84 



rsrcHE. 



[September — October 1SS3. 



pastures new,' they, contrary to their 

 orilinary habits, cross land or sea, ar- 

 riving, of course, very often, in some 

 inhospitable clime, where — if not at 

 once captured — they very likely soon 

 fall victims to some pitiless storm of 

 wind and rain. But supposing both 

 these risks to be avoided, the moth — 

 if an impregnated female — in due 

 course lays its eggs, which most proba- 

 bly hatch. If the temperature happens 

 to be lower or the weather wetter than 

 the natural constitution of the species 

 is able to endure, the young larvae die 

 without even attempting to feed, but if 

 matters are more favourable, the strong- 

 est of them struggle along, and if fairly 

 favoured by the weather a few of them 

 may reach the perfect state ; if quite 

 unusually fiivored by the weather a large 

 proportion of them maj'. do so, pro- 

 ducing those remarkable instances of 

 the sudden appearance in numbers of 

 a species usually rare. Such good for- 

 time rarely extends to a second season 

 and the species becomes a rarity again 

 or is even probably exterminated, to be 

 renewed at some future time by the same 

 instinct of migration. In cases such 

 as these it appears to me that sunshine 

 means life, and its absence destruction, 

 to the larvae, and that by this simple 

 and obvious influence the extension of 

 species beyond their assigned limits is 

 practically prohibited. 



" It also happens sometimes that the 

 immigrant, following instinctively its 

 inherited habit, attempts to produce an 

 additional brood in the year, over what 

 the climate will allow." For instance, 

 pupae of the second brood of Colias 



cdusa. in England, showed in Decem- 

 ber the \ello\v color of the wings, 

 ■' which only shows itself when the 

 insect is nearly ready to emerge," thus 

 ••following inherited habit so as to 

 hibernate, as they are well known to 

 do [on the continent of Europe] in the 

 perfect state, but from insufficient 

 warmth and sunshine were unable to 

 muster sufficient strength," and died. 



Again, in the autumn of 1880, in 

 which year there had been a \vonderful 

 immigration of Vanessa cardui into 

 England, evidently a portion of the 

 vast army that migrated across Europe, 

 larvae were found tolerably common. 

 feeding, at the beginning of October, 

 on yoimg thistle plants, close to the 

 ground, making their nests among the 

 radical leaves, all the tall thistles being 

 dead. From some of these larvae two 

 pupae were obtained, in doors, 17 and 

 20 October, and one imago, 20 Novem- 

 ber. •' The rest died. This failure of 

 instinct on the part of the immigrants 

 surely explains, in some degree, the 

 fact that last year [iSSi] the insect was 

 more than usnallv scarce, hardlv any 

 appearing to have hibernated, and also 

 Avhy an insect with such power of 

 increase in a suitable climate is so un- 

 certain and variable in its a]5pearances 

 in one that is unfavorable. 



'• With reference to the second class 

 of cases — those in which a species 

 always present is periodically common 

 or scarce — much has been written. 

 excessive rain being usually assigned 

 as the cause of diminution in numbers, 

 sunshine as the cause of increase. 

 Without doubt these causes act to a 



