Sect. V.—Gradation of Groups.
15. It has been already noticed (last [chapter],) that even that vague application of the idea of resemblance which gives rise to the terms of common language, introduces a subordination of classes, as man, animal, body, substance. Such a subordination appears in a more precise form when we employ this idea in a scientific manner as we do in Natural History. We have then a series of divisions, each inclusive of the lower ones, which are expressed by various metaphors in different writers. Thus some have gone as far as eight terms of the series[31], and have taken, for the most part, military names for them; as Hosts, Legions, Phalanxes, Centuries, Cohorts, Sections, Genera, Species. But the most received series is Classes, Orders, Genera, and Species; in which, however, we often have other terms interpolated, as Sub-genera, or Sections of genera. The expressions Family and Tribe, are commonly appropriated to natural groups; and we speak of the Vegetable, Animal, Mineral Kingdom; but the other metaphors of Provinces, Districts, &c., which this suggests, have not been commonly used[32].
[31] Adanson, p. cvi.
[32] Sub-Kingdom has recently been employed by some naturalists.
It will of course be understood that each ascending step of classification is deduced by the same process from the one below. A Genus is a collection of Species which resemble each other more than they [131] resemble other species; an Order is a collection of Genera having, in like manner, the first degree of resemblance, and so on. How close or how wide the Degrees of Resemblance are, must depend upon the nature of the objects compared, and cannot possibly be prescribed beforehand. Hence the same term, Class and Order for instance, may imply, in different provinces of nature, very different degrees of resemblance. The Classes of Animals are Insects, Birds, Fish, Beasts, &c. The Orders of Beasts are Ruminants, Tardigrades, Plantigrades, &c. The two Classes of Plants (according to the Natural Order[33]) are Vascular and Cellular, the latter having neither sexes, flowers, nor spiral vessels. The Vascular Plants are divided into Orders, as Umbelliferæ, Ranunculaceæ, &c.; but between this Class and its Orders are interposed two other steps:—two Sub-classes, Dicotyledonous and Monocotyledonous, and two Tribes of each: Angiospermiæ, Gymnospermiæ of the first; and Petaloideæ, Glumaciæ of the second. Such interpolations are modifications of the general formula of subordination, for the purpose of accommodating it to the most prominent natural affinities.
[33] Lindley.
16. Species.—As we have already seen in tracing the principles of the Natural Method, when by the intimate study of plants we seek to give fixity and definiteness to the notion of resemblance and affinity on which all these divisions depend, we are led to the study of Organization and Analogy. But we make a reference to physiological conditions even from the first, with regard to the lowest step of our arrangement, the Species; for we consider it a proof of the impropriety of separating two Species, if it be shown that they can by any course of propagation, culture, and treatment, the one pass into the other. It is in this way, for example, that it has been supposed to be established that the common Primrose, Oxlip, Polyanthus, and Cowslip, are all the same species. Plants which thus, in virtue of external circumstances, as soil, [132] exposure, climate, exhibit differences which may disappear by changing the circumstances, are called Varieties of the species. And thus we cannot say that a Species is a collection of individuals which possess the First Degree of Resemblance; for it is clear that a primrose resembles another primrose more than it does a cowslip; but this resemblance only constitutes a Variety. And we find that we must necessarily include in our conception of Species, the notion of propagation from the same stock. And thus a Species has been well defined[34]: ‘The collection of the individuals descended from one another, or from common parents, and of those which resemble these as much as these resemble each other.’ And thus the sexual doctrine of plants, or rather the consideration of them as things which propagate their kind, (whether by seed, shoot, or in any other way,) is at the basis of our classifications.
[34] Cuv. Règne Animal, p. 19.
17. The First permanent Degree of Resemblance among organized beings is thus that which depends on this relation of generation, and we might expect that the groups which are connected by this relation would derive their names from the notion of generation. It is curious that both in Greek and Latin languages and in our own, the words which have this origin (γένος, genus, kind,) do not, in the phraseology of science at least, denote the nearest degree of relationship, but have other terms subordinate to them, which appear etymologically to indicate a mere resemblance of appearance (εἶδος, species, sort); and these latter terms are appropriated to the groups resulting from propagation. Probably the reason of this is, that the former terms (genus, &c.) had been applied so widely and loosely before the scientific fixation of terms, that to confine them to what we call species would have been to restrict them in a manner too unusual to be convenient.
18. Varieties. Races.—The Species, as we have said, is the collection of individuals which resemble each other as much as do the offspring of a common [133] stock. But within the limits of this boundary, there are often observable differences permanent enough to attract our notice, though capable of being obliterated by mixture in the course of generation. Such different groups are called Varieties. Thus the Primrose and Cowslip, as has been stated above, are found to be varieties of the same plant; the Poodle and the Greyhound are well marked varieties of the species dog. Such differences are hereditary, and it may be long doubtful whether such hereditary differences are varieties only, or different species. In such cases the term Race has been applied.