THE EVOLUTION OF THE EYE. 81 



sacs. Which Ascidian was the wiser in his generation, I for one 

 can hardly tell : whether a life in the sea, where one was fixed 

 beyond the power of storms to move, and daily food came with 

 every ocean current ; or the fierce struggle for existence, and cruel 

 sufferings of the weak, which higher reason brings. 



An interesting history of tribal descent is shown in the eye of 

 the Dibranchiate Molluscs, as compared with the eye of Nautilus. 

 The eye of Nautilus is among the most interesting structures of 

 that remarkable animal — a sole survivor of a long extinct species. 

 No other animal so high in organisation has so simple an eye as 

 that of Nautilus. When looked at from the surface, no metallic 

 lustre, no transparent coverings, are seen. It is simply a slightly 

 projecting hemispherical box like a kettle-drum, half an inch in 

 diameter, and in the middle of the drum membrane is a minute 

 hole. It was very naturally thought some membrane had covered 

 this hole during life, and had been ruptured in the specimen 

 studied by Owen. But further researches showed that this hole is 

 a natural opening into the globe of the eye, which is accordingly 

 filled with sea water during life. In short, of all the parts which 

 in common parlance are called the eye, none exists in this most 

 primitive optical apparatus, which is arranged to form an image on 

 the principle of the " pin-hole " camera. There is no cornea and 

 no lens, and the naked retina is bathed by sea water on one side, 

 and on the other receives the fibres of the optic nerve. 



The most interesting consideration connected with this eye of 

 Nautilus is this — that the elaborate lens-bearing eyes of the 

 Dibranchiate Molluscs, such as the well-known cuttle-fish, pass 

 through an embryonic stage of development, in which they have 

 exactly the structure of the eye of Nautilus — namely, of simple 

 open sacs. Such, too, is the structure of the eye in the limpet. 



I come now to one of the most recent and curious discoveries 

 in the history of descent. As every zoologist knows, a special 

 interest attaches to all transitional forms ; the most apparently 

 insignificant animal becomes invested with the highest value, 

 where it can be said to form a link between the different branches 

 of the animal kingdom. The Ascidian, the lancelet, the mud-fish 

 of Australia, are all examples in point. 



A learned professor is even now in New Zealand investigating 



Vol. VI. G 



