Chap. XX. SELECTION. 223 



Beagle could certainly see distant objects more distinctly than 

 our sailors with all their long practice ; I do not know whether 

 this depends on nervous sensitiveness or on the power of 

 adjustment in the focus; but this capacity for distant vision 

 might, it is probable, be slightly augmented by successive modi- 

 fications of either kind. Amphibious animals, which are enabled 

 to see both in the water and in the air, require and possess, as 

 M. Plateau has shown, 92 eyes constructed on the following plan : 

 " the cornea is always flat, or at least much flattened in front of 

 " the crystalline and over a space equal to the diameter of that 

 " lens, whilst the lateral portions may be much curved." The 

 crystalline is very nearly a sphere, and the humours have nearly 

 the same density as water. Now, as a terrestrial animal slowly 

 became more and more aquatic in its habits, very slight changes, 

 first in the curvature of the cornea or crystalline, and then in 

 the density of the humours, or conversely, might successively 

 occur, and would be advantageous to the animal whilst under 

 water, without serious detriment to its power of vision in the air. 

 It is of course impossible to conjecture by what steps the fun- 

 damental structure of the eye in the Vertebrata was originally 

 acquired, for we know absolutely nothing about this ^ organ in 

 the first progenitors of the class. With respect to the lowest 

 animals in the scale, the transitional states through which the 

 eye at first probably passed, can by the aid of analogy be indi- 

 cated, as I have attempted to show in my « Origin of Species.' 93 



92 On the Vision of Fishes and Amphibia, translated in « Annals and Mag. of 

 Nat. Hist.,' vol. xviii., 1866, p. 469. ™ Fourth edition, 1866, p. 215° 



