THE ARGUMENT FROM EMBRYOLOGY 



109 



eye of a cuttle-fish, we find that the eye, although a 

 complicated one, yet passes in its own development 

 through all the above series of stages, from the 

 slight depression of skin, through the stages of a pit 



Fig. 18 



PATELLA 



HALIOTIS 



FISSURELLA 



Diagram of eyes of Patella, Haliotis, and Fissurella. 



In Patella no image is formed. In Haliotis an indistinct image is 

 formed by narrowing of the mouth of the pit. In Fissurella a distinct 

 image is formed by the lens. 



with large and small mouth ; lens and finally eyelids 

 being developed. (Fig. 19.) 



The important point here is that we are able to 

 show that the series fulfils our conditions, that all 

 stages are possible ones, and that each is a distinct 

 step onwards and an improvement on its pre- 

 decessor ; and furthermore, that each stage is retained 

 as the actual permanent condition in some actually 

 living mollusc. 



It is not always possible to point out so clearly as 



