SENSATION. 91 



Amphibia and Reptiles, are not much, if at all, in advance of 

 these senses as they occur in Fish. 



Among Birds the sense of sight is proverbially keen, and 

 in point of fact the animal kingdom has no parallel to the 

 excellence of the organ of vision as it occurs in some species 

 of this class. Whether we consider the eye of a Hawk, which 

 is able to distinguish from a great height a protectively 

 coloured animal from the surface of the ground which it so 

 closely imitates ; or the eye of a Solen Goose, which is able 

 from a height of a hundred feet in the air to see a fish at the 

 depth of many fathoms in the water ; or the eye of a Swift, 

 which is able so suddenly to form its adjustments ; we must 

 alike conclude that the visual apparatus has attained to its 

 highest perfection among birds. And in this connection it is 

 of interest to note that protective colouring has attained its 

 highest degree of perfection among animals which constitute 

 the prey of birds. So surprising, indeed, is the perfection to 

 which protective colouring has attained in some of these 

 cases, that it has been adduced as a difficulty against the 

 theory of evolution ; for it seems incredible that such perfec- 

 tion should have been attained by slow stages through natural 

 selection before the species exhibiting it had been extermi- 

 nated by the birds. The answer to this difficulty is that 

 the visual organs of the birds cannot be supposed to have 

 been always so perfect as they are now, and therefore that a 

 degree of protective colouring which might have afforded 

 efficient protection at an earlier stage in the evolution of 

 those organs would not supply such protection at the present 

 day. In other words, the evolution of the eyes of birds ami 

 of the protective coloration of their prey must be supposed 

 to have progressed pari passu, each stage in the one acting 



i cause in the succeeding stage of the other. The crystal- 

 line lens is flat in birds which are remarkable for long sight, 

 Midi as the vulture; rounder in owls, which are very near- 

 sighted ; and heroines progressively more spherical hi aquatic 

 bilda, according to their aquatic habits. 



All birds are able to hear, and it is in this class that we 

 liist meet with definite evidence of an ear capable ul' appre- 

 ciating with delicacy differences of pitch. Among many 

 species of hud- the delicacy of such appreciation (as well ;) s 

 that of timbre) is so remarkable that, it may he questioned 



whether even human c.u.. are more efficient in this respect. 



