In this way the language of Natural History not only expresses, but inevitably implies, general laws of nature; and words are thus fitted to aid the progress of knowledge in this, as in other provinces of science.

Aphorism XII.

If terms are systematically good, they are not to be rejected because they are etymologically inaccurate.

Terms belonging to a system are defined, not by the meaning of their radical words, but by their place in the system. That they should be appropriate in their signification, aids the processes of introducing and remembering them, and should therefore be carefully attended to by those who invent and establish them; but this once done, no objections founded upon their etymological import are of any material weight. We find no inconvenience in the circumstance that geometry means the measuring of the earth, that the name porphyry is applied to many rocks which have no fiery spots, as the word implies, and oolite to strata which have no roelike structure. In like manner, if the term pœcilite were already generally received, as the name of a certain group of strata, it would be no valid ground for quarrelling with it, that this group was not always variegated in colour, or that other groups were equally variegated: although undoubtedly in introducing such a term, care should be taken to make it as distinctive as possible. It often happens, as we have seen, that by the natural progress of changes in language, a word is steadily confirmed in a sense quite different from its etymological import. But though 302 we may accept such instances, we must not wantonly attempt to imitate them. I say, not wantonly: for if the progress of scientific identification compel us to follow any class of objects into circumstances where the derivation of the term is inapplicable, we may still consider the term as an unmeaning sound, or rather an historical symbol, expressing a certain member of our system. Thus if, in following the course of the mountain or carboniferous limestone, we find that in Ireland it does not form mountains nor contain coal, we should act unwisely in breaking down the nomenclature in which our systematic relations are already expressed, in order to gain, in a particular case, a propriety of language which has no scientific value.

All attempts to act upon the maxim opposite to this, and to make our scientific names properly descriptive of the objects, have failed and must fail. For the marks which really distinguish the natural classes of objects, are by no means obvious. The discovery of them is one of the most important steps in science; and when they are discovered, they are constantly liable to exceptions, because they do not contain the essential differences of the classes. The natural order Umbellatæ, in order to be a natural order, must contain some plants which have not umbels, as Eryngium[37]. ‘In such cases,’ said Linnæus, ‘it is of small import what you call the order, if you take a proper series of plants, and give it some name which is clearly understood to apply to the plants you have associated.’ ‘I have,’ he adds, ‘followed the rule of borrowing the name à fortiori, from the principal feature.’

[37] See Hist. Ind. Sc. b. xvi. c. iv. sect. 5.

The distinction of crystals into systems according to the degree of symmetry which obtains in them, has been explained elsewhere. Two of these systems, of which the relation as to symmetry might be expressed by saying that one is square pyramidal and the other oblong pyramidal, or the first square prismatic and the second oblong prismatic, are termed by Mohs, the first, Pyramidal, and the second Prismatic. And it may 303 be doubted whether it is worth while to invent other terms, though these are thus defective in characteristic significance. As an example of a needless rejection of old terms in virtue of a supposed impropriety in their meaning, I may mention the attempt made in the last edition of Haüy’s Mineralogy, to substitute autopside and heteropside for metallic and unmetallic. It was supposed to be proved that all bodies have a metal for their basis; and hence it was wished to avoid the term unmetallic. But the words metallic and unmetallic may mean that minerals seem metallic and unmetallic, just as well as if they contained the element opside to imply this seeming. The old names express all that the new express, and with more simplicity, and therefore should not be disturbed.

The maxim on which we are now insisting, that we are not to be too scrupulous about the etymology of scientific terms, may, at first sight, appear to be at variance with our [Fourth] Aphorism, that words used technically are to retain their common meaning as far as possible. But it must be recollected, that in the Fourth Aphorism we spoke of common words appropriated as technical terms; we here speak of words constructed for scientific purposes. And although it is, perhaps, impossible to draw a broad line between these two classes of terms, still the rule of propriety may be stated thus: In technical terms, deviations from the usual meaning of words are bad in proportion as the words are more familiar in our own language. Thus we may apply the term Cirrus to a cloud composed of filaments, even if these filaments are straight; but to call such a cloud a Curl cloud would be much more harsh.

Since the names of things, and of classes of things, when constructed so as to involve a description, are constantly liable to become bad, the natural classes shifting away from the descriptive marks thus prematurely and casually adopted, I venture to lay down the following maxim. 304

Aphorism XIII.