Because its characteristics are equally those of a lizard and a fish.
Sir Humphry Davy, in his Consolations in Travel, (to which work we have already referred in Popular Chemistry), describes this extraordinary animal as " a far greater wonder of nature than any of those which the Baron Valvasa detailed to the Royal Society, a century and a half ago, as belonging to Carniola, with too romantic an air for a philosopher." Sir Humphry saw the proteus in a lake, in the beautiful grotto of Maddalena, at Adelsburg, in Illyria. "At first you might suppose it to be a lizard, but it has the motions of a fish. Its head, and the lower part of its body, and its tail, bear a strong resemblance to those of the eel; but it has no fins; and its curious bronchial (or lunglike) organs are not like the gills of fishes; they form a singular vascular (net-like) structure, almost like a crest round the throat, which may be removed without occasioning the death of the animal, who is likewisefurnished with lungs. With this double apparatus for supplying air to the blood, it can live either below or above the surface of the water. Its fore feet resemble hands, but they have only three claws or fingers, and are too feeble to be of use in grasping or supporting the weight of the animal; the hinder feet have only two claws or toes, and in the larger specimens are found so imperfect as to be almost obliterated. It has small points in place of eyes, as if to preserve the analogy of nature. It is of a fleshy" whiteness, and transparency in its natural state, but when exposed to light, its skin gradually becomes darker, and at last gains an olive tint. Its nasal organs appear large; and it is abundantly furnished with teeth, from which it may be concluded, that it is an animal of prey; yet, in its confined state, it has never been known to eat, and it has been kept alive for many years, by occasionally changing the water in which it was placed."
Sir Humphry does not think the proteus is bred in the lake in the grotto: " in dry seasons they are seldom found here, but after great rains they are often abundant. I think it cannot be doubted, that their natural residence is an extensive, deep, subterranean lake, from which, in great floods, they sometimes are forced through the crevices of the rocks into this place." We have not space for this great philosopher's theory of the proteus; but we may state its conclusion: " the problem of the re-production of the proteus, like that of the common eel, is not yet solved; but ovaria have been discovered in animals of both species, and in this instance, as in all others belonging to the existing order of things, Harvey's maxim of ' omne vivum ab ovo,' (every animal from an egg) will apply." The curious reader should turn to the " Consolations" for Sir Humphry's ingenious speculations on this and many other striking phenomena of nature: indeed, every page of that work is penned in a delightful strain of deep-souled philosophy.