XXVII. 



DIGONEUTISM IN BUTTERFLIES 



In all plural brooded butterflies with an exten- 

 sive distribution in latitude, the number of genera- 

 tions varies with the length of the season, and this 

 will account for the apparent waste we often see 

 as winter approaches, for such changes must be 

 gradual, and in intermediate districts irregular, 

 dependent upon the season. Where, as is some- 

 times the case, some chrysalids of each brood live 

 until the following spring, it manifestly makes lit- 

 tle difference how short the season may be, or how 

 suddenly and effectually any brood may be cut off ; 

 these chrysalids, and so the species, will survive. 

 That this feature is more common than is generally 

 supposed is shown by the increasing number of 

 proofs brought forward of lethargic tendencies in 

 caterpillars and of persistent torpor in many mid- 

 summer chrysalids. It is also indicated by the 

 variation in the numerical proportions of different 

 broods ; the winter is the severest season, and con- 



