Chap. VI.] OF NATUBAL SELECTION. 



237 



from a common progenitor, ilr. 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 resem- 

 blance, there is hardly any real similarity 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 ganglion in- 

 cluded within the membranes of the eye. The relations 

 of the muscles are as different as it is possible to con- 

 ceive, 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 de- 

 veloped 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 accordance with 

 this view of their manner of formation. As two men 

 have sometimes independently hit on the same inven- 

 tion, so in the several foregoing cases it appears that 

 natural selection, working for the good of each being, 



