specially applicable to these branches of science. What more I may be able to say upon the subject will be better said, if ever, when I am able to take up the closelyconnected subject of Scientific Nomenclature, Terminology, and Descriptive Representation. In the meantime, I have wished to show, in a negative point of view, that natural classification in the animal and vegetable kingdoms is a special problem, and that the special methods and difficulties to which it gives rise are not those common to all cases of classification, as so many physicists have supposed. Genealogical resemblances are only a special case of resemblances in general.

Unique or Exceptional Objects.

In framing a system of classification in almost any branch of science, we must expect to meet with unique or peculiar objects, which are so called because they seem to stand alone, having few analogies with other objects. They may also be said to be sui generis, each unique object forming, as it were, a class by itself; or they are called nondescript, because in thus standing apart it is difficult to find terms in which to explain their properties. The rings of Saturn, for instance, form a unique object among the celestial bodies. We have indeed considered this and many other instances of unique objects in the preceding chapter, on Exceptional Phenomena. Apparent, Singular, and Divergent Exceptions especially, are analogous in nature to unique objects.

In the classification of the elements, Carbon stands apart as a substance entirely unique in its powers of producing compounds. It is considered to be a quadrivalent element, and it obeys all the ordinary laws of chemical combination. Yet it manifests powers of affinity in such an exalted degree that the substances in which it

appears are more numerous than all the other compounds known to chemists. Almost the whole of the substances which have been called organic contain carbon, and are probably held together by the carbon atoms, so that many chemists are now inclined to abandon the name Organic Chemistry, and substitute the name Chemistry of the Carbon Compounds. It used to be believed that the production of the so-called organic compounds was due solely to the action of a vital force, or some inexplicable cause involved in the phenomena of life, but it is now found that chemists are able to commence with the elementary materials, pure carbon, hydrogen, and oxygen, and by strictly chemical operations, combine these together so as to form complicated organic compounds. So many compounds have already been thus formed that the probability is very great that many others will be so formed in the course of time, and we might be inclined to generalize, and infer that all so-called organic compounds might ultimately be produced without the agency of living beings. Thus the distinction between the organic and the inorganic kingdoms seems to be breaking down, but our wonder at the peculiar powers of carbon must increase at the same time.

In considering generalization, the law of continuity was applied chiefly to physical properties capable of mathematical treatment. But in the classificatory sciences, also, the same important principle is often beautifully exemplified. Many objects or events seem to be entirely exceptional and abnormal, and in regard to degree or magnitude they may be so termed. We might adduce examples on the one hand of such extreme cases, but it is often easy to show, on the other hand, that they are connected by intermediate links with other apparently different cases.

In the organic kingdoms of nature there is a common

groundwork of similarity running through all classes, but particular actions and processes present themselves conspicuously in particular families and classes. Tenacity of life is most marked in the Rotifera, and some other kinds of microscopic organisms, which can be dried and boiled without loss of life. Reptiles are distinguished by torpidity, and the length of time they can live without food. Birds, on the contrary, exhibit ceaseless activity and high muscular power. The ant is as conspicuous for intelligence and size of brain among insects as the quadrumana and man among vertebrata. Among plants the Leguminosæ are distinguished by a tendency to sleep, folding their leaves at the approach of night. In the genus Mimosa, especially the Mimosa pudica, commonly called the sensitive plant, the same tendency is magnified into an extreme irritability, almost resembling voluntary motion. More or less of the same irritability probably belongs to vegetable forms of every kind, but it is of course to be investigated with special ease in such an extreme case. In the Gymnotus and Torpedo, we find that organic structures can act like galvanic batteries. Are we to suppose that such animals are entirely anomalous exceptions; or may we not justly expect to find less intense manifestations of electric action in all animals and plants?

In the animal world we find many phenomena which seem to be peculiar to certain classes, but are afterwards found to differ but in degree from what is always present. The lower animals, for instance, seem to differ entirely from the higher ones in the power of reproducing lost limbs. A kind of crab has the habit of casting portions of its claws when much frightened, but they soon grow again. There are multitudes of smaller animals which, like the Hydra, may be cut in two and yet live and develop into new complete individuals. No mammalian animal can repro

duce a limb, and in appearance there is no analogy. But it was suggested by Blumenbach that the healing of a wound in the higher animals really represents in a lower degree the power of reproducing a limb. That this is true may be shown by adducing a multitude of intermediate cases, each adjoining pair of which are clearly analogous, so that we pass gradually from one extreme to the other. Darwin holds, moreover, that any such restoration of parts is closely connected with that perpetual replacement of the particles which causes every organized body to be after a time entirely new as regards its constituent substance. In short, we approach to a great generalization under which all the phenomena of growth, restoration, and maintenance of organs are effects of one and the same power. It is perhaps still more surprising to find that the complicated process of sexual reproduction in the higher animals may be gradually traced down to a simpler and simpler form, which at last becomes undistinguishable from the budding out of one plant from the stem of another. By a great generalization we may regard all the modes of reproduction of organic life as alike in their nature, and varying only in complexity of development".

Limits of Classification.

Science can extend only so far as the power of accurate classification extends. If we cannot detect resemblances, and assign their exact character and amount, we cannot have that generalized knowledge which constitutes science; we cannot infer from case to case. It will readily be

x Darwin, The Variation of Animals and Plants,' vol. ii. pp. 293, 359, &c.; quoting Paget, 'Lectures on Pathology,' 1853, pp. 152, 164. y Ibid. vol. ii. p. 372.

observed that classification is the opposite process to discrimination. If we feel that two tastes differ, for instance, the tastes of two specimens of wine, the mere fact of difference existing prevents inference. The detection of the difference saves us, indeed, from false inference, because so far as difference exists, all inference is impossible. But classification consists in detecting resemblances of all degrees of generality, and ascertaining exactly how far such resemblances extend, while assigning precisely at the same time the points at which difference begins. It enables us, then, at once to generalize and make inferences where it is possible, and it saves us at the same time from going too far. Full classifications constitute a complete record of all our knowledge of the objects or events classified, and the limits of exact or scientific knowledge are identical with the limits of classification.

It must by no means be supposed that every group of natural objects will be found capable of rigorous classification. There may be substances which vary by insensible degrees, consisting, for instance, in varying mixtures of simpler substances. Granite is a mixture of quartz, felspar, and mica, but there are hardly two specimens in which the proportions of these three constituents are alike, and it would be impossible to lay down definitions of distinct species of granite without finding an infinite variety of intermediate species. The only true classification of granites, then, would be founded on the proportions of the constituents present, and a chemical or microscopic analysis would be requisite, in order that we should assign any specimen to its true position in the series. Granites vary, again, by insensible degrees, as regards the magnitude of the crystals of felspar and mica. Precisely similar remarks might be made concerning the classification of other plutonic rocks, such as syenite, basalt, pumice-stone, lava, tuff, &c.