98 



satisfactory evidence that would lead me to suppose that this special 

 form of variation is in any sense a case of retrogression. 



On the other hand, the prevailing conditions of a given district 

 may be such as to render the existence of a species in its ordinary 

 form precarious ; such conditions might induce slight modifications 

 in colour, or markings, or habits, that would be beneficial to the 

 species, and the individuals acquiring them would possess an ad- 

 vantage as compared with those not having them. The circum- 

 stances that would in the first place induce such modifications 

 would, if continued, probably tend to increase them in succeeding 

 generations, until the point was reached at which the species had 

 assumed the form in which it could take the fullest advantage of 

 such conditions. Such a case would be progressive. 



The form of T. comes most frequent throughout the whole range 

 of its distribution is that which I have referred to as the clay- 

 coloured form. This I take to be the normal form, and it is only as 

 we reach the northern or western verge of the range of the species 

 that we find any great amount of deviation from it. This, I think, 

 points to the southern part of Europe as being the true home of the 

 species. I am strengthened in this view by experiments that I have 

 carried out by feeding up the larvae in confinement. Under the 

 natural conditions of our climate the larva, on leaving the egg at the 

 end of August or early in September, feeds for about a month, and . 

 then goes into partial hybernation, not reaching full growth until the 

 following June \ but by keeping the larva at a somewhat higher tem- 

 perature as the autumn advances, probably analogous to that which 

 it would experience in the south of Europe when it was at a corre- 

 sponding stage of its growth, it shows no disposition to hybernate, 

 but feeds up readily,* suggesting the probability of a succession of 

 broods as the normal habit of the species, and that the partial hyber- 

 nation is an acquired habit that enables it to keep its footing under 

 altered climatic conditions. 



If this surmise be a correct one, it may perhaps, to some extent, 

 account for the darker forms assumed by the species in its more 

 northerly habitats. Weismann, Merrifield, and others have shown 

 us that modifications of colour may be artificially produced in some 

 species by subjecting them to abnormal conditions in their earher 

 stages, one of the most remarkable results obtained being the dark- 

 ening of the colour of the imago by the prolonging of the pupal 

 stage by subjecting the pupa to a low temperature. Possibly some 

 such conditions may be accountable for the increase of the redder 

 tone noticed in the colour of the species in its more northerly dis- 



* Ova from Scotch (Forres) parents . 



