Metamorphosis of structure in Amphibia and in Flat-fishes corresponds to the change of conditions of life in the free-living animal. In the case of the eyes of the Cave-fishes the conditions in respect of absence of light are constant throughout life, and we find only an embryonic development of the eye taking place by heredity. The question arises whether, when there is no embryonic recapitulation, it must be concluded that apparent adaptations are due to mutation and not to function or external conditions. One case of this kind is that of the limbs of Snakes, where, if we except the vestiges of hind limbs in the Pythons, there is no trace of limbs either in the embryo or after hatching. There are several similar cases among Reptiles and Amphibia. The Slow-worm (_Anguis fragilis_) is limbless, and so are the members of the sub-class Apoda among the Amphibia. In these also rudiments of limbs are entirely absent in the embryos or larval stages. Considering the recent evolution of Snakes as compared with the origin of lungs and loss of gills and gill slits in terrestrial Vertebrates in general, we have here a remarkable contrast which shows in the first place the difference resulting when the change in habits and conditions in the one case takes place from one stage of life to another, and in the other case the new habits are constant throughout life from the moment of hatching. It seems to me that in the present state of our knowledge we cannot form a decisive opinion on the question whether the absence of limbs in such cases is the result of mutation or of disuse--that is, absence of functional stimulation.