THE VARIATION OF THK IMAGINES OF THE LEPIDOPTERA. 73 



acting on the organism has brought about the result we see. If the 

 result produced by these internal activities be such as to cause positive 

 injury to the race, by rendering the individuals conspicuous, more 

 palatable, etc., then natural selection will direct the variations that 

 exist in the newly-formed race, into such lines, as will necessarily be of 

 advantage to it. 



But it may happen that a larva may exist under very distinctly 

 favourable conditions until pupation takes place, and that, then, the 

 pupa may be subjected to unusual conditions. We have already seen 

 that the pupal period is that in which the wing-scales are entirely 

 formed, and their contained pigment entirely elaborated. It is well- 

 known that the pigmentary material goes through a regular (and for 

 each species, fixed) genetic sequence before the mature colour is reached. 

 It is evident, therefore, that changes of colour, due either to modification 

 of scale-structure, or pigment, will be more readily effected in this stage 

 than any other. The most powerful factor in bringing about a direct 

 change has, thus far, been temperature. By exposing pupae to tem- 

 peratures to which they are not normally subjected, Merrifield has 

 produced definite changes in the colours of certain species. These 

 changes have been particularly marked in those species which have 

 normally two seasonal spring and summer forms, varying in colour. 

 Other species have, however, responded to the stimulus somewhat 

 readily. No general results, however, have yet been deduced. In 

 some instances (Kutjonia polycldoros), as we have already seen, a low 

 temperature produces a darkening of the normal colour ; in others 

 (Chrysoplianw phlaeas), a high temperature produces a similar effect. 

 What is the nature of the difference in the pigment-factor of the scales 

 of these two insects that makes them thus respond in such similar ways 

 to different stimuli ? 



It is well-known that, within the area of distribution of a species, 

 there is a certain part in which the environment is more perfectly 

 fitted than in the remainder, by food supply, climatic conditions, etc., 

 for the development of the species in its most vigorous form. Outside 

 this limited area the species exists under less completely favourable 

 conditions ; the food-supply may partially fail, the climatic conditions, 

 or other external factors of environment, may be less suitable, and, as a 

 result, the insect produced may be less vigorous, less highly developed, 

 either as regards size or colour, and may altogether show considerable 

 difference from individuals developed under the most favourable 

 conditions. 



The darkness (or brightness) of Chrysopkanus phlaeas seems to be 

 due to climatic (temperature) conditions. It is well known that, as 

 regards acclimatisation, some species succeed better in cold and 

 others in hot, some in wet and others in dry, seasons. Whether this 

 be due to the fact that some insects have spread to us from more 

 northern, others from more southern latitudes, or to other causes, it 

 is rendered highly probable that the same amount of heat may act 

 prejudicially on one insect and advantageously on another. In the 

 latter case, increased heat may be expected to produce effects that 

 show an increase of vitality, whilst in the former, cold will produce 

 the same result. A great excess of either heat or cold would, of course, 

 be injurious to any species. To an insect that exists in Britain, at a 

 mean temperature, say of 54, but prefers 60, any decrease of tempe- 

 rature will be injurious, whilst increased temperature will affect it 



