In this connection a few words on another source of variation may not be out of place. The patterns of butterflies are often very sensitive to changes in the conditions to which they are exposed during later larval and pupal life. Many moths and butterflies in temperate climates are double brooded. The eggs laid by the late summer brood hatch out, hibernate in the larval or pupal state, and emerge in the following spring. This spring brood produces the summer brood during the same year. In these cases it often happens that the two broods differ in appearance from one another, a phenomenon to which the term "Seasonal Dimorphism" has been applied. A well-marked instance is that of the little European Vanessid, _Araschnia levana_. The so-called _levana_ form which emerges in the spring is a small black and orange-brown butterfly (Pl. VI, fig. 10). From the eggs laid by this brood is produced another brood which emerges later on in the summer, and is, from its very different appearance, distinguished as the _prorsa_ form (Pl. VI, fig. 9). It is very much darker than the spring form and is characterised by white bands across the wings. The eggs laid by the _prorsa_ form give rise to the _levana_ form which emerges in the following spring. It has been shewn by various workers, and more especially by the extensive experiments of Merrifield[84], that the appearance of the _levana_ or the _prorsa_ form from any batch of eggs, whether laid by _prorsa_ or _levana_, is dependent upon the conditions of temperature under {131} which the later larval and early pupal stages are passed. By cooling appropriately at the right stage _levana_ can be made to produce _levana_ instead of the _prorsa_ which it normally produces under summer conditions. So also by appropriate warming _prorsa_ will give rise to _prorsa_. Moreover, if the conditions are properly adjusted an intermediate form _porima_ can be produced, a form which occurs occasionally under natural conditions. The pattern is, in short, a function of the temperature to which certain earlier sensitive stages in this species are submitted. What is true of _A. levana_ is true also of a number of other species. In some cases temperature is the factor that induces the variation. In other countries where the year is marked by wet and dry seasons instead of warm and cold ones moisture is the agent that brings about the change. In some of the South African butterflies of the genus _Precis_ the seasonal change may be even more conspicuous than in _A. levana_. In _Precis octavia_, for example, the ground colour of the wet season form is predominantly red, while in the dry season form of the same species the pattern is different, blue being the predominating colour (cf. Pl. VI, figs. 11 and 12). Such examples as these are sufficient to shew how sensitive many butterflies are to changes in the conditions of later larval and earlier pupal life. The variations brought about in this way are as a rule smaller than in the