"Never shall I forget," says Agassiz, "the impression which the sight of the Pterichthys, provided with appendages resembling wings, produced upon me, when I assured myself that it belonged to the class of fishes. It was an entirely new type, which was about to figure, for the first time since it had ceased to exist, in the series of beings—again to form a link which nothing of all that had been revealed up to the time with regard to extinct creations, would have led us ever to suspect the existence of—showing forcibly that observation alone can lead us to the recognition of the laws of development of organized beings; and how much we should guard against all those systems of transformation of species, which the imagination invents with as much facility as reason refutes them."