DIFFICULTIES OF THE THEORY 155 



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 throw- 

 ing an image at the back of a darkened chamber. Beyond this 

 superficial resemblance, there is hardly any real similarity between 

 the eyes of cuttle-fish and vertebrates, as may be seen by consult- 

 ing Hensen's admirable memoir on these organs in the Cephalo- 

 poda. 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 disposi- 

 tion to what occurs in the vertebrata. The retina is wholly dif- 

 ferent, 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 

 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 de- 

 scribing 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 

 accordance 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 being, and taking advantage of all 

 favorable variations, has produced similar organs, as far as func- 

 tion is concerned, in distinct organic beings, which owe none of 

 their structure in common to inheritance from a common pro- 

 genitor. 



Fritz Muller, in order to test the conclusions arrived at in this 

 volume, has followed out with much care a nearly similar line of 

 argument. Several families of crustaceans include a few species, 

 possessing an air-breathing apparatus and fitted to live out of the 

 water. In two of these families, which were more especially exam- 

 ined by Muller, and which are nearly related to each other, the 



