In the following attempt to make clear the relationship between simple climatic variation and seasonal dimorphism, I shall concern myself only with such undoubted climatic varieties. A case of this kind, in which the winter form of a seasonally dimorphic butterfly occurs in other habitats as the only form, i.e., as a climatic variety, has already been adduced in a former paragraph. I allude to the case of _Pieris Napi_, the winter form of which seasonally dimorphic species occurs in the temperate plains of Europe, whilst in Lapland and the Alps it is commonly found as a monomorphic climatic variety which is a higher development of the winter type, viz., the var. _Bryoniæ_. Very analogous is the case of _Euchloe Belia_, a butterfly likewise belonging to the _Pierinæ_, which extends from the Mediterranean countries to the middle of France, and everywhere manifests a very sharply pronounced seasonal dimorphism. Its summer form was, until quite recently, described as a distinct species, _E. Ausonia_. Staudinger was the first to prove by breeding that the supposed two species were genetically related.[25] This species, in addition to being found in the countries named, occurs also at a little spot in the Alps in the neighbourhood of the Simplon Pass. Owing to the short summer of the Alpine climate the species has in this locality but one annual brood, which bears the characters of the winter form, modified in all cases by the coarser thickly scattered hairs of the body (peculiar to many Alpine butterflies,) and some other slight differences. The var. _Simplonia_ is thus in the Alps a simple climatic variety, whilst in the plains of Spain and the South of France it appears as the winter form of a seasonally dimorphic species. This _Euchloe_ var. _Simplonia_ obviously corresponds to the var. _Bryoniæ_ of _Pieris Napi_, and it is highly probable that this form of _E. Belia_ must likewise be regarded as the parent-form of the species surviving from the glacial epoch, although it cannot be asserted, as can be done in the case of _Bryoniæ_, that the type has undergone no change since that epoch, for _Bryoniæ_ from Lapland is identical with the Alpine form,[26] whilst _E. Simplonia_ does not appear to occur in Polar countries.