DIFFICULTIES OF THE THEORY 263 



will be found that although the general appearance and 

 function of the organs may be the same, yet fundamental 

 differences between them can always be detected. For 

 instance, the eyes of cephalopods or cuttle-fish and of 

 vertebrate animals appear wonderfully alike; and in such 

 widely sundered groups no part of this resemblance can 

 be due 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 super- 

 ficial resemblance, 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 

 retiDa is wholly different, with an actual inversion of 

 the elemental parts, and with a large nervous ganglion 

 included 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 Yertebrata. 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 varia- 

 tions; but if this be admitted in the one case, it is 



