VIII.] THE COMMON FROG. 127 



into the throat, so that there come to be three Eusta- 

 chian openings. 



Can the resemblance between Pipa and Dactylethra 

 in this matter be taken as a serious indication of 

 genetic affinity, in spite of the wide, deep, and pro- 

 bably ancient Atlantic which rolls between the two 

 species now ? 



This is a question which cannot be confidently 

 answered, seeing in how many other instances struc- 

 tural pecuHarities have evidently had an independent 

 origin. Nevertheless, the fact that these two genera 

 agree also in the small size of the eyes, rudimentary 

 eyelids, and vastly expanded sacral transverse pro- 

 cess would seem to point to some ancestral and 

 fundamental relationship. If so, however, it is re- 

 markable that no other such forms, or no inter- 

 mediate ones should have been preserved, seeing that 

 neither kind can be suspected of having migrated to 

 its own habitat from the existing habitat of the 

 other ; and therefore that forms similar to that from 

 which we may, if we please, conceive both to have 

 been derived must have had a more or less widely 

 extended geographical distribution and have been 

 numerous in order to have given origin to genera in 

 many respects so different as the two in question. 



