Weak or defective eyesight is by no means rare as a spontaneous
variation in animals, "the great French veterinary Huzard going so far
as to say that a blind race [of horses] could soon be formed." Natural
selection evolves blind races whenever eyes are useless or
disadvantageous, as with parasites. This may apparently be done
independently of the effects of disuse, for certain neuter ants have
eyes which are reduced to a more or less rudimentary condition, and
neuter termites are blind as well as wingless. In one species of ant
(_Eciton vastator_) the sockets have disappeared as well as the eyes. In
deep caves not only would natural selection cease to maintain good
eyesight but it would persistently favour blindness--or the entire
removal of the eye when greatly exposed, as in the cave-crab--and as Dr.
Ray Lankester has indicated,[38] there would have been a previous
selection of animals which through spontaneous weakness, sensitiveness,
or other affection of the eye found refuge and preservation in the cave,
and a subsequent selection of the descendants whose fitness for relative
darkness led them deeper into the cave or prevented them from straying
back to the light with its various dangers and severer competition.
Panmixia, however, as Weismann has shown, would probably be the most
important factor in causing blindness.