pigment from the lower side--are not structurally correlated with one another at all as changes in different parts of the organism in a mutation are said to be, but are all closely related to their functions in the new position of the body. A mutation consisting in general asymmetry would be comprehensible, but the head of the Pleuronectid is not asymmetrical in a general sense, but only so far as to allow of the changed position of the eyes. The posterior end of the skull is as symmetrical as in any other fish, and in some cases the mouth and jaws are also symmetrical, entirely unaffected by the change in the position of the eyes. In other cases the jaws are asymmetrical in a direction opposite to that of the eyes, there is no change of position but a much greater development of the lower half of the jaws, reduction, with absence of teeth, of the upper half. In the latter case the fish feeds on worms and molluscs living on the ground and seized with the lower half of the jaws, in the former the food consists of small fish swimming above the Flat-fish and seized with the whole of the jaws (Turbot, Halibut, etc.). I contend, then, that the mode in which the normal Flat-fish develops is quite different from that in which mutations arise. T. H. Morgan [Footnote: _A Critique of the Theory of Evolution_ (1916), p. 18.] states that a variation arising in the germ-plasm, no matter what its cause, may affect any stage in the development of the next individuals that arise from it. In certain cases this is true, that is to say, when there are very distinct stages already. For example, a green caterpillar becomes a white butterfly with black spots. A mutation might affect the black spots, an individual might be produced which had two spots on each wing instead of one, and no sign of this mutation would be evident in the caterpillar. But my contention is that when this mutation occurred, the original condition of one spot would not be first developed and then gradually split into two. Morgan proceeds to state clearly what I wish to insist upon concerning mutations. He writes that in recent times the idea that variations are discontinuous has become current. Actual experience, he tells us, shows that new characters do not add themselves to the line of existing characters, but if they affect the adult characters, they change them without as it were passing through and beyond them.