( lxxxix ) 



and begin to move actively towards emergence, the pace at 

 which they do so is accelerated by warmth, and retarded by 

 cold. 



Nor do I suggest that all individual Lepidoptera belong, 

 throughout their lives, or even throughout any one meta- 

 morphic stage, to one or other of these two classes, nor that 

 the element of personal variability does not come in largely, 

 so that there are many species and individuals which may be 

 described as intermediate, though often very discontinuously so. 



Physiological dimorphism. 



I shall have to advert, rather fully, to that very numerous 

 and remarkable class called double-brooded (digoneutic and 

 bivoltine), which in one of their "phases" — to adopt the 

 term suggested by Professor Poulton — belong to the sensi- 

 tive, in the other phase to the insensitive, thus exemplify- 

 ing the kind of diversity to which Dr. Dixey has given the 

 apt name of " physiological dimorphism." 



Ron seasonal Lepidoptera. 



The climates of the world may be divided into the non- 

 seasonal and the seasonal, the former consisting of those equa- 

 torial or tropical regions where, with a temperature averaging 

 something over 80° F. (27° C.),* not much above and very little 

 below, and with adequate humidity all the year round, these 

 conditions continuing the same year after year, species abound 

 whose " little lives are rounded " in a month, regions where 

 at all times and in all their stages — egg, larva, pupa, and 

 imago — they are to be found simultaneously and in nearly 

 equal abundance — such species as the Heliconias and Ithomias 

 of tropical South America, and the Ypthimas, etc., of the Indo- 

 Australian region. I do not know the life histories of these 

 species, but, looking to the regularity with which generation 

 succeeds generation, it may perhaps be assumed that so far as 

 the operation of the temperature — itself so regular — is con- 

 cerned, there is nothing in a nature bred under these con- 

 ditions to prevent that general effect which I have adverted to 



* For convenience I have in all cases given, approximately, the 

 Centigrade degrees. 



