208 SELECTION. Chap. XX. 



has often been advanced as an excellent instance of adapta- 

 tion. Short-sight, which is often inherited, permits a person 

 to see distinctly a minnte object at so near a distance that it 

 would be indistinct to ordinary eyes ; and here we have a 

 capacity which might be serviceable under certain conditions, 

 abruptly gained. The Fuegians on board the Beagle could 

 certainly see distant objects more distinctly than our sailors 

 with all their long practice; I do not know whether this 

 depends upon 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 modifications 

 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 the 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 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 advan- 

 tageous 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 fundamental 

 structure of the eye in the Yertebrata was originally acquired, 

 for we know nothing about this organ in the first progenitors 

 of the class. \Yith 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 indicated, as I have 

 attempted to show in my ' Origin of Species.' 93 



92 On the Vision of Fishes and p. 469. 

 Amphibia, translated in 'Annals and 93 Sixth edition, 1872, p, 144. 



Mag. of Nat. Hist.,' vol. xviii., 1866, 



