THE CONQUEST OF THE DRY LAND 187 



Land 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, ex- 

 cept the first one, which becomes a tube (the 

 Eustachian tube, named after an old anato- 

 mist) 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 un- 

 sounded letters in words, which tell us part of 

 the history of the word. Thus the unsounded 

 o in leopard tell us that this animal used to 

 be regarded as a cross between 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 lit- 

 tle way below the surface. On this drum the 



