There is no evidence that these abnormalities are due to abnormal conditions of life. One specimen of Plaice of this type was kept alive in the aquarium, and it lay on its side, buried itself in the sand, and when disturbed swam horizontally, like a normal specimen. The abnormalities are undoubtedly mutations of gametic origin. The development of one of these abnormal specimens from the egg has not to my knowledge been traced, but there is no reason to suppose that the fish develops first into the normal asymmetrical condition and then changes gradually to the abnormal condition described. On the contrary, everything points to the conclusion that the abnormality is an arrest or incomplete occurrence of the normal process of development, _i.e._ of the normal metamorphosis. T. H. Morgan, in a volume published some years ago, [Footnote: _Evolution and Adaptation_.] put forward the extraordinary view that the Pleuronectidae arose from symmetrical fishes by a mutation which was entirely gametogenetic and entirely independent of habits or external conditions, and then finding itself with two eyes on one side of its head, and no air-bladder, adopted the new mode of life, the new habit of lying on the ground on one side in order to make better use of its asymmetrically placed eyes. According to this view habits have been adapted to structure, not structure to habits. We are thus to believe that Amphibia came out of the water and breathed air because by an accidental mutation they possessed lungs and a pulmonary circulation capable of atmospheric respiration. Such is the result of applying conclusions derived from phenomena of one kind to phenomena of a totally different kind. One of the chief differences between structural features and correlations which are adaptive from those which are not is the process of metamorphosis, where we see the structure changing in the individual life history as the mode of life changes. The egg of the Flat-fish develops into a symmetrical pelagic larva similar to that of many other marine fishes. The larva has an eye on each side of its head and swims with its plane of symmetry in a vertical position: it has also colour on both sides equally. When the skeleton begins to develop the transformation takes place: the eye of one side, left in some species, right in others, moves gradually to the edge of the head and then on to the other side. The dorsal fin extends forward, preserving its original direction, and so passes between the eye that has changed its position and the lower side of the fish, on which that eye was originally situated. In some cases this extension of the fin takes place earlier and the eye passes beneath the base of the fin to reach the other side. Any one who takes the trouble to make himself acquainted with the facts will see that the three chief features of the Pleuronectid--namely, the position of the eyes, the extension of the dorsal and ventral fins, and the absence of