A CHAPTER IN DARWINISM 



19 



The frog's egg first gives rise to a little aquatic 

 creature with external gills and a tail — the tadpole — 

 which gradually loses its gills and its tail and acquires 

 in their place lungs and four legs (Fig. 2), so as now 



Fig, 2. — Tadpoles and young of the Common Frog. 1, Eeceutly hatched (twice 

 natural size) ; 2 and 2a, same enlarged to show the external gills ; 3 and 4, 

 later stages with gill-slits covered by a membrane leaving only the spiracle 

 (see Fig. 16) as an exit for the respired water ; 5, with hind legs appearing ; 

 6, with both fore and hind legs ; 7, atrophy of the tail ; 8, young frog. 



to be fitted for life on dry land. From what we other- 

 wise know of the structure of the frog and the animals 

 to which it is allied, we are justified in concluding that 

 the tadpole is a recapitulative phase of development, 



