DIGONEUTISM 243 



sequently the spring broods are ordinarily, and 

 under simple conditions would always be, less nu- 

 merous than the summer or autumn broods ; gen- 

 erally the broods go on increasing in individuals 

 as the season advances ; but in some it is not so, 

 and it may be presumed that these are species 

 which have not long enjoyed the privilege of a sec- 

 ond brood, or, in other words, those in which a 

 part of the chrysalids fail to persist until the fol- 

 lowing spring. In the case of our Tiger Swallow- 

 tail, which is found from Alaska to Florida, we 

 have a butterfly which is single brooded in the 

 north and double brooded in New England ; but 

 the second brood is much less abundant than the 

 first, and the change as we go north is probably 

 effected by the lingering development of some 

 caterpillars and the disposition of chrysalids to 

 winter early. Wherever in a double brooded but- 

 terfly the second brood is less abundant than the 

 first, it is probable that the butterfly is partly 

 single and partly double brooded — that is, that 

 the early brood of a given year is made up of the 

 direct descendants of each brood of the preceding- 

 year. 



Occasionally, the difference in the number of 

 broods affects the mode of hibernation. The Black 



