It has been suggested, therefore, that the occasional sinistral individual of Helix arises from an egg cleaving in the reverse direction, and there is nothing improbable in an assumption of this kind. No attempt has been made as yet to explain why, in some cases, the cleavage spiral is turned in one direction, and in other cases in the reverse direction; but even leaving this unaccounted for, the assumption of the unusual form of Helix being the result of a reversal of the cleavage throws some light as to how it is possible for the complete reversal of the organs of the adult to arise. If it is assumed that in the early embryo the cells on each side of the median line are alike, and at this time capable of forming adult structures, a simple change of the spiral from right to left might determine on which side of the middle line the mesodermal cell would lie, and its presence on one side rather than on the other might determine which side of the embryo would develop, and which would not. This possibility removes much of the mystery which may appear to surround a sudden change of this sort. It seems to me that we shall not go far wrong if we assume that it is largely a matter of indifference whether an individual snail is a right-handed or a left-handed form, as far as its relation to the environment is concerned. One form would have as good a chance for existing as the other. If this is granted, we may conclude that, while in most species a perfectly definite type is found, a right or a left spiral, yet neither the one nor the other has been acquired on account of its relation to the environment. This conclusion does not, of course, commit us in any way as to whether the spiral form of the visceral mass has been acquired in relation to the environment, but only to the view that, if a spiral form is to be produced, it is indifferent which way it turns. From the evolutionary point of view this conclusion is of some importance, since it indicates that one of the alternatives has been adopted and has become practically constant in most cases without selection having had anything to do with it. Somewhat similar conditions are found in the flounders and soles. As is well known, these fishes lie upon one side of the body on the bottom of the ocean. Some species, with the rarest exceptions to be mentioned in a moment, lie always on the right side, others on the left side. A few species are indifferently right or left. At rare intervals a left-sided form is found in a right-sided species, and conversely, a right-sided form in a left-sided species. In such cases the reversed type is as perfectly developed in all respects as the normal form, but with a complete reversal of its right and left sides.