I have not thought it necessary to pursue this reasoning into higher divisions than "families;" but of course it may be extended to any amount,--so as to shadow forth, equally, the compartments of _primary_ significance. Nor would I wish to imply, by the above similes, that I regard a _lineal_ method of arrangement as the correct one. Every zoologist is aware, that in Nature such does not exist: but the mode of illustration which I have selected is applicable to all systems alike, so far as the _principle_ is concerned. It will consequently be seen, from what has been said, that the terms "genus" and "species" not only differ very considerably in _importance_, but in signification also. Whilst the former is merely suggestive of a particular _position_ which a creature occupies in a systematic scale (a position, however, which depends upon the various structural peculiarities which it possesses _in common with other beings_,--which thus more or less resemble it); the latter expresses the actual creature itself: so that while one applies to _several_ animals (of distinct natures and origins, though bound together by a certain bond of imitation), the other belongs to _a single race alone_, which it therefore exclusively indicates. But if such be the case, it will perhaps be asked,--Why then insist upon a generic name at all, if the specific one be sufficient to denote all that is required, namely, the _animal itself_? To which, however, we may reply, that the binomial nomenclature is demanded for two elementary reasons,--first, because it is founded upon a natural truth, which (to say the least) it would be unwise to violate; and, secondly, because it is _convenient_, both for simplification and analysis. We should assuredly be surprised were a man to object to his surname, as unnecessary, because he has a christian (or specific[79]) one which is the exponent of him _alone_. True it is that his family (or generic) title applies to the rest of his kin also; but, since there are other people (of other families) who may have the same _individual_ appellation as himself, it is clearly desirable, even as a matter of expediency alone, that patronymic and christian name should be alike retained. We need not, however, plead expediency, in favour of this acceptance of what has been so long tested, and shown to be correct; we appeal to a higher tribunal,--that of experience,--in proof that it draws its origin from Nature itself, and is implied by the very existence, or reality, of _natural groups_. The 'Methode Mononomique' has indeed been attempted[80]; and it has failed,--or at any rate it has shown itself to be inferior, both ideally and in practice, to the plan commonly in use: and if I might be pardoned a passing conjecture on its ultimate success, I should be inclined, since it is contrary to the canon of the organic world, to regard its case as utterly hopeless.