In several species of _Taeniocampa_, especially _instabilis_, the multiplicity of forms is extreme, while _cruda_ (= _pulverulenta_) is a comparatively constant species. The genus _Plusia_ contains a number of constant species, but in _Plusia interrogationis_ we meet the fact that the central silvery mark undergoes endless variation. "Truly no two are alike," says Mr. Tutt, "and to look down a long series of _interrogationis_ is something like looking at a series of Chinese characters." In contrast to this we have the fact that in _Plusia gamma_ the very similar silvery mark is by no means variable. I have taken this series of cases from the Noctuid moths, but it would be as easy to illustrate the same proposition from the Geometridae or the Micro-Lepidoptera.[15] I have a long series of _Peronea cristana_, for example, which was given to me by Mr. W. H. B. Fletcher, of Bognor. All were beaten out of the same hedge, and their polymorphism is such that no one unaccustomed to such examples could suppose that they belonged to a single species. Another common form, _P. schalleriana_, which lives in similar circumstances, exhibits comparatively slight variability. It should be expressly noted that the variation of which I am speaking is a genuine polymorphism. Several of the species enumerated exhibit also geographical variation, possessing definite and often strikingly distinct races peculiar to certain localities; but apart from the existence of such local differentiation, stands out the fact upon which I would lay stress, that some species are excessively variable while others are by comparison constant, in circumstances that we may fairly regard as comparable.