( xcix ) 



and Dr. E. Fischer, who used for the most part temperatures 

 more extreme than those which I employed. This is not the 

 occasion for setting forth the different theories ably stated by 

 those gentlemen in explanation of the results obtained, both 

 invoking to some extent " arrested development " as their 

 cause, and, as development is arrested by abnormal heat as 

 well as by abnormal cold, the theories are strongly supported 

 by the fact that there is a striking similarity between 

 many of the results of the application of directly opposite 

 temperatures. 



The difference in size between the two phases is often so 

 great that in the pupal stage there can be no complete 

 conversion in this respect, and I think it may be doubted 

 whether in this stage even the summer phase is ever converted 

 physiologically, i. e, so as to give to a summer pupa the ten- 

 dency to lie over for many months, as distinguished from the 

 mere temporary retardation imposed as the normal effect of 

 a low temperature.* 



Conversion in earlier than pupal stages. 



There is evidence, however, that in stages earlier than the 

 pupal the conversion of individuals may be complete. Dr. 

 Chapman in the paper I have adverted to mentions that he finds 

 about 5 per cent, pretty regularly of the larva; of a brood of 

 A. caja to be " forwards," i. e. to be ready (under the very slight 

 encouragement it would appear of an indoor life, with its 

 somewhat higher temperature, and perhaps without it) to feed 

 up rapidly in about three months, and emerge the same 

 summer instead of hibernating as larvre ; and he records that, 

 of a brood which he sent to me for experiment and which 

 were forced by me, 75 per cent, so fed up in about two 

 months. 



But it must not be assumed from this that conversion from 

 the winter to the summer phase can generally succeed with 

 half-grown larvae. In 1895, from the 22nd to the 29th 



* As regards size, it is noteworthy that the summer insect is often the 

 smaller, usually so, I think, in the Geometric! moths ; in the Selenias the 

 size is much less. In A. levana, on the other hand, the butterfly from 

 the summer pupa is the larger. 



