208 HUXLEY AND NATURAL SELECTION 



Precis, allied to Kalliiua, possess seasonal forms closely 

 resembling those of the latter. In other species, however, 

 the phases are more distinct than those of any known 

 butterfly. Naturalists were fairly astounded when, in 

 1898, Mr. Guy A. K. Marshall first bred the black and 

 blue dry season Precis sesamus from the black and red 

 wet season P. natalensis.^ The two butterflies differ in 

 size, form, pattern, colours, relation of upper to under 

 surface, and habits. But for the fact that very rare inter- 

 mediate examples were known, they would not have been 

 looked upon as closely allied. The under side of the dry 

 form is very dark and well-adapted to hide the insect in 

 the sheltered places it frequents ; the under side of the 

 wet form resembles the upper side, save that it is even 

 more conspicuous, and the individuals of this phase 

 furthermore seek exposed and open ground. The same 

 alternation between a dry form with under side strongly 

 adapted for concealment, and a wet form in which it is 

 highly conspicuous, has been established in other species 

 by Mr. Marshall, by the same incontrovertible evi- 

 dence. Of these Precis antilope and P. actia are almost, 

 if not quite, as remarkable as P. sesamus. In Precis 

 archesia, which he has not yet succeeded in breeding from 

 one form to the other, the difference is particularly instruc- 

 tive. A mid-rib-like stripe is, perhaps, the most effective 

 detail in the dead-leaf-like appearance of the dry phase 

 under side, and this very feature isemphasized broadened, 

 lightened in tint, and with sharp outlines into the most 

 conspicuous character of the wet form. The very cha- 

 racter which chiefly promotes the concealment of one 

 phase is the principal element in the conspicuousness 

 of the other. In attempting to understand these astonish- 

 ing alternations it is necessary to study the relationship 

 between the seasons and the struggle for existence which 

 goes on in them. The dry season is a time of far greater 

 pressure than the wet ; for although enemies of insects are 

 fewer, the insects themselves are proportionately even more 

 reduced. Large groups of insects bury themselves in the 

 ground, and so disappear altogether. Hence the struggle 

 1 Ann. Mag. Nat. Hist., ser. 7, vol. ii, July 1898, p. 30. 



