CHAPTER IV.
An extinct animal recognized by its tooth: how came this to be possible?—The task of classification.—Artificial and natural methods.—Linnæus, and his baptism of the animal kingdom: his scheme of classification.—What is there underlying all true classification?—The chief groups.—What is a species?—Re-statement of the question respecting the fixity or variability of species.—The two hypotheses.—Illustration drawn from the Romance languages.—Caution to disputants.
I was one day talking with Professor Owen in the Hunterian Museum, when a gentleman approached with a request to be informed respecting the nature of a curious fossil, which had been dug up by one of his workmen. As he drew the fossil from a small bag, and was about to hand it for examination, Owen quietly remarked:—“That is the third molar of the under-jaw of an extinct species of rhinoceros.” The astonishment of the gentleman at this precise and confident description of the fossil, before even it had quitted his hands, was doubtless very great. I know that mine was; until the reflection occurred that if some one, little acquainted with editions, had drawn a volume from his pocket, declaring he had found it in an old chest, any bibliophile would have been able to say at a glance: “That is an Elzevir;” or, “That is one of the Tauchnitz classics, stereotyped at Leipzig.” Owen is as familiar with the aspect of the teeth of animals, living and extinct, as a student is with the aspect of editions. Yet before that knowledge could have been acquired, before he could say thus confidently that the tooth belonged to an extinct species of rhinoceros, the united labours of thousands of diligent inquirers must have been directed to the classification of animals. How could he know that the rhinoceros was of that particular species rather than another? and what is meant by species? To trace the history of this confidence would be to tell the long story of zoological investigation: a story too long for narration here, though we may pause awhile to consider its difficulties.
To make a classified catalogue of the books in the British Museum would be a gigantic task; but imagine what that task would be if all the title-pages and other external indications were destroyed! The first attempts would necessarily be of a rough approximative kind, merely endeavouring to make a sort of provisional order amid the chaos, after which succeeding labours might introduce better and better arrangements. The books might first be grouped according to size; but having got them together, it would soon be discovered that size was no indication of their contents: quarto poems and duodecimo histories, octavo grammars and folio dictionaries, would immediately give warning that some other arrangement was needed. Nor would it be better to separate the books according to the languages in which they were written. The presence or absence of “illustrations” would furnish no better guide; while the bindings would soon be found to follow no rule. Indeed, one by one all the external characters would prove unsatisfactory, and the labourers would finally have to decide upon some internal characters. Having read enough of each book to ascertain whether it was poetry or prose: and if poetry, whether dramatic, epic, lyric, or satiric; and if prose, whether history, philosophy, theology, philology, science, fiction, or essay: a rough classification could be made; but even then there would be many difficulties, such as where to place a work on the philosophy of history—or the history of science,—or theology under the guise of science—or essays on very different subjects; while some works would defy classification.
Gigantic as this labour would be, it would be trifling compared with the labour of classifying all the animals now living (not to mention extinct species), so that the place of any one might be securely and rapidly determined; yet the persistent zeal and sagacity of zoologists have done for the animal kingdom what has not yet been done for the library of the Museum, although the titles of the books are not absent. It has been done by patient reading of the contents—by anatomical investigation of the internal structure of animals. Except on a basis of comparative anatomy, there could have been no better a classification of animals than a classification of books according to size, language, binding, &c. An unscientific Pliny might group animals according to their habitat; but when it was known that whales, though living in the water and swimming like fishes, were in reality constructed like air-breathing quadrupeds—when it was known that animals differing so widely as bees, birds, bats, and flying squirrels, or as otters, seals, and cuttle-fish, lived together in the same element, it became obvious that such a principle of arrangement could lead to no practical result. Nor would it suffice to class animals according to their modes of feeding; since in all classes there are samples of each mode. Equally unsatisfactory would be external form—the seal and the whale resembling fishes, the worm resembling the eel, and the eel the serpent.
Two things were necessary: first, that the structure of various animals should be minutely studied, and described—which is equivalent to reading the books to be classified;—and secondly, that some artificial method should be devised of so arranging the immense mass of details as to enable them to be remembered, and also to enable fresh discoveries readily to find a place in the system. We may be perfectly familiar with the contents of a book, yet wholly at a loss where to place it. If we have to catalogue Hegel’s Philosophy of History, for example, it becomes a difficult question whether to place it under the rubric of philosophy, or under that of history. To decide this point, we must have some system of classification.
In the attempts to construct a system, naturalists are commonly said to have followed two methods: the artificial and the natural. The artificial method seizes some one prominent characteristic, and groups all the individuals together which agree in this one respect. In Botany the artificial method classes plants according to the organs of reproduction; but this has been found so very imperfect that it has been abandoned, and the natural method has been substituted, according to which the whole structure of the plant determines its place. If flying were taken as the artificial basis for the grouping of some animals, we should find insects and birds, bats and flying squirrels, grouped together; but the natural method, taking into consideration not one character, but all the essential characters, finds that insects, birds, and bats differ profoundly in their organization: the insect has wings, but its wings are not formed like those of the bird, nor are those of the bird formed like those of the bat. The insect does not breathe by lungs, like the bird and the bat; it has no internal skeleton, like the bird and the bat; and the bird, although it has many points in common with the bat, does not, like it, suckle its young; and thus we may run over the characters of each organization, and find that the three animals belong to widely different groups.
It is to Linnæus that we are indebted for the most ingenious and comprehensive of the many schemes invented for the cataloguing of animal forms; and modern attempts at classification are only improvements on the plan he laid down. First we may notice his admirable invention of the double names. It had been the custom to designate plants and animals according to some name common to a large group, to which was added a description more or less characteristic. An idea may be formed of the necessity of a reform, by conceiving what a laborious and uncertain process it would be if our friends spoke to us of having seen a dog in the garden, and on our asking what kind of dog, instead of their saying “a terrier, a bull-terrier, or a skye-terrier,” they were to attempt a description of the dog. Something of this kind was the labour of understanding the nature of an animal from the vague description of it given by naturalists. Linnæus rebaptized the whole animal kingdom upon one intelligible principle. He continued to employ the name common to each group, such as that of Felis for the cats, which became the generic name; and in lieu of the description which was given of each different kind, to indicate that it was a lion, a tiger, a leopard, or a domestic cat, he affixed a specific name: thus the animal bearing the description of a lion became Felis leo; the tiger, Felis tigris; the leopard, Felis leopardus; and our domestic friend, Felis catus. These double names, as Vogt remarks, are like the Christian- and sur-names by which we distinguish the various members of one family; and instead of speaking of Tomkinson with the flabby face, and Tomkinson with the square forehead, we simply say John and William Tomkinson.
Linnæus did more than this. He not only fixed definite conceptions of Species and Genera, but introduced those of Orders and Classes. Cuvier added Families to Genera, and Sub-kingdoms (embranchements) to Classes. Thus a scheme was elaborated by which the whole animal kingdom was arranged in subordinate groups: the sub-kingdoms were divided into classes, the classes into orders, the orders into families, the families into genera, the genera into species, and the species into varieties. The guiding principle of anatomical resemblance determined each of these divisions. Those largest groups, which resemble each other only in having what is called the typical character in common, are brought together under the first head. Thus all the groups which agree in possessing a backbone and internal skeleton, although they differ widely in form, structure, and habitat, do nevertheless resemble each other more than they resemble the groups which have no backbone. This great division having been formed, it is seen to arrange itself in very obvious minor divisions, or Classes—the mammalia, birds, reptiles, and fishes. All mammals resemble each other more than they resemble birds; all reptiles resemble each other more than they resemble fishes (in spite of the superficial resemblance between serpents and eels or lampreys). Each Class again falls into the minor groups of Orders; and on the same principles: the monkeys being obviously distinguished from rodents, and the carnivora from the ruminating animals; and so of the rest. In each Order there are generally Families, and the Families fall into Genera, which differ from each other only in fewer and less important characters. The Genera include groups which have still fewer differences, and are called Species; and these again include groups which have only minute and unimportant differences of colour, size, and the like, and are called Sub-species, or Varieties.