86 THE GENESIS OF SPECIES. [CHAP. 



auditory organ in a very inferior condition of development 

 to what we find in the dibranch : thus we have not only 

 evidence of the independent high development of the. 

 organ in the former, but also evidence pointing towards a 

 certain degree of comparative rapidity in its development. 



Such being the case with regard to the organ of hearing, 

 we have another yet stronger argument with regard to the 

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

 Murphy. 1 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, viz. 

 that which includes all insects, spiders, crabs, &c., 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 vertebrate type on 

 the other. 



In the cuttle-fishes we find an eye constructed even 

 more completely on the vertebrate type than is the ear. 

 Sclerotic, retina, choroid, vitreous humour, lens, aqueous 

 humour, all are present. The correspondence is wonder- 

 fully 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 independent instances by me.rely indefinite and minute 

 accidental variations, is an improbability which amounts 

 virtually to impossibility. Moreover, we have here again 

 the . sume imperfection of the four-gilled cephalopod, as 

 compared with the two-gilled, and therefore (if the latter 

 proceeded from the former) a similar indication of a cer- 



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



