924 THE BUTTERFLIES OF NEW ENGLAND. 



but in 80inc it is not go, and it may be preeuinod that tliese are species 

 which have not long enjoyed the privilege of a second 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 

 cflfected by the lingering development of some caterpillars and the dispo- 

 sition 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 swallow-tail, for instance, is triple brooded in 

 the south, and hibernates as a butterfly and pei'hajjs also as a chrysalis ; 

 in the north it is double brooded, and hibernates only as a chrysalis. 



Digoneutism or polygoneutism, then, is either a device of nature for 

 the better perpetuation of the species, by varying the conditions of its exis- 

 tence at any given time, and so multiplying the chances of successfully 

 meeting opposing or unfavorable agencies ; or it is simply taken advan- 

 tage of by nature as a means thus to vary the conditions. The particular 

 problem diflBcult of solution which it carries in its train is this : In 

 some species — and it would appear to be no very uncommon occurrence, 

 and to be found among moths as well as butterflies — the result of sum- 

 mer dormancy in the caterpillar or prolonged life in the chrysalis is that, 

 by some unexplained common impulse, the caterpillars or the chrysalids 

 that arouse after lethargy, and do not hibernate, more frequently than 

 otherwise do this at such a time that the resulting butterfly flies with its 

 nephews and nieces instead of with its brethren and sisters, i. e., it 

 bridges over with considerable accuracy the interval between two genera- 

 tions in mid-summer, just as happens from easily perceived causes when 

 winter intervenes. Who solves this problem will win deserved renown. 



What may be the exact climatic features which determine the number 

 of generations of a butterfly has not yet been studied ; but there are some 

 curious difficulties in the way of understanding them. Vanessa cardui, for 

 instance, is double brooded in New England, both in the districts where 

 the contrasts of heat and cold, moisture and drought, are excessive — that 

 is, where the climate has those peculiarities which are termed "continental" ; 

 and also on islands such as Nantucket in southern New England, where 

 a much greater evenness prevails and the climate partakes of an "insular" 

 character ; yet in the valleys of Switzerland, where perhaps of all places 

 in Europe the climate presents the greatest and most sudden inequalities, 



