VAKIATION CONSIDERED BIOLOGICALLY. 185 



possible, and the tissues are formed witliout having sufficient, time to 

 mature, as they woukl under normal conditions (we may here suppose 

 these to be those which are most beneficial to the species) ; the surplus 

 material is rapidily 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 magnitude. 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 

 Chri/so2)hanus phloeas, must be looked upon as simply a fortuitous cir- 

 cumstance, inasmuch as it appears to be largely due to the dark ground 

 coloration of the scales, for Vanessa pohjchloros becomes darker by the 

 subjection of its pupa to a low temperature. Probably the physioloo-ical 

 result is much the same in both cases ; heat in the case of C. phloeas, cold 

 in the case of V. pohjchloros, being detrimental to the development of the 

 most highly specialised individuals of these species. 



I take it, therefore, that, within the limits of existence, the possi- 

 bilities of the germ are such, that the determinants of the scales, owing 

 to the action of natural selection, present a range of variation 

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

 influences determine, through their physiological action, which of the 

 three factors shall come to the fore in the final production of the 

 scales. 



It is difficult, therefore, to say that Weismann's selection of C. 

 phloeas, as a case in which an organism can be affected by external 

 influences for which it is not adapted in advance, is pertinent to the 

 question. It appears to me that the insect must be more or less 

 adapted to its climatic environment in all the countries in which it 

 exists. It may be better adapted to some localities than to others, but 

 still it is adapted to all. If it shows some definite external character on 

 the confines of its range wherein it differs conspicuously from tlie more 

 highly developed forms produced in the most favourable areas of its 

 geographical distribution and if this difference be a difference of scale- 

 coloration, then the scale-determinants must be considered as ran^-ino 

 between those exhibited under the most favourable and under the 

 least favourable environment, and to this extent it must be conceded 

 that C. phloeas is adapted in advance, and that the illustration fails so 

 far as it positively states that it is not so ada])ted. It is clear that the 

 species is adapted to hot countries because it is widely spread over 

 them. It may not be so well adapted, but yet at the same time we 

 cannot say that it is not adapted, and it is clear that the limit of 

 variation in the determinants of the scales must lie between the dark 

 coloration produced under one condition of environment and the bright 

 coloration produced under the other. That this is the result of natural 

 selection acting on the development of the insect is obvious. We may 



