ON SEASONAL DIMORPHISM IN BRITISH BUTTERFLIES. 273 



brood assume more of the character of those of the second or 

 summer generation. This^is especially noticeable as regards 

 Pieris hrassiccBy specimens of which both in size and intensity of 

 markings resemble very closely those of the summer brood, so 

 that it is really difficult to distinguish them with certainty. 

 Pieris rapes does not seem to be so similarly affected as regards 

 size, but the markings are generally more distinct; and in the 

 female an ochreous tinge is frequently visible, this being one of 

 the characters which females of the second generation often 

 assume, especially in hot summers, and has been more frequently 

 observed by me upon the Continent than in this country. 



Now, as regards the summer broods of these three butterflies. 

 I find the distinctive characters of these similarly affected in 

 accordance with the temperature prevailing at the time or just 

 preceding that of their emergence. In very wet seasons, when 

 the temperature is as a consequence low, the butterflies partake 

 more of the characters of the spring emergence ; and this is 

 particularly the case with Pieris napi, the under sides of the 

 wings being more strongly veined and clothed with dusky spots 

 than those of the typical summer specimens. I have indeed 

 found specimens of all three species in July which could not be 

 distinguished at all from typical specimens of the spring brood, 

 but whether these are only late emergences of the first genera- 

 tion or not, I am at present unprepared to say. In hot summers, 

 on the other hand, the distinctive characters of the second broods 

 become much more intensified. These manifest themselves in 

 the darker markings on the upper side and in the lighter 

 colouring upon the under side of the wings ; this latter appear- 

 ance being due in large measure to the diminution in the number 

 of dusky scales. In very fine hot summers, when third broods 

 are able to make their appearance, these characters make them- 

 selves more prominent still, and the size of the specimens in 

 addition undergoes an increase, as the series in my own collection 

 amply testify. 



What conclusions are we able to draw from these facts ? 

 They seem to be due to the indirect influence of the temperature 

 upon the chrysalis, in accordance with the views of Weismann, 

 and this in the latter stage of its existence. Whether they were 

 originally produced by temperature it is difficult to decide ; but 

 if one may be permitted to judge from the more recent of the 

 chameleon-like theories advanced by the learned Professor above- 

 named, the seasonal characters have been rendered permanent 

 by means of natural selection for protective purposes, — if I 

 may employ a metaphorical expression, — the assistance of the 

 climate, that is, the temperature of the season, bringing into 

 activity those particular germs which will enable the perfect 

 insect to assume morphological characters in harmony with the 

 surroundings. 



