D EG EN ERA TION. 



23 



The frog's ^g'g 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 to be fitted for life on dry land. From 

 what we otherwise know of the structure of the frog; 



Fig. 3. — Adult shrimp of the genus Peneus. 



and the animals to which it is allied, we are 

 justified in concluding that the tadpole is a recapi- 

 tulative phase of development, and represents to us 

 more or less closely an ancestor of the frog which 

 was provided with gills and tail in the adult state, 

 and possessed neither legs nor lungs. 



A less familiar case is that of a certain kind of 



