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 hon 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 
