Chap. VI.] OF NATURAL SELECTION. 237 



to inheritance from a common progenitor. Mr. Mivart 

 has advanced this case as one of special difficulty, but I 

 am unable to see the force of his argument. An organ 

 for vision must be formed of transparent tissue, and 

 must include some sort of lens for throwing an image 

 at the back of a darkened chamber. Beyond this 

 superficial resemblance, there is hardly any real simi- 

 larity between the eyes of cuttle-fish and vertebrates, as 

 may be seen by consulting Hensen's admirable memoir 

 on these organs in the Cephalopoda. It is impossible 

 for me here to enter on details, but I may specify a few 

 of the points of difference. The crystalline lens in the 

 higher cuttle-fish consists of two parts, placed one 

 behind the other like two lenses, both having a very 

 different structure and disposition to what occurs in the 

 vertebrata. The retina is wholly different, with an 

 actual inversion of the elemental parts, and with a large 

 nervous ^andion included within the membranes of the 

 eye. The relations of the muscles are as different as it 

 is possible to conceive, and so in other points. Hence 

 it is not a little difficult to decide how far even the 

 same terms ought to be employed in describing the eyes 

 of the Cephalopoda and Vertebrata. It is, of course, 

 open to any one to deny that the eye in either case 

 could have been developed through the natural selection 

 of successive slight variations ; but if this be admitted 

 in the one case, it is clearly possible in the other ; and 

 fundamental differences of structure in the visual organs 

 of two groups might have been anticipated, in accord- 

 ance with this view of their manner of formation. As 

 two men have sometimes independently hit on the same 

 invention, so in the several foregoing cases it appears 

 that natural selection, working for the good of each 



