70 BRITISH LEPIDOPTERA. 



which are not particularly affected by differences of temperature. On 

 the other hand, temperature, possibly, has some effect as a factor in 

 determining general variation in tropical countries, where it is, how- 

 ever, overshadowed by moisture. There are, probably, many other 

 external factors besides "moisture and drought," and " high and low 

 temperatures," which react on insects in such a manner as to deter- 

 mine variation, but these are the factors which have, at present, been 

 most studied. 



It may now be understood how a species, which exists under two 

 very different environments, may produce two very different-looking 

 imagines, so different, indeed, that their specific identity may be 

 doubted. In the European fauna, this is well illustrated by species that 

 exist both at the sea-level and also on mountains at a high elevation. 

 The specimens of Melitaea aurinia from the plains are large, brigbtly 

 coloured and ample-winged. Those from the mountains are small, 

 ill-pigmented and narrow-winged, and are known as var. mero-pe. 

 That the factor that determines this change of size is food, we think very 

 possible ; that moisture and temperature have also something to do with 

 the matter, is exceedingly probable. In this manner we get "lowland " 

 and "alpine" forms of the same species; "northern" and "southern" 

 forms similarly occur when a species is spread over several degrees of 

 latitude; "eastern" and "western" forms, when specimens of a 

 species caught in Western Europe are compared with specimens of the 

 same species taken in Japan, and so on, differing often in size, shape 

 or colour, or even all combined, and such races — known as geographical 

 races— are often so distinct, that an expert can often tell at a glance 

 the exact area or district from which the specimens have come. 



The differences that mark these various races have an important 

 bearing on the question of the origin of species. We believe that these 

 differences are often correlated with variations that exist in the 

 organism itself ; sometimes, indeed, that they are the manifestations of 

 such variation, and, if the conditions which are thus set up, and in- 

 sisted upon by the environment year after year, be intensified, as, by 

 the localisation and isolation of these races, they must be, the differ- 

 ences may often become permanent and acquire specific value. 

 Differences in habitat — altitude for example — often permanently alter 

 the time of appearance of the insect in the imaginal state, and thus 

 the life-cycle is modified, the particular form is isolated, and its special 

 features become, as it were, more and more fixed. 



Thus far we have dealt only with the external manifestations of 

 variation, as they are presented to our observation. Dixey has shown* 

 us by what sequence of modifications the patterns of the wings of the 

 Nymphalid butterflies have been formed from a more primitive type. 

 He has also shown f us how the mimicking Pierids have attained the 

 markings by means of which they so closely resemble the nauseous 

 Nymphalids they mimic ; but he has given us no clue as to the phy- 

 siological processes underlying these changes. 



Starting from the basis that every portion of an insect's wing has 

 in it, from the germ, endless possibilities in the direction of variation, 

 Weismann argues that utility determines the particular form of variation 

 which will be acquired by the individual. We have already criticised, % 



* Trans. Ent. Soc. Lond., 1890, pp. 89 et seq. f I^id, 1894, pp. 249 et seq. 

 \ Entom. Record, etc., vol. viii., pp. 1 et seq. 



