RECAPITULATION 227 



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 com- 

 pared 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. 



The power of flight is an excellent example of 

 adaptation. It has been evolved independently in 

 Pterodactyls, Bats, and Birds. In the two first 

 groups, and to a slight degree in the third, the expanse 

 of the wing is formed by an extension of the skin into 

 a thin membrane, supported by the fore-limbs. It is 

 not necessary to argue in detail that the evolution of 

 this membrane and of the modifications of bones and 

 muscles by which it is supported and moved, can be 

 satisfactorily explained on the theory that modifica- 

 tions due to mechanical and functional stimulation 

 are ultimately inherited. In birds, however, the 

 surface of the wing is supplied chiefly by feathers, 

 and consideration of the matter affords no reason for 

 supposing that the evolution of feathers was due to 

 any external or functional stimulation. It is often 



