BY THE PPRESIDENT. 
127 
to obtain single vision. In other words the eye may learn not to 
see, and ends in not being able to see. The absence of light, or 
what is called cavern darkness, which is associated with the con¬ 
ditions of existence of the adult stage of many animals, has caused 
in them the same visual degeneration. The function of the organ 
has become useless. Thus for instance the crabs* which live in the 
branchial cavities of many Mollusca gradually become blind, and, 
when old, lose all trace of eyes, which were well developed before 
they entered their dark habitat. Thus, too, the eyes of the mole 
are in course of retrogression and sometimes vary in structure in 
the same individual. Thus, too, the blind Amphibian— Proteus — 
living in the subterranean waters of Carniola and Dalmatia, has 
eyes in a rudimentary stage, deeply seated in the head and covered 
with skin. There are many such illustrations, and to this incomplete 
list maybe added the reptiles with a retrograded “ pineal eye ” f 
situated on the median line on the top of the head, and which 
appears to have been possessed in full development by some of the 
animals of the Pre-tertiary periocls. The exact function of this 
organ is not known, and we can only guess at the conditions which 
have led to its disuse and degeneration, but it may have been 
limited to the acute discernment of the direction whence came rays 
of heat or light, and have guided animals to clear atmosphere out 
of dense mists. In fact, just as the ocelli of moths may enable 
these insects to avoid that excess of light which renders them 
temporarily blind as to form by the contraction of the pigment of 
their compound eyes, J so the “ pineal eye ” may have enabled extinct 
animals to reach places where their true organs of vision could 
perform their function. As the atmosphere of our earth assumed 
its present condition, so the “ pineal eye ” would have become of 
little use, and would have degenerated until it reached the various 
stages of retrogression seen in some of the reptiles of our time. 
It is clear that there is a relation between the absence of light 
and the degeneration of visual organs, but it is equally true that 
many animals inhabiting dark caves, and the Pacific Ocean at a 
depth of nearly three thousand fathoms, have well-developed eyes, 
while others have not. It has been suggested that the bottom of 
the ocean is illuminated with phosphorescent light diffused by deep- 
sea creatures, but of this there is no direct evidence, any more than 
* These crabs belong to the family Pinnotheridse. 
t The pineal eye is of the Invertebrate type. See “ Pineal Eye in Lacertilia,” 
by W. Baldwin Spencer, ‘ Quarterly Journ. of Microscopical Science,’ vol. xxvii, 
new series. 
X See supra p. 112, and my paper on “ Instinct” in the ‘ Transactions ’ of this 
Society, Vol. Ill, p. 133. 
