One of the most familiar cases of degeneration is blindness, associated with life in the dark. The most striking cases are those of cave animals, but this is only an extreme example of what is found everywhere amongst animals that live concealed during the day under stones, etc. The blind fish and the blind crayfish of the Mammoth Cave, the blind proteus of the caves of Carniola, the blind mole that burrows underground, the blind larvæ of many insects that live in the dark, are examples most often cited. Some nocturnal animals, like the earthworm, have no eyes, although they are still able to distinguish light; and some of the deep-sea animals, that live below the depth to which light penetrates, have degenerate eyes. The workers of some ants, that remain in the nests, are blind, but the males and the queens of these forms have well-developed eyes, although the eyes may be of use to them at only one short period of their life, namely, at the time of the marriage flight. This fact is significant and is underestimated by those who believe that disuse accounts for the degeneration of organs. The wings of the ostrich and of the kiwi are rudimentary structures no longer used for flight, and many insects, belonging to several different orders, have lost their wings, as seen in fleas, some kinds of bugs, and moths, and even in some grasshoppers. A curious case of degeneration is found in the abdomen of the hermit crab, which is protected by the appropriated shell of a snail. The appendages of one side of the abdomen have nearly disappeared in the male, although in the female the abdominal appendages are used to carry the eggs as in other decapod crustaceans. The abdomen, instead of being covered by a hard cuticle, as in other members of this group, is soft and unprotected except by the shell of the snail. Cases of these kinds could be added to almost indefinitely, and the explanation of these degenerate structures has been a source of contention amongst zoologists for a long time. The most obvious interpretation is that the degeneration has been the result of disuse. But as I have already discussed this question, and given my reasons for regarding it as improbable that degeneration has arisen in this way, we need not further consider this point here. The selectionists have offered several suggestions to account for degeneration. In fact, this has been one of the difficulties that has given them most concern. They have suggested, for example, that when an organ is no longer of use to its possessor it would become a source of danger, and hence would be removed through natural selection. They have also suggested that since such organs draw on the general food supply they would place their possessor at a disadvantage, and hence would be removed. Weismann has attempted to meet the difficulty by his theory of “Panmixia,” or universal crossing, by which means the useless structures are imagined to be eliminated.