226 Genetical Studies in Moths 



but of this I have no proof. If we classify them according to their period 

 of emergence seven appear between March and June (both inclusive) 

 and always distinctly later than the same species at sea level or further 

 south, whilst 0. helvola and X. circellaris emerge in autumn, slightly 

 in advance of their relatives elsewhere ; their displacement, although 

 in the same direction, is not so pronounced as that of 0. autumnata. 

 Nevertheless we have gained some sort of clue to the solution of our 

 problem ; the, anomaly is connected with species hibernating as ova, 

 aestivating as pupae and with autumnal emergence. 



Increased temperature acting on the pupae may at once be ruled 

 out; forcing in the genus acts as a retarding influence. But is the 

 position the same if the ova hatch earlier than the normal ? Is this 

 early hatching accompanied by accelerated emergence ? These points 

 are readily put to the test. I took autumnata ova, forced them out in 

 February and had them in pupae by March 28, exactly eleven weeks 

 before their brethren in nature. And these pupae, kept under as 

 natural conditions as possible, yielded their imagines simultaneously 

 with their wild relatives ! Still, precocious hatching ova may play their 

 part as we shall discover later. 



What then determines the development of Oporahia imagines if not 

 increased temperatures ? They emerge in response to exposure to a 

 stimulus of a progressive fall in mean daily temperature, as I proved by 

 submitting half of a brood of pupae to a gradual fall in low temperatures 

 and the other half to a similar fall in higher ones. The former de- 

 livered their imagines first. 



Here, apparently, we have got at the root of the matter, for owing to 

 the dense masses of moss, pine needles and decaying Lastraea fronds 

 retaining their moisture, and aided by the closer canopy overhead, the 

 pinewood attains a lower maximum summer temperature and a higher 

 winter one than the open, less moss-grown birchwood — and the difference 

 is perceptible at once as one enters them. In consequence, the optimum 

 period of exposure to low temperatures capable of stimulating pupal 

 development is experienced sooner in the pinewood, and the insect 

 emerges earlier. This dragging backward of the period of aestivation 

 is bound to be aided by early hatching ova. Furthermore, such early ova 

 would be more advantageously placed in the larchwood than amongst 

 the birches, for the latter leafs later in the Clevelands as the following 

 dates for two successive years will show : larch April 15, birch April 23 ; 

 larch April 24, birch May 5. Thus any precocious individuals would die 

 on the one and succeed on the other. 



