( xcvii ) 



other within about six months. They then came to an end 

 without giving rise to a winter phase. 



" Splitting." 



I have treated the tendency to be single-brooded, or the 

 reverse, as usually congenital in the individual ; and I think 

 this is proved by the common phenomenon which I have called 

 " splitting," met with in nature and in home-breeding, when 

 part of a brood from the same parent, all bred under similar 

 conditions, divides, one portion feeding up hastily, and within 

 the year, the other portion feeding slowly and the pupa 

 resting peacefully till the following year. This separation, I 

 think, most usually shows itself in the larval stage, when the 

 " forwards," to use Dr. Chapman's name for them in his 

 experiments with Arctia caja* feed up rapidly and voraciously, 

 the others biding their time ; his explanation being that A. caja, 

 as we find it, is a mongrel having two or more races mingled 

 together in its composition. 



Though the character of the winter phase is in many cases 

 impressed so deeply on the individual as to render it immov- 

 able in its habits, there is a vast number of cases in which 

 it is not so. The books are full of accounts of Lepidoptera, 

 ordinarily single-brooded in England, which give out a second 

 or even a third brood in a hot summer, and the degree of 

 instability in this respect varies greatly not only as between 

 different species, but as between different individuals. 



Conversion of one phase into the other. 



Reverting to those individuals on which the physiological 

 character, i. e. that of the summer or winter, is strongly 

 impressed, it seems proper to say something on the difficult 

 problems connected with the conversion, complete or partial, of 

 one of these phases into the other. 



Conversion in the pupal stage. 



To deal in the first instance with conversion in the pupal 

 stage, let me point out that it appears to be usually easier to 

 convert the summer phase into the winter than the opposite, 

 * Ent. Record, vol. iv, pp. 265 et seq., and vol. v, p. 33 et seq. 



