THE RESTING HABIT OP INSECTS, ETC. 7 



But there are reasons for supposing that species continuously 

 inhabiting the same climate have varied as to the stage at which they 

 hybernate. It is difficult to suppose that Arctia, hybernating as a 

 larva, Avent to the tropics, gave up hybernating, and came back as 

 Sjpilosoma to hybernate as a pupa and vice versa. 



When once a species has acquired the hybernating habit, it has 

 acquired it. Sjnlosoma menthastri must hybernate as a pupa, and 

 does so under ordinary conditions, and tries to do so (or perishes in the 

 attempt) under any conditions. It needs no special incentive in tem- 

 perature, or food, or other external influence, to make it do so. It 

 cannot be prevented from doing so. You can no more make it hyber- 

 nate as a larva, like Arctia caia, than you can make it develop the red 

 underAving of that species. You could conceivably make it do either, 

 only by a rare and exceptional sport, or by a long course of selection. 



The alternate broods, when a summer brood does not hybernate, 

 and the winter brood does, give room for much speculation and conjec- 

 ture, and in these Ave see that the tAVO forms are, in some instances, 

 most persistently alternate, in spite of variations of environment ; in 

 other instances any brood may be the one or the other, according to 

 temperature or other conditions. A. caia (vide papers passim, 

 Eat. Becord, A'ols. iv.-v.) has both of these forms, crossed, mixed, and 

 interbred together, so that some larvfe in a brood are of the one form 

 and some of the other, in spite of anything, but the mass takes either 

 form, according to temperature. Hybernation, then, is a habit of a 

 species, as much engrained in it (and no more) as any other habit, 

 any marking or other character ; it is similarly produced by natural 

 selection, and is subject to variation under changes of condition, 

 in greater or less degrees in different species, just as are any other 

 characters. The need of a resting period selected those that rested best, 

 beginning Avith small needs and small capacities Avhich developed 

 together, and this Avhether the need Avas cold, heat, starvation, drought. 

 Sec, but, Avhen deA'eloped, the conditions haA'e no closer causal connec- 

 tions to each other than they had at first. The conditions continuing, 

 the habit has to be kept in full force ; but they do not determine it ; 

 that is done by the inherent conditions developed during many 

 generations. 



I haA^e already said that " the adaptation which Ave find to exist 

 everyAvhere among organic beings to a very high degree (although such 

 adaptation must of necessity be considered relatively rather than 

 absolutely perfect), proves that the internal forces of the organism have 

 helped to bring about the desired result, but at the same time, only in 

 response to an outside stimulus, Avhich, as it Avere, determines the lines 

 on Avhich the inherent forces must act." If Ave assume that tempera- 

 ture (indirectly), andAvantof food (directly), were the combined outside 

 stimuli Avhich originally produced, in many larvae or imagines, the phe- 

 nomenon of hybernation, it Avould be interesting to attempt to examine 

 the internal forces of the insect Avhich Avere acted upon, and Avhich 

 responded in so large a measure to the outside stimuli as to ensure 

 success. 



That the Avant of food Avas the main factor in inducnig hybernation 

 appears clear, for whereas almost all arboreal and deciduous shrub- 

 feeding larvae, which exist in that stage in temperate regions, through- 

 out the Avinter, become totally lethargic throughout the winter months, 



