THE VARIATION OF THE IMAGINES OF THE LEPIDOPTERA. 75 



energy. The insect, therefore, does not develop under such favour- 

 able conditions ; it needs what energy it possesses to build up its vital 

 organs, and so fails in perfectly building up the secondary tissues. 

 This failure is in direct proportion to the degree in which the vitality 

 is lessened. If the temperature during the period of active develop- 

 ment be below a certain degree, the vital force ceases to act at all, and 

 death results. Heat, greater than that to which the insect is normally 

 subjected, instead of reducing the vitality to the lowest ebb at which life 

 can be sustained, affects the histolysis and histogenesis, usually, in a di- 

 rectly opposite manner. Under its influence the vital processes are carried 

 on at express speed, Energy is expended at the fastest rate possible, 

 and the tissues are developed without having sufficient time to mature, 

 as they would under normal conditions (we may here suppose these 

 to be those that are most beneficial to the species) ; the surplus 

 material is rapidly utilised, with the result that as marked an abnor- 

 mality is produced under the one condition as under the other, 

 although in an opposite direction. It is conceivable that to insects 

 which normally mature at a low temperature, a moderately high 

 temperature might be fatal, and that the pupal tissues would not form 

 at all. It is clear, however, that all changes in the environment of 

 the pupa must necessarily produce some effect on its development.. If 

 the change be sufficiently extreme, then the effect is death ; anything 

 short of such an extreme will produce an effect proportioned to its magni- 

 tude. If a pupa be thoroughly acclimatised to a given range of 

 temperature, then excessive heat or cold must be injurious. 



The fact that an increased temperature produces dark specimens of 

 Chrysophamts phlaean, must be looked upon as simply a fortuitous 

 circumstance, inasmuch as it appears to be largely due to the dark 

 ground coloration of the scales, for, as we have seen, Ewjonia poly- 

 chloros becomes darker by the subjection of its pupa to a low tem- 

 perature. Probably the physiological result is much the same in both 

 cases ; heat in the case of C. phlaeas, cold in the case of E. polychloros, 

 being detrimental to the development of the most highly specialised 

 individuals of these species. 



We consider, therefore, that within the limits of existence, the 

 possibilities of the germ are such, that the determinants of the scales 

 (under the influence of intra-selection), present a range of variation 

 within the extreme limits possible to the species, and that external 

 influences determine, through their action on the organism, which of 

 the three before-mentioned factors shall come to the fore in the final 

 production of the scales. 



There can be no doubt that it is in this direction that experiment 

 and observation are particularly wanted, if we are to obtain any real, 

 as apart from a theoretical, knowledge of the factors underlying variation. 

 These problems relate rather to vital activities, and to physiological 

 phenomena, than to anatomical structures, or the external guiding 

 influence exerted by natural selection, and it is on these lines, it 

 appears, that the laws governing variation will finally have to be 

 worked out. 



