The Bionomics of South African Insects. 415 



differences in the insect life of the two seasons, will all be 

 shown to be consistent with the above hypothesis. 



The results of Mr. Mai\shairs experiments as to the 

 nature of the stimulus by which the change is started in 

 any individual will be discussed, and further lines of 

 investigation suggested. The much greater size and 

 weight of the dry phases will be shown to have an import- 

 ant bearing upon the inquiry, indicating, as it does, that 

 the phase must be predetermined in the larval stage. 



Finally, it will be argued that the facts proved by Mr. 

 Marshall, although most startling and indeed astounding, 

 are not subversive of any of the principles of the science 

 of systematics. 



B. Historical. 



In his great work on " South African Butterflies " 

 (London, 1887, vol. i), Mr. Roland Trimen describes several 

 intermediate varieties between Precis nataletisis and P. sesa- 

 mns, and records Mr. F. N. Streatfeild's capture of the two 

 butterflies incoitu. He concludes {loc. cit. pp. 231 and 233), 

 " It is only to such occasional unions, and to their fertility, 

 that the origin of the intermediate examples imder notice 

 can be attributed." 



Mr. Trimen also makes a similar suggestion as to the 

 intermediate varieties between iJclasgis and archesia, which 

 are also recorded as having been taken in coitu (loc. cit, p. 

 235). 



Mr. Guy A. K. Marshall first pubhshed in 1896 the 

 suggestion that a group of South African butterflies 

 described and known as different species of the genus 

 Freds or Jimonia, were in reality the seasonal phases of a 

 comparatively limited number of species. He pointed 

 out, however, that octavia and amestris (s. I.) had been 

 previously considered as two forms of a single species by 

 M. Charles Oberthiir of Kennes (Ann. Mus. Genov. xviii, 

 1883, p. 721), and also that Mr. C. N. Barker, the distin- 

 guished Natal naturalist, had been long convinced of the 

 existence of these seasonal phases, and especially of the 

 most remarkable case of all, P. sesamns, and its wet-season 

 form, natal ensis. 



Mr. Marshall's general description of the differences be- 

 tween the two phases is as follows : " The dry-season form 

 is smaller, and usually assumes a duller type of colouring' 

 on the upper-side, sometimes of quite a different hue; the 



