152 ACQUIRED CHARACTERS sec. 



orange-coloured spot on its anterior ^vings (var. Cleopatra). 

 Further, not rarely a long period of dry or moist weather 

 exercises a considerable influence on the size of the following- 

 generation. Immediately after a continuously dry summer 

 Ijutterflies are always smaller than after a moderately moist 

 season. The second generation of Argynnis Selene, which 

 takes flight in the height of summer, is always smaller than 

 the first generation, wliicli is produced in spring, and so on. — 

 " If, then, such important facts are observed in many species of 

 our small continent, why should not similar effects be possible 

 in extra- European species, whicli are spread over so large a 

 territory where they are influenced by the most varied climates, 

 differently composed soils, and to some extent quite different 

 food plants ? In a hemisphere where the conditions of tem- 

 perature are so varied, why should not the tropical sun, under 

 which varieties are generally acknowledged to occur, be also 

 the chief cause of variation ? Thus the origin of the immense 

 Lepidoptera of the East Indies is to be ascribed to the moisture 

 and heat of the climate {e.g. the Ornithoptera) ; and, on the 

 other hand, the small size of the Australian forms is to be 

 attributed to the dryness of the Australian climate. 



" If Lepidoptera pass during the rainy season in their 

 normal habitat into another region where the rainy season 

 has not begun and where great heat and a dry atmosphere 

 prevail, or if they pass from the monsoon region, which is 

 neither deficient in water nor in rain, into the ever-dry and 

 waterless continent of Australia, diminution of size in the 

 very next generation is sure to take place. But as the climate 

 in Australia is permanently dry, the diminution which occurs 

 remains permanent in all subsequent generations in their 

 new habitat, and the deteriorated condition of the species is 

 established for ever." 



Thus permanent local varieties are produced. There are 

 therefore numerous Lepidoptera which in moist climates are 



