1-12 Mr. F. Meri'ifield's systematic temperature 



siderably darker than the rest, and make the nearest 

 approach in colouring, and I think, on the whole, in 

 markings, to the spring emergence. In both classes 

 those that were longest in pnpa were, as a rule, darkest, 

 even in the case of such of the pupae as were never at 

 a lower temperature than about 60°, which seems to 

 indicate that retardation may be a cause of the difference 

 in colour and markings. 



ic) Causes of the difference in the two cases. — These 

 results are so far in general accord with those recorded 

 by Prof. Weismann as regards A. levana, and by Mr. 

 Edwards as regards P. ajax, as to show a significant 

 difference between the readiness of the summer pupa to 

 bear and be influenced by winter conditions and the 

 resistance offered by the autumn pupa to summer con- 

 ditions. Prof. Weismann explains this difference by 

 the hypothesis that the winter-pupating generation is 

 the ancient and more firmly established form, and 

 expresses the opinion that other disturbances of the 

 summer pupa, such as extreme heat or mechanical 

 motion, may cause a reversion to the older form. My 

 experiments seem to indicate further a direct effect of 

 temperature in altering colour, &c., in hotlt broods of the 

 seasonally dimorphic illustraria, causing the summer 

 pupa to yield a much darker moth and the autumn pupa 

 a much paler one, the darkness or the j^aleness in either 

 case depending in a great degree on the length of the 

 exposure. And the very marked effects of a similar 

 kind produced on the pupa of autumnaria, which I believe 

 is nowhere double-brooded, also appear to indicate a 

 similar direct effect of temperature on colour : I do not 

 know if there is any reason for ascribing it in the case of 

 this species to reversion. 



5. htage in which temperature most operative. — There 

 is no doubt a strong predisposition, in an individual 

 belonging to a double-brooded species, at some period of 

 its development, towards one of the two different desti- 

 nations, i. e., the emerging in the summer and with the 

 summer colouring, or the lying over until the spring 

 and then emerging in the spring colouring. The experi- 

 ments lead me to think that in the species operated on 

 by me the predisposition has become so decided in the 

 larval stage that no treatment of the pupa can after- 

 wards entirely alter it, but that in the early larval stage 



