200 ORIGIN OF SPECIES 



different parts of the body, offer, under our present state of 

 ignorance, a difficulty almost exactly parallel with that of the 

 electric organs. Other similar cases could be given; for in- 

 stance in plants, the very curious contrivance of a mass of 

 pollen-grains, borne on a foot-stalk with an adhesive gland, 

 is apparently the same in Orchis and Asclepias, — genera al- 

 most as remote as is possible amongst flowering plants; but 

 here again the parts are not homologous. In all cases of be- 

 ings, far removed from each other in the scale of organisa- 

 tion, which are furnished with similar and peculiar organs, 

 it will be found that although the general appearance and 

 function of the organs may be the same, yet fundamental dif- 

 ferences between them can always be detected. For instance, 

 the eyes of cephalopods or cuttle-fish and of vertebrate ani- 

 mals 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. Be- 

 yond 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 dift'erent structure and disposition 

 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 mem- 

 branes of the eye. The relations of the muscles are as dif- 

 ferent 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 



