344 H. M. BERNARD. 
proper. These eyes are constructed on different plans, and 
must either, according to the theory, be different modifications 
of the same kind of skin, or else modifications of two different 
types of skin. I shall endeavour to show that the latter 
assumption best accords with the facts ; that the change which 
undoubtedly took place in the character of the skin as the 
Vertebrata arose out of their Invetebrate ancestors is sufficient 
to explain the difference between these two types of eye. 
Following the lines of the theory, I make two assumptions :— 
(1) The retina is but a specialised portion of the epithelial 
layer of the skin, between the cells of which the pigment 
granules from the subjacent chromatophoral layer stream 
outwards under the action of light. (2) The retina and the 
chromatophoral layer must have been in intimate and insepar- 
able association through all the stages of the evolution of the 
eye. I have, then, to try to show how far the facts relating 
first to the structure, and secondly to the development of the 
Vertebrate eyes can be harmonised with these assumptions ; how 
far, indeed, it is possible to deduce the Vertebrate eyes directly 
and continuously from the skin. 
A glance at the diagrams (Plate 22) shows that a striking 
parallel between the eyes and the skins, out of which I assume 
them to have arisen, can be easily instituted, but the details 
are by no means easy to work out, and still more difficult to 
demonstrate. The greatest difficulty, of course, lies in the 
fact that, according to their ontogenetic histories, the eyes 
either wholly or partly developed from the brain. It is true 
that no one has yet succeeded in elucidating these embryological 
records ; nevertheless they hold the field, even though amount- 
ing to little more than bald assertions that the eyes developed 
from the brain and not from the skin. Before critically exa- 
mining this adverse testimony I propose to marshal all the 
available facts which seem to connect the eyes directly with 
the skin, and to suggest possible explanations of the various 
modifications and specialisations of the tissues of which our 
eyes are constructed. Only after having shown that it is 
possible to deduce the eyes as continuous structural modifica- 
