22 



DEGESERA TION. 



here and there in these histories of growth from the 

 ^g^ most valuable assistance in the attempt to recon- 

 struct the genealogical tree. The history of the 



Fig. 2. — Tadpoles and young of the Common Frog, i, Recently hatched (twice 

 natural size) ; 2 and ■za, 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. 

 t6) 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. 



development of the common frog is a good illustia- 

 tion of the kind of evidence in question. 



