362 H. M. BERNARD. 
unworkable it becomes. Whence, for instance, came the optic 
nerve of such an eye before it was engulfed in the medullary 
groove? If from some primitive ganglionic centre, how came 
it to be transferred to the definitive centre formed by the 
medullary groove? How did this optic nerve manage to 
coincide with the stalk of the invagination? Further, it is 
not easy to deduce in this way the specialised association of 
the chromatophoral layer with the retinal layer. The actual 
evidence against the primary optic vesicles having been me- 
chanically forced in by the lens is very strong, quite apart 
from the difficulty of imagining why a lens-like body should 
crush in the optic vesicle, and, as if by a happy accident, create 
a highly organised eye. Why, again, was the pigment con- 
fined to the (originally) external hemispheres of these buried 
optic pits? Lastly, we do not know to what extent the me- 
dullary groove is historical. Nervous systems usually develop 
from thickenings of the ectoderm; and if a large amount of 
material is required, the thickening may easily become an 
invagination or a groove, and it is by no means necessary to 
suppose that such invagination has any phylogenetic significa- 
tion at all; it may be purely an adaptive process for the supply 
of formative tissue. Indeed, in the Cyclostomata and bony 
fishes, which are very low down in the Vertebrate phylum, 
the central nerve-strand actually arises as a solid thickening. 
But, admitting for the present that the medullary groove may 
have some historical value, eyes are not very likely to let 
themselves be tucked in so as to be functionless, unless there 
is some obstacle to prevent them from shifting. According to 
our theory, the pigment will always be drawn in the direction 
of the strongest illumination, and on changes taking place in 
the form of the body the eyes will shift—through this attrac- 
tive action of the light upon the pigment—into the positions 
in which they can best function as organs of sight.) This 
1 Perhaps one of the most remarkable instances of the power of the eyes to 
follow light is that recorded by Carl Chun (‘ Biologisches Centralblatt,’ xiii, 
1893). Certain Crustaceans have luminous organs which throw light upon 
the ground beneath them, This light has actually drawn down a portion of 
