SOME ASPECTS OF HIBERNATION. 171 



A phenomenon analogous to this is the variability in the duration 

 of the pupal condition that is well known to occur in many species. 

 In some of these cases the pupre of the same generation may be 

 physiologically differentiated into two groups, maturing at different 

 rates, and giving two distinct emergences of the perfect insect ; one 

 perhaps in the same summer, the other in the following spring. The 

 instances of hibernating larvte, referred to by Mr. Tutt, where some 

 individuals of any given brood will occasionally complete, or attempt 

 to complete, the whole series of changes within a single season, are 

 also due to a similar physiological dimorphism. The advantages to 

 a species of such dimorphism are obvious — on the one hand it is in 

 favour of the continued existence of the species to get through as 

 many successive broods as possible in the course of the year ; on the 

 other hand, there is a risk under these, circumstances of the whole of 

 one generation being cut off by an abnormally early winter, or by 

 some other adverse condition. If, however, a part of each brood is 

 as it were held in reserve, a provision is thereby afforded against the 

 incidence of such catastrophes, and the species, by means of its 

 dimorphism, is enabled to tide over the serious thinning of its 

 numbers that they involve. If such an arrangement exists in the 

 case of P. cardui, this would to some extent account for the curious 

 facts relating to its intermittent prevalence in this country, as 

 recounted by Mr. Tutt. 



The main point to be observed with regard to all these cases is, 

 that they are invariably such modifications of habit as may well be 

 supposed to have arisen under the influence of natural selection. 

 Each species meets the exigences of the case in its own way ; some 

 may be supposed to be still undergoing adjustment, others have 

 attained a state of comparative equilibrium. The actual time of year 

 at which a species goes into hibernation, the stage of development 

 in which its period of repose occurs, and the uniformity, or otherwise, 

 in the behaviour of the members of each brood, are all .points that 

 may be expected to difier in relation with other circumstances which 

 it is needless to enumerate. Natural selection, in pointing out, so to 

 speak, the best or most practicable road for each species to take, will 

 give full consideration to all the circumstances under which that 

 species has already found it possible to maintain its existence. If 

 the foregoing be allowed, it will be seen that the question of the 

 actual nature of the inducing stimulus becomes of minor importance. 

 The stimulus may differ in every case. Under natural selection 

 anything may be taken to serve as the stimulus, provided only that 

 it touches off the appropriate mechanism at the right moment for the 

 species. It may be an external change in one case, an internal 

 condition in another. It is probably, in many instances, the simple 

 lowering of the temperature ; in others it may be repletion with a 

 certain diet ; in others, again, mere contact with the outer air on 

 escaping from the pupal integuments. It is even conceivable that in 

 some instances no stimulus is required at all. 



Many of these points have been clearly put by Mr. Tutt in the 

 latter part of his paper ; he has, however, as it seems to me, some- 

 what failed to appreciate the distinction between (1) cold considered 

 as an external condition, exercising, whether directly or indirectly, 

 a selective influence upon organisms, and in that sense leading to 



