48 Studies in the Theory of Descent. 



which extends from the Mediterranean countries 

 to the middle of France, and everywhere mani- 

 fests a very sharply pronounced seasonal dimor- 

 phism. Its summer form was, until quite recently, 

 described as a distinct species, E. Ausonia. Stau- 

 dinger was the first to prove by breeding that the 

 supposed two species were genetically related. 2 

 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 corre- 

 sponds to the var. Bryonicz of Fieri s 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 Bryonicz, that the type has undergone 



3 [Eng. ed. In 1844, Boisduval maintained this relationship 

 of the two forms. See Speyer's " Geographische Verbreit. d. 

 Schmetterl.," i. p. 455.] 



