114 THE ORIGIN OF VERTEBRATES 



the retina is formed. On the other hand, eyes with an inverted 

 compound retina are not entirely unknown among invertebrates, for 

 the eyes of Pecten and of Spondylus possess a retina which is 

 inverted after the vertebrate fashion and still may be spoken of as 

 compound rather than simple. It is clear that an invagination, the 

 effect of which is an inversion of the retinal layer, would lead to 

 the same result, whether the retinal optic nerves were short or long, 

 whether, in fact, a retinal ganglion existed or not. Undoubtedly the 

 presence of the retinal ganglion tends greatly to obscure any process 

 of invagination, so that, as already mentioned, many observers, with 

 Parker, consider the retina of the crustacean lateral eye to be 

 formed by a thickening only, without any invagination, while 

 Peichenbach says an obscure invagination does take place at a very 

 early stage. So in the vertebrate eye most observers speak only of 

 a thickening to form the retina, but Gotte's observation points to an 

 invagination of the optic plate at an early stage. So also in the eye 

 of Pecten, Korschelt and Heider consider that the thickening, by 

 which the retina is formed according to Patten, in reality hides an 

 invagination process by means of which, as Biitschli suggests, an 

 optic vesicle is formed in the usual manner. The retina is 

 formed from the anterior wall of this vesicle, and is therefore 

 inverted. 



The origin of the inverted retina of the vertebrate eye does not 

 seem to me to present any great difficulty, especially when one 

 takes into consideration the fact that the retina is inverted in the 

 arachnid group, only in the lateral eyes. The inversion is 

 usually regarded as associated with the tubular formation of the 

 vertebrate retina, and it is possible to suppose that the retina became 

 inverted in consequence of the involvement of the eye with the gut- 

 diverticulum. I do not myself think any such explanation is at all 

 probable, because I cannot conceive such a process taking place with- 

 out a temporary derangement — to say the least of it— of the power of 

 vision, and as I do not believe that evolution was brought about by 

 sudden, startling changes, but by gradual, orderly adaptations, and 

 as I also believe in the paramount importance of the organs of 

 vision for the evolution of all the higher types of the animal kingdom, 

 I must believe that in the evolution from the Arthropod to the 

 Gephalaspid, the lateral eyes remained throughout functional. I 

 therefore, for my own part, would say that the inversion of the 



