VARIATION IN EUROPEAN LEPIDOPTERA. 



83 



points of resemblance which would suggest that they are often nearly 

 related to one another — at least ancestrally. But such conclusions 

 must necessarily be merely speculative. We have firmer ground to 

 go upon, however, when we deal with the evolution of species at 

 present in progress. We find that this order of insects of which I 

 have been speaking is subject to many kinds of variation, some 

 attached to one sex only, others depending on the season of 

 emergence ; some which are constant and hereditary, and others 

 which are capricious and irregular in their appearance. Some, too, 

 there are which I have termed generic ; that is to say, that many species 

 of the same genus show a common tendency to vary alike in some 

 one particular, and this tendency seems to point to their derivation 

 originally from a common stock. Whatever the exciting causes may 

 be of deviation from the typical pattern — whether climate, soil, quan- 

 tity or quahty of food, we have seen that the law of heredity is of 

 paramount influence in developing variation so produced, and in 

 transmitting and stereotyping it, as is clearly shown by the results of 

 close breeding, as induced sometimes in nature by isolation, as in the 

 case of an island or an Alpine valley ; or artificially, as in the instance 

 already cited of the aberrant forms of T. Crepuscularia and Biundu- 

 laria ; which, moreover, have become naturally permanent varieties, if 

 not species, I am given to understand, in a wood in the neighbour- 

 hood of Barnsley, where they have replaced the normal type. Of the 

 remote and primary causes of variation we as yet know little with any 

 certainty, except in the case of the presence or absence of sun-light, 

 and heat, which, without doubt, is most potent in the development 

 or degeneration of colour and distinctness of pattern, acting, as we 

 have seen, inversely in the case of the nocturnal and diurnal divisions 

 of Lepidoptera. 



As to the effect of diversity of food, I have ventured to express 

 my belief that, with certain exceptions, its effect is not directly 

 apparent in individuals, though in common with other causes it may 

 eventually leave its impress on a race; but that its abundance or 

 scarcity acts powerfully on the size and vigour of the insect in 

 every stage is undeniable. 



As to those strange topographical characters, two cases of which 

 I have brought under your notice, which a number of insects 

 peculiar to a particular district possess in common, although belong- 

 ing to different genera, I do not hazard a suggestion, unless that in 

 some way the law of mimicry, that most inexplicable of all natural 

 powers, has asserted itself. 



I shall now close with a few practical remarks. 



Nov. 1884. 



