Blind Cave-Animals
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.