236 MAMMALS IN THE WATER 



pollard-tree, dig holes, dive in salt water, travel fast on 

 land, and run at the bottom of the water. 



Compared with the aquatic powers of civilized man 

 the only mammal, except a monkey, which does not 

 swim naturally these feats are very surprising ; but 

 the list of land animals which are expert swimmers is 

 very much larger than might be supposed. It also 

 embraces many classes of animals, and the number of 

 the aquatic or semi-aquatic members of very different 

 families suggests that the aquatic habit was at first only 

 accidental, and that very many creatures which do not 

 by habit swim and dive might, under other circum- 

 stances, have become aquatic. Judging from our own 

 experience, one of the most difficult * adaptations ' 

 of habit encountered in the amphibious life is that 

 of keeping the eyes open under water, with no special 

 protection. It is disagreeable in fresh and painful in 

 salt water. Conceding that the really amphibious 

 creatures have learnt to do this gradually otters, 

 water-voles, water-shrews, polar bears, and seals 

 how are we to account for the aquatic dexterity ot 

 a creature like the land-rat ? A common brown rat 

 can see under water as well as a water-rat can ; it 

 swims and dives equally well, and can burrow into a 

 bank below the water. This is less creditable engineer- 

 ing than the sub-aquatic work of the beaver, which 

 buries logs and fixes the foundations of the dam under 



