170 DIFFICULTIES OF THE THEORY 



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 tke 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 inven- 

 tion, 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 function is concerned, in distinct organic 

 beings, which owe none of their structure in common to 

 inheritance from a common progenitor. 



Fritz Mtiller, 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 Mtiller, and which 

 are nearly related to each other, the species agree most 

 closely in all important characters : namely in their sense 

 organs, circulating systems, 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 impor- 

 tant air-breathing apparatus would have been the same ; for 

 why should this one apparatus, given for the same purpose. 



