, V t &3T&DIJBS IN LIFE AND SENSE. 



4 just that amount of distinction and variation 



wHich' tHe evolutionist would expect to find in products and structures 

 of two varied and divergent twigs of the tree of descent. Darwin 

 remarks, in reply to Mr. Mivart, that Hensen's memoir on the cuttle- 

 fish eye incontestably shows the difference between that organ and 

 the eye of Vertebrata. The only likeness is that implied in vision 

 of all kinds a transparent organ, containing a lens " for throwing an 

 image at the back of a darkened chamber." When the two eyes are 

 carefully compared, the differences become prominent and apparent. 

 Thus, as Mr. Lankester has shown, the eye of the cuttlefish begins 

 its development as a pit (fig. 7, D) in the epiblast or outer layer of the 

 embryo. Around this pit grows a fold which, as its edges meet in 

 the middle line (E), shuts off the pit from the exterior. The epiblast 

 lining the front of the pit or vesicle becomes the ciliary body and 

 processes of the eye, whilst that lining the back of the vesicle gives 

 origin to the retina (R). Then the pit becomes a closed sac, and a 

 third layer (mesoblast) grows between the outer epiblast and its wall. 

 The lens of the eye now forms in two pieces. The inner piece grows 

 from the front wall of the pit or vesicle into its cavity, and ultimately 

 the lens exhibits the characteristic double structure (B, L 1 L 2 ) of the 

 adult cuttlefish eye. The iris (IR) grows outside the optic vesicle in 

 front, in the shape of two folds, whilst external to the iris other two 

 folds (s s) form the front chamber of the eye. This front chamber 

 may or may not remain closed ; usually it opens externally by a small 

 aperture (B) which persists in the middle of the cornea. 



Thus there can be little hesitation in affirming that a study of the 

 eyes of the cuttlefishes teaches two important lessons : Firstly, that 

 their development adds another proof to the already overwhelming 

 amount of testimony which supports the doctrine of evolution. In 

 the course of its development the eye of one of the higher or two- 

 gilled cuttlefishes (B) passes through stages which correspond with 

 the permanent condition of the eye in the nautilus (A), in which 

 there is neither lens, vitreous humour, nor cornea, the eye being 

 merely a vesicle or sac lined by the retina (R), and opening ex- 

 ternally by a very small aperture. Just before the optic pit becomes 

 closed (E), the permanent state of the nautilus eye is duly figured 

 forth. Again, at a later stage, when the vesicle is closed (c) and 

 when the lens (L) projects into it, the condition of eye common in 

 the adult gasteropod is imitated. The development of a single 

 higher cuttlefish eye (B) is, in fact, a panorama of the evolution of 

 molluscan eyes at large. 



A second lesson taught us by the investigation of the organ of 

 sight in cuttlefishes is that of hesitation in assuming or rejecting the 

 genetic relationship of living forms, or in criticising the possibilities 

 of evolution, until exact research has placed the determination of 



