"If any swimming birds, such as the swan and the goose, whose legs are short, nevertheless have a very long neck, it is because these birds in swimming on the surface of the water have the habit of plunging their head down as far as they can, to catch aquatic larvæ and different animalcules for food, and because they make no effort to lengthen their legs. "When an animal to satisfy its wants makes repeated efforts to elongate its tongue, it will acquire a considerable length (the ant-eater, green wood-pecker); when it is obliged to seize anything with this same organ, then its tongue will divide and become forked. That of the humming-birds, which seize with their tongue, and that of the lizard and serpents, which use it to feel and examine objects in front of them, are proofs of what I advocate. "Wants, always occasioned by circumstances, and followed by sustained efforts to satisfy them, are not limited in results, in modifying--that is to say, in increasing or diminishing--the extent and the faculties of organs; but they also come to displace these same organs when certain of these wants become a necessity. "The fishes which habitually swim in large bodies of water, having need of seeing laterally, have, in fact, their eyes placed on the sides of the head. Their bodies, more or less flattened according to the _species_, have their sides perpendicular to the plane of the water, and their eyes are placed in such a way that there is an eye on each flattened side. But those fishes whose habits place them under the necessity of constantly approaching the shores, and especially the shelving banks or where the slope is slight, have been forced to swim on their flattened faces, so as to be able to approach nearer the edge of the water. In this situation, receiving more light from above than from beneath, and having a special need of being always attentive to what is going on above them, this need has forced one of their eyes to undergo a kind of displacement, and to assume the very singular situation which is familiar to us in the _soles_, _turbots_, _dabs_, etc. (_Pleuronectes_ and _Achirus_). The situation of these eyes is asymmetrical, because this results from an incomplete change. Now, this change is entirely completed in the rays, where the transverse flattening of the body is entirely horizontal, as also the head. Also the eyes of the rays, both situated on the upper side, have become symmetrical.