_Variation._--The discrimination of the species in most of the genera, offers very great difficulties. I cannot too strongly impress on any one intending to study this class, not to trust to external characters: he must separate and clean, and carefully examine the internal structure and form of the compartments, and more especially of the opercular valves. After considerable experience, when numerous varieties of a species have been carefully examined, the eye acquires a sort of instinctive knowledge, by which it can recognise the species, though the character cannot be defined by language; but I have found that no amount of experience with some of the commonest species, will save frequent and grave errors, as long as external characters alone are trusted to. Not only does every external character vary greatly in most of the species, but the internal parts very often vary to a surprising degree; and to add to the difficulty, groups of specimens not rarely vary in the same manner. After having given up several years to the study of this class, I must express my deliberate conviction that it is hopeless to find in any species, _which has a wide range, and of which numerous specimens from different districts_ are presented for examination, any one part or organ,--which from differing in the different species is fitted for offering specific characters,--absolutely invariable in form or structure. I may in one respect even go further, and affirm, that, if in a species, any part or organ differs remarkably from the same part in its congeners, then if many specimens are examined, especially when collected from different districts, such part or organ will be found _eminently_ variable. I may instance the antenniformed third pair of cirri in _Chthamalus antennatus_, the teeth on the posterior cirri in _Acasta sulcata_, the terga in _Pyrgoma dentatum_, the adductor ridge of the scuta in _Pyrgoma cancellatum_ and in Creusia, and other such cases: hence it will not do to found a species on a slight, or sometimes even on a considerable difference, in _any single point or organ_. On the other hand, I am far from asserting, that if only half-a-dozen specimens of some rare species of Cirripede be brought from some one quarter of the world, characters beautifully defined may not be readily discovered. In determining what forms to call varieties, I have followed one common rule; namely, the discovery of such closely-allied, intermediate forms, that the application of a specific name to any one step in the series, was obviously impossible; or, when such intermediate forms have not actually been found, the knowledge that the differences of structure in question were such as, in _several allied forms_, certainly arose from variation,--for instance, in the case of two shells otherwise identical, one being longitudinally ribbed and the other smooth, a character which we know to vary,--but I have always used this argument from analogy with great caution.