SENSATIOX. 91 



Amphibia and Eeptiles, 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 

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

 which is able so suddenly to form its adj ustments ; 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 ^'isual 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 jDrotection at the present 

 day. In other w^ords, the evolution of the eyes of birds and 

 of the protective coloration of their 23rey must be supposed 

 to have progressed ;pari j^ctssic, each stage in the one acting 

 as a cause in the succeeding stage of the other. The crystal- 

 line lens is flat in birds which are remarkable for long^ sig'lit, 

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

 sighted ; and becomes progressively more spherical in aquatic 

 birds, according to their aquatic habits. 



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

 first meet with definite evidence of an ear capable of appre- 

 ciating with delicacy differences of pitch. Among many 

 species of birds the delicacy of such appreciation (as well as 

 that of timbre) is so remarkable that it may be questioned 

 whether even human ears are more efficient in this respect. 



