THE CONQUEST OF THE DRY LAND 187 



animals carry about in their bodies the tell- 

 tale evidences of a marine, or at least of an 

 aquatic, ancestry. Thus all the embryos of 

 reptiles, birds, and mammals have gill-clefts on 

 the sides of their neck, opening into the pharynx 

 (the beginning of the food-canal, just behind 

 the mouth), and in two or three cases, in reptile 

 and bird, tuft-like traces of the gills themselves 

 have been recently discovered. These gill- 

 clefts are of no use for breathing in reptiles, 

 birds, and mammals ; indeed, we cannot say that 

 they are of any use at all, except the first one, 

 which becomes a tube (the Eustachian tube, 

 named after an old anatomist) leading from the 

 ear-passage to the back of the mouth. But 

 these gill-clefts are always present, and they 

 must be regarded as historic relics. As Darwin 

 said, they are like unsounded letters in words, 

 which tell us part of the history of the word. 

 Thus the unsounded o in leopard tells us that 

 this animal used to be regarded as a cross be- 

 tween a lion and a tiger (or pard). So there 

 are vestiges in land animals which betray their 

 aquatic ancestry. 



In the ear-passage of a mammal there is a 

 drum or tympanum stretched across just a little 

 way below the surface. On this drum the 



