History of biology

by L. C. Miall

Natural groups, large or small, result from the long-continued operation of divergence, the survival of some, and the extinction of others; they are to be respected as facts; they are not created by definitions, which only serve to indicate and remind; any character, however trifling, will suffice, if only it is constant and distinctive.

The conflict between natural classification and logic is apparent only. Logicians say that in classifying books, for instance, you may take any property you please, subject, size, etc., as the basis of your arrangement, but having made your choice, you must adhere to it for all divisions of the same rank. Naturalists seem to say something different, for they are agreed that what they call "single-character classifications," in which one property is adhered to throughout, are unnatural. The fact is that a natural classification always rests upon one and the same property—viz. affinity, i.e. relative nearness of descent from some common ancestor. Every natural classification, like every logical classification, proceeds upon a single basis, and the failure of the single-character classifications is due to their displacing affinity by some definition.

The effect of the Origin of Species upon zoological and botanical systems has been revolutionary. Furnished with a new and intelligible meaning of the word natural, and with new criteria of naturalness, systematists have during the last fifty years worked hard to create classifications which admit of being thrown into the form of genealogical trees. Wide gaps in the geological history of life render the task difficult beyond expression, but much has already been accomplished. Newly discovered forms (especially the fossil Archæopteryx and the Cycadofilices) and more fully investigated forms, far too numerous to be specified, have established links between groups which formerly seemed to be wholly independent. Unnatural assemblages based on pre-determined characters (Radiates, Entozoa, Birds of prey, etc.) have been replaced by groups which are at least possible on evolutionary principles. Almost every working naturalist will admit that the progress of zoological and botanical system during the last two generations has done much to fortify the Darwinian position.

2. Embryology.—Baer in 1828 was possessed of all the embryological facts which Darwin used in support of his theory of evolution; in particular, he was well acquainted with the most striking fact of all—viz., the presence in embryo mammals and birds of a series of paired clefts along the sides of the neck, between which run vessels arranged as in gill-breathing vertebrates. The vessels had been figured by Malpighi; the clefts had been discovered by Rathke, who had no hesitation in calling them gill-clefts and the vessels gill-arches. Nor had Baer, who nevertheless to the end of his long life refused to accept the one explanation which gives meaning to the facts—viz., that remote progenitors of mammals and birds breathed by gills. Few embryologists have since felt such a scruple. The adaptation to gill-breathing is obvious; is gill-breathing now practised by any mammal or bird? Certainly not. Is it destined to be practised by their descendants at some future time? To say nothing of the danger of putting forth any such prophecy, it involves all the consequences of descent with modification. The opponent of evolution may as well admit at once that the gill-breathing was practised in time past. As an example of the same kind taken from plants, we may quote the trifoliate leaves of the furze-seedling, which, though absent from the full-grown furze, are frequent in the family (Leguminosæ) to which it belongs. The general similarity of vertebrate embryos, of insect-embryos and of dicotyledonous seedlings, is also worthy of note. We may suppose that early embryos, being largely or wholly dependent on food supplied by the parent, and perhaps protected by the parent as well, escape the pressure of the struggle for existence, and are often not urgently impelled to produce adaptations of their own. In these circumstances it is intelligible that features inherited from remote ancestors should persist. If, however, early independence is demanded by the conditions of life, the embryo may develop temporary adaptations, wanting in the parent and in embryos of allied groups. Larval adaptation is as much a part of the economy of nature as the retention of ancestral structures which have been lost by the adult.

3. Morphology.—Let us next consider the light which the Origin of Species throws upon homologous parts. No example will serve our purpose better than the very familiar one of the fore-limbs of different vertebrates, the arm and hand of man, the wing of the bat, the wing of the bird, the pectoral fin of the fish, and the paddle of the whale. These limbs, adapted for actions so diverse as grasping, running, flying, and swimming, nevertheless exhibit a common plan, evident at a glance, except in the pectoral fin of the fish. But why a common plan? Of what advantage is it to an animal that its wing, paddle, or hand should reproduce the general plan of a fore-foot? Why should the digits of the land vertebrates never exceed five? Why should the thumb never have more than two free joints? It would be hard to find a satisfactory answer to these questions in any book earlier than the Origin of Species; no student of the Origin of Species finds any difficulty in answering them all. The common plan has been transmitted from type to type by inheritance, and its features are derived from an unknown common ancestor.

The new conception, that structures inherited from remote ancestors may be incessantly modified by the conditions of life and by mutual competition, is the key to the chief problems of morphology. No limited collection of examples can substantiate so wide a proposition as this. Those who have made themselves familiar with old text-books of comparative anatomy will recollect how dry, or else how inconclusive, was pre-evolutionary morphology, how vague were the references to some ideal archetype, or to climate, or to the ancient conditions of the earth's surface; how often exclamations of admiration for the marvels of nature or Providence were substituted for clear explanations. Cuvier, it is true, was both precise and reasonable; but how little he was in a position to explain! His "empirical" comparative anatomy could throw no direct light upon origins or transformations; his "rational" comparative anatomy was practicable only in a few easy cases.

4. Geographical Distribution.—The facts of distribution were handled in the Origin of Species with great originality. It was shown that they support, and indeed require, some doctrine of organic evolution. The succession in the same area of the same types—armadillos succeeding armadillos in South America, marsupials succeeding marsupials in Australia—was enough of itself to render independent creation highly improbable. This was not all. Darwin's mind being charged with facts and reasonings, the accumulations of many years of travel and meditation, he sketched in rapid outline conclusions which have given a new form to the distribution question. The subject had hitherto been treated by collecting masses of facts and interpreting them by recent physical geography; Darwin showed that the history of the continents and islands may be far more influential than soil, elevation, or climate.

The scientific discussion of the facts of distribution is as old as the sixteenth century, when L'Obel pointed out that the mountain plants of warm countries descend to low levels in the north. Linnæus remarked that fresh-water plants and alpine plants are often cosmopolitan. Another early and well-founded generalisation is the statement of Linnæus that the plants common to the old and the new world are all of northern range. Buffon made the same remark about the animals, and offered the probable explanation—viz., that since the two great land-masses approach one another only in high latitudes, it is only there that animals have been able to cross from one to the other.

In the nineteenth century theories involving prodigious changes of land and sea were much in the minds of naturalists. Darwin lost his temper (a rare thing with him) over the land-bridges, hundreds, or even thousands, of miles long, which were created in order to explain trifling correspondences in the population of distant countries. A belief in the comparative stability of the great continents and oceans has since prevailed, and it is now recognised that the means of dispersal of species are greater than was once supposed.

The discovery, about the year 1846, of the marks of ancient glaciers in all parts of northern Europe, and the acceptance of an Ice Age, had a still greater influence upon the teaching of naturalists. Edward Forbes[46] put forth a glacial theory to account for the present distribution of plants of northern origin. Glacial cold, he maintained, had driven the arctic flora far southward. When more genial conditions returned, most of the northern plants retreated towards the Pole, but some climbed the mountains and gave rise to an isolated alpine flora. Darwin, whose unpublished manuscripts had anticipated Forbes's theory, believed that the whole earth became chilled during the Ice Age, and that the fauna and flora of the temperate zone reached the tropics. His argument, which is contained in chap. xi. of the Origin of Species, is now generally accepted in principle, though opinions differ on many points of detail. Some think that he extended too widely the effects of glacial cold, exaggerated the resemblance of the arctic and alpine fauna and flora, and attributed the extinction of the northern species in the intermediate plains too exclusively to climatic causes.