134 ANIMAL MECHANISM. 



adapted for seeing equally well in air and water, can have 

 but imperfect vision in either. These conclusions are in 

 conformity with what is known of the power of vision in 

 those animals that live partly on the land and partly in 

 the water. The seal (phoca) is one of those animals that 

 live in both elements. But the seal has but imperfect 

 vision in the air. Rosenthal in his memoir on the organs 

 of the senses of seals, says, ' we have convinced ourselves 

 by careful observation with living seals, of the species 

 Phoca Grypus of Faber, that the animal is always short- 

 sighted in the air ; for when we held before it fish and 

 other bodies, as pieces of wood or stones, it did not dis- 

 tinguish them accurately, until they were brought so 

 near, that the organ of smell could be called into activity. 

 I have the most satisfactory evidence of the short-sighted- 

 ness of seals, from a series of experiments and observa- 

 tions, made in Boston harbor. My duties requiring me 

 to be floating in a boat, from vessel to vessel, many 

 months of the year, I have been so often accompanied by 

 seals, alongside and astern, as to establish the fact, that 

 they can see but a few yards in the air, and then very 

 obscurely. Scoresby remarks, ' Whales are observed to 

 discover one another, in clear water, when under the 

 surface, at an amazing distance. When at the surface, 

 however, they do not see far.' Scoresby' 1 s Arctic Regions, 

 vol. i. p. 456. Faber, in his very interesting work on 

 the habits and manners of birds that inhabit high north- 

 ern latitudes, remarks that Divers (Colymbus) do not see 

 so well above water as Grebes (Podiceps,) but better 

 under water, because it is there they obtain their food. 



It also appears, that birds which see well in one ele- 

 ment, do not see so welt in the other. Faber proposes 

 the question, ' Is it the case that divers, when under 

 water, draw their nictitating membrane over the eye, as 

 they do when looking towards the sun, in order to pre- 

 vent the contact of the water ?' It would appear, from 

 the observations of Treviranus, from whose excellent 

 work, the observations on vision we are now detailing 

 are principally extracted, that, by drawing the nictitating 

 membrane over the eye, divers, and all other land animals 

 which seek their food under water, are enabled, not only 



