152 Difficulties of the Theory Chap. vi. 



throwing 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 

 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 included within the membranes of the 

 eye. The relations of the muscles are as different as it is pos- 

 sible 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 struc- 

 ture in the visual organs of two groups might have been anti- 

 cipated, 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 advan- 

 tage of all favourable variations, has produced similar organs, as 

 far as function is concerned, in distinct organic beings, which 

 owe none of their structure in common to inheritance from a 

 common progenitor. 



Fritz Miiller, 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 

 examined by Miiller, and which are nearly related to each other, 

 the species agree most closely in all important characters ; namely 

 in their sense-organs, circulating system, in the position of the 

 tufts of hair within their complex stomachs, and lastly in the 

 whole structure of the water-breathing branchiae, even to the 

 microscopical hooks by which they are cleansed. Hence it might 

 have been expected that in the few species belonging to both 

 families which live on the land, the equally-important air-breathing 

 apparatus would have been the same ; for why should this one 

 apparatus, given for the same purpose, have been made to differ, 



