74 Studies in the Theory of Descent. 



magnitude of the difference between the tempera - 

 ture which acts >on ;thq pupse of the primary and 

 that which acts 'on those of the secondary form ; 

 it rather depends: on the absolute temperature 

 which the pupae .experience. This follows without 

 doubt from the fact that many species, such as 

 our common Swallow-tail (Papilio Mac/iaon), and 

 also P. Podalirius, in Germany and the rest of 

 temperate Europe, show no perceptible difference 

 of colour between the first generation, the pupae 

 of which hibernate, and the second generation, 

 the pupal period of which falls in July, whereas the 

 same butterflies in South Spain and Italy are to 

 a small extent seasonally dimorphic. Those 

 butterflies which are developed under the in- 

 fluence of a Sicilian summer heat likewise show 

 climatic variation to a small extent. The follow- 

 ing consideration throws further light on these 

 conditions. The mean summer and winter tem- 

 peratures in Germany differ by about i4.9R. ; 

 this difference being therefore much more pro- 

 nounced than that between the German and 

 Sicilian summer, which is only about 3-6R. 

 Nevertheless, the winter and summer generations 

 of P. Podahrius are alike in Germany, whilst the 

 Sicilian summer generation has become a climatic 

 variety. The cause of this change must therefore 

 lie in the small difference between the mean 

 summer temperatures of 15.0 R. (Berlin) and 

 19.4 R. (Palermo). According to this, a given 



