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 
Reichenbach 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 Gétte’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 
Cephalaspid, the lateral eyes temained throughout functional. I 
therefore, for my own part, would say that the inversion of the 
