VARIATION CONSIDEnEI) BIOLOGICALL\. 183 



Here I join issue with the learned Professor ; for I believe that the 

 determinants of the scales are so " arranged in advance by natural 

 selection " that tho}'' must produce l)la(^k under the intlucnco of a hi^h 

 temperature. 



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-su])i)ly, 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, climatic conditions, or other external factors of environ- 

 ment may })artially fail, and as a result the insect produced may be less 

 vigorous, less highly developed either as regards size or colour (for it 

 must be remembered that in insects the scales and the colour are as 

 much structural as the wing membrane itself), and may altogether 

 show considerable difference from individuals developed under the 

 most favourable conditions. 



The darkness or otherwise of Chrysophanus phloeas 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 otliers in dry seasons. Whether this be due to the 

 fact that some insects have spread to us fi'om more northern, others 

 from more southern latitudes, or to other causes, it renders it 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 

 gi-eat excess of either heat or cold would, of course, be injurious to any 

 species. To an insect that exists in Britain, say, in a mean temperature 

 of 51° but prefers GO^, any decrease of temperature will be injurious, 

 whilst increased temperature will affect it beneficially until it reaches 

 iW, and will not affect it prejudicially probably until it exceeds 06°. 



It is a fact that the largest, most vigorous, and brightest coloured 

 specimens of Chri/i^ophaam phloeas are obtained in the temperate parts 

 of the Pala^arctic area, and that, as we pass south, the insect becomes less 

 brilliant, darker, and often smaller. This tends to show that it is one 

 of those species which prefer an environment more like that of our 

 temperate climes, and that a higher temperature affects it more or less 

 prejudicially. The most easily marked evidence of this prejudicial 

 action appears to be seen in the scaling, for, even in Britain, a very hot 

 summer like 1893 always produces a fair proportion of dark specimens, 

 even in those localities where, in cooler seasons, the colour is most 

 brilliant. This is sufficient to prove that the range of variation in the 

 determinants of the scales is such as may enable the insect to be either 

 black, or of a bright ruddy golden colour, and the external stimulus 

 which brings one or other of these extreme conditions to the fore, appears 

 to be tliat of temperature. This conclusion, I need hardly point out, 

 is diametrically opjwsite to that of Professor Weismann. 



But it is interesting to pursue this matter farther, and to encjuire 

 what particular i)hysiological processes are concerned in the production 

 of this colour phenomenon in Chrysophanus phloeas. The colours of 

 insects are, I consider, largely dependent on three factors : — (1) I'he 

 pigment in the scales themselves. (2) The shape, ikc, of tlie scales, giving 

 rise, eitlier by diffraction or interference, to tlie non-pigmentary colours. 



