ATTEMPT TO DEDUCE VERTEBRATE EYES FROM THE SKIN. 361 
ascertain (1) whether any plausible phylogenetic explanation 
can be given them? and (2) whether they cannot be more 
simply explained as adaptations? With regard to the former 
test, I confess at the outset that it seems to me almost waste 
of time to discuss the possibility of these complicated in- 
foldings and fusions of separate parts indicating the history 
of such a singularly compact organ as the eye, especially if we 
are to assume, what we are justified in assuming, an unbroken 
continuity of function from the earliest rudiments to its most 
complicated development. We are, in the first place, pre- 
cluded from believing that the functional eye developed con- 
tinuously from the brain towards the skin which then formed 
a lens, as we should have no explanation of the bending back 
of the nerve-fibrils. In order to get over this difficulty it is 
suggested that the primary optic vesicle, i.e. the invagination 
from the brain, may represent a primitive eye of the type 
known as an “optic pit.” It is true that, according to our 
interpretation of the facts, we have evidence in the pineal eye 
or eyes that such structures did at one time exist on the heads 
of the ancestors of the Vertebrata. Ifa pair of such eyes were 
caught and eventually folded in by the medullary groove, they 
might, it is thought, give rise to these “ primary optic vesicles,” 
with the retine in the right position to become the retine of 
the paired Vertebrate eyes. And, at first sight, there seems 
to be some possibility of deducing the Vertebrate eyes from a 
pair of engulfed, and therefore vestigial, and for the time 
being functionless eyes, which may have started into life once 
more by coming into contact on each side with some thicken- 
ing, or perhaps glandular invagination, of the ectoderm. This 
latter, by condensing the light upon the pigment still present 
in these buried eyes, might once more set it in regular move- 
ments, which would record the variations in the intensity of 
the light. Some such hypothesis, it is claimed, would explain 
what is called the inversion of the eye, for the light would 
now shine through what was formerly the under side of the 
retina. 
The closer, however, this suggestion is examined, the more 
