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mediate forms result. In the same way it is possible that 

 the yellow aberration lutescens of Callimorpha hera is in reality 

 more closely allied to the scarlet type than is the inter- 

 mediate ab. saturnina, the yellow being the simplest chemical 

 change that can take place in the normal scarlet pigment. 

 In this way probably the colour of the ab. saturnina is due to 

 the ability of the two germs (represented by a yellow and 

 scarlet form respectively) to mix organically and produce a 

 tint that appears intermediate to our senses. In other words, 

 we may suppose that certain conditions produce the typical 

 red form ; other conditions (acting through the organism m 

 its early stages) produce the yellow form, cross the red and 

 yellow form, and red, or yellow, or intermediate forms may 

 result. 



The question at once arises. Would under any conditions 

 the yellow form become a species ? It is quite clear, by the 

 persistence and abundance of the yellow form in certain 

 areas of the western distribution of this species, and its 

 absence in other districts, that something in its environment 

 is the cause of its development. It appears to be confined 

 almost entirely to western France, the Channel Islands, and 

 south-western England. Only once out of some hundreds 

 (nay, thousands) of examples have I observed it abroad, and 

 then in Piedmont (at Susa). Neither in the Pyrenees Orien- 

 tales nor Asturias, in Savoy nor Dauphiny, near Aosta, the 

 Austrian Tyrol, Bregenz, nor other localities in which we 

 have seen this species (sometimes in amazing numbers) 

 have we ever observed a yellow example. What conditions 

 then occur in the west of France, the Channel Islands, and 

 the south-west of England that do not occur in southern 

 and central Europe ? There are less sun, more moisture, a 

 later spring, and a longer summer, with their resultant effects 

 on vegetation in these western districts. There are as results 

 the more rapid feeding up of the larva, and more rapid 

 metabolism in the pupal stage, in central and southern 

 Europe. Not that the yellow' aberration never occurs in 

 southern Europe ; we have already noticed one at Susa 

 among thousands of typical examples, and this shows that 

 the physiological peculiarities that result in the production 

 of the yellow coloration are within the limits of variation 

 possible to the species even in its most typical form. If, 

 however, this peculiar phase of the insect's organisation has 

 become so far developed, owing to changed environment, as 

 to produce a very large percentage of individuals so con- 

 stituted in western Europe, it is clear that we have arrived 



