When the young flounders leave the egg, they swim in an upright position, as do ordinary fishes, with both sides equally developed. There cannot be any doubt that the ancestors of these fish were bilaterally symmetrical. Therefore, within the group, both right-handed and left-handed forms have appeared. It seems to me highly improbable that if a right-handed form had been slowly evolved through the selection of favorable variations in this direction, the end result could be suddenly reversed, and a perfect left-sided form appear. Moreover, as has been pointed out, the intermediate stages would have been at a great disadvantage as compared with the parent, and this would lead to their extermination on the selection theory. If, however, we suppose that a variation of this sort appeared at once, and was fixed,—a mutation in other words,—and that whether or not it had an advantage over the parent form, it could still continue to exist, and propagate its kind, then we avoid the chief difficulty of the selection theory. Moreover, we can imagine, at least, that if this variation appeared in the germ and was, in its essential nature, something like the relation seen in the snail, the occasional reversal of the relations of the parts presents no great difficulty. In this same connection may be mentioned a curious fact first discovered by Przibram and later confirmed by others. If the leg carrying the large claw of a crustacean be removed, then, at the next moult, the leg of the other side that had been the smaller first leg becomes the new big one; and the new leg that has regenerated from the place where the big one was cut off becomes the smaller one. Wilson has suggested that both claws in the young crustacean have the power to become either sort. We do not know what decides the matter in the adult, after the removal of one of the claws. Some slight difference may turn the balance one way or the other, so that the smaller claw grows into the larger one. At any rate, there is seen a latent power like that in the egg of the snail. Zeleny has found a similar relation to exist for the big and the little opercula of the marine worm, Hydroides.