90 THE GENESIS OF SPECIES. [CHAP. 



the organ of sight, as has been well pointed out by Mr. J. 

 J. Murphy. 12 He calls attention to the fact that the eye 

 must have been perfected in at least " three distinct lines 

 of descent," alluding not only to the molluscous division 

 of the animal kingdom, and the division provided with a 

 spinal column, but also to a third primary division, namely, 

 that which includes all insects, spiders, crabs, etc., which 

 are spoken of as Annulosa, and the type of whose structure 

 is as distinct from that of the molluscous type on the one 

 hand, as it is from that of the type with a spinal column 

 (i. e., the vertebrate type) on the other. 



In the cuttle-fishes we find an eye even more complete- 

 ly constructed on the vertebrate type than is the ear. 

 Sclerotic, retina, choroid, vitreous humor, lens, aqueous hu- 

 mor, all are present. The correspondence is wonderfully 

 complete, and there can hardly be any hesitation in saying 

 that for such an exact, prolonged, and correlated series of 

 similar structures to have been brought about in two inde- 

 pendent instances by merely indefinite and minute acci- 

 dental variations, is an improbability which amounts' prac- 

 tically to impossibility. Moreover, we have here again 

 the same imperfection of the four-gilled cephalopod, as com- 

 pared with the two-gilled, and therefore (if the latter pro- 

 ceeded from the former) a similar indication of a certain 

 comparative rapidity of development. Finally, and this is 

 perhaps one of the most curious circumstances, the process 

 of formation appears to have been, at least in some re- 

 spects, the same in the eyes of these molluscous animals as 

 in the eyes of vertebrates. For in these latter the cornea 

 is at first perforated, while different degrees of perforation 

 of the same part are presented by different adult cuttle- 

 fishes large in the calamaries, smaller in the octopods, 

 and reduced to a minute foramen in the true cuttle-fish 

 sepia. 



12 See "Habit and Intelligence," vol. i., p. 321. 



