180 DIFFICULTIES OF THE THEORY 



The luminous organs which occur in a few insects, 

 belonging to widely different families, and which are sit- 

 uated in diiferent parts of the body, offer, under our pres- 

 ent state of ignorance, a difficulty almost exactly parallel 

 with that of the electric organs. Other similar cases could 

 be given; for instance in plants, the very curious contriv- 

 ance of a mass of pollen-grains, borne on a foot-stalk with 

 an adhesive gland, is apparently the same in Orchis and 

 Asciepias, genera almost as remote as is possible among 

 flowering plants; but here again the parts are not 

 homologous. In all cases of beings, far removed from 

 each other in the scale of organization, which are fur- 

 nished 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 differences 

 between them can always be detected. For instance, the 

 eyes of Gephalopods or cuttle-fish and of vertebrate ani- 

 mals appear wonderfully alike; and in such widely sun- 

 dered 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 difficult}^, 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 superficial re- 

 semblance, there is hardly any real similarity between the 

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

 sulting 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 

 wnat 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. Tlie relations of the muscles 

 are as different as it is possible to conceive, and so in 

 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 



