226 METAMORPHOSIS AND 



But the condition of the Dipnoi, which possess lungs 

 but do not walk on land, does not support this 

 supposition, for they possess fins which are either 

 filamentous or fin-like, having a central axis with 

 rays on each side. There can be little doubt that 

 the digits of the terrestrial limb are homologous 

 with endoskeletal fin-rays, but the evolution of the 

 axis of the limb is not to be ascertained either from 

 development or palaeontology. The absence of meta- 

 morphosis here may perhaps be due to the fact that 

 the lateral fins ceased to function in the earlier aquatic 

 stages, only the caudal fin being used for swimming. 

 If this were the case the absence of metamorphosis in 

 the legs is itself an adaptation, the disuse of the 

 paired limbs in the larva having caused the earlier 

 fin-like stages of these limbs to disappear, while the 

 terrestrial leg was developed later by heredity, just 

 as the legs have disappeared in the larvae of many 

 insects, though fully developed in the adult. 



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. 



