( cii ) 



The abundance of Lepidoptera in warmer regions, and the 

 vast number of their individuals, arising out of the frequency 

 of their broods, would tend to overspills, and the evidence of 

 actual migration is in favour of its having been mostly in a 

 direction that would point to these regions as the source.* 



I do not suggest that in any of the cases the difficulties are 

 insuperable, and one can see that even when there is so great 

 a physiological gulf as that between the summer pupa of a 

 double-brooded insect and the winter pupa of a single-brooded 

 one, there are stepping-stones across it, such as variability of 

 climate in the same locality, variability in individuals, and 

 possibility of overlapping resulting in fusion by mating of 

 different generations. And many species are probably at all 

 times in an unfixed, perhajDS intermediate and transitional 

 condition, such as those single-brooded species, which frequently 

 under the stimulus of a hot season bring forth wholly or 

 partially a second brood, often ineffectively, it is true.t 



Altering pattern bij temperature. 



I have referred in general terms to the effect of the 

 application of temperature in the pupal stage in altering 

 the colour and pattern of the wings of Lepidoptera. To 

 attain this result in a high degree it is usually necessary to 

 apply the temperature from the time when the newly-formed 



Dr. Chapman, Ent. Record, vol. xvii, p. 266. As to hibernation, it is not 

 suggested that this is only caused by cold, or aestivation only by heat or 

 draught. There is an interesting discussion on this by Mr. Tutt, Mr. 

 de Vismes Cane, Dr. Kidding, Dr. Dixey and others in Ent. Record, vols, 

 vii and viii. 



* See articles by Tutt in Ent. Record, vol. xiv, pp. 262, 292, and 

 31. r >. " Broadly Bpeaking, the dispersal movements of insects . . . are . . . 

 from sub-tropical to temperate regions. . . . The general trend ... in the 

 Noil hern hemisphere is northerly and in the Southern hemisphere is 

 southerly." I have only mentioned migration, which is operative before 

 our eyes, but I nni not unmindful of another liotent way in which climate 

 may affect organisms, and one that has doubtless been highly effective in 

 the past, and that is where, instead of the organism migrating to the 

 climate, the climate comes to the organism, in the great secular changes 

 which have taken place; heat and cold having at different periods char- 

 acterized most of the Palsearctic region, and having had their turn in 

 starting or moulding the forms that have descended to our times. 



t Warm climates where there are. considerable mountains would seem 

 suited to qualify species for acclimatization] though, as we see, many 

 of the species generated in such countries remain obstinately single- 

 brooded. 



