal, as in the bagadottes (No. 5) with crooked bill.