432 Mr. G. A. K. Marshall on 



extraordinary seasonal phases of Precis were to be inter- 

 preted along the lines suggested in 1887 — that we have to 

 do with a set of somewhat distasteful species which can 

 only exist in the keen struggle of the dry African winter 

 when food palatable to insect-eaters is relatively scarce, by 

 a very high standard of protective disguise associated with 

 the appropriate instincts, but gain the recognized advan- 

 tages of aposematic colouring by producing markedly 

 conspicuous generations during the moist summer, when 

 insect-eating animals have a much greater variety and 

 abundance of suitable food. 



F. The severity of the Struggle for Existence among Inserts 

 in the African Dry Season as compared vAth the Wet. 

 The relation of the Seasonal Changes in Precis to 

 those of other Butterflies. 



As soon as the idea expressed in the concluding para- 

 graph of the last section of this memoir occurred to me, I 

 wrote to Mr. Marshall asking for his experience on the 

 subject, and also inquiring whether any of the admittedly 

 unpalatable African butterflies exhibited seasonal changes, 

 such that the Avinter generations became comparatively 

 inconspicuous. 



His deeply-interesting reply is printed in extenso below. 



"Salisbury, Jan. 8, 1899. — As to your query about the 

 keenness of the struggle for existence at the two seasons, 

 in my own mind I had never felt any 'doubt that the dry 

 season is certaiidy the more critical period for insects, and 

 this I referred to incidentally in my paper on Frecis (Ann. 

 and Mag. Nat. Hist., ser. 7, vol. ii, July 1898, p. 36). It 

 is true that insectivorous birds are far more numerous 

 during the summer, but this I think would be more than 

 outbalanced by the increase of such insects as Coleoptera, 

 Hymenoptcra, Diptera, etc., apart from the fact that the 

 summer broods of the perennial butterflies are undoubtedly 

 larger and much more numerous (some Pierimv have a fresh 

 brood every four or five weeks), and that a number of 

 additional species make their appearance at that season 

 only. On the other hand, during the dry season, although 

 a number of migratory birds depart northwards, yet we 

 have a considerable number of resident insectivorous birds, 

 including rollers, drougos, shrikes, flycatchers, bash-king- 



