corresponding change from nauseousness to palatability. But there 

 is really no great difficulty here. It is true that provision for con- 

 cealment generally goes with edible qualities, but there is good 

 evidence that distasteful forms exist which adopt the cryptic mode 

 of defence at one time, and the warning, or as Professor Poulton 

 calls it, the aposematic mode at another. If this is true of the life 

 of the individual, there can be no reason why it should not also 

 obtain in the successive phases of the species, and we may therefore, 

 I think, conclude that as regards these forms of Preih at least, we 

 have gone some way towards finding the meaning of the seasonal 

 change. In the wet season, when insect food is plentiful and 

 enemies are fastidious, it is safe to display warning colours — whether 

 true or false we need not now enquire — and to push them to the 

 front. In the dry season, when food is scarce and enemies will eat 

 almost anything they can get, the best chance is to keep out of their 

 notice altogether. The contrast between the two forms of defence 

 is perhaps best seen in Precis archesia, where, as Professor Poulton 

 has pointed out, the very same feature, viz., the diagonal stripe 

 from tip to posterior angle, becomes at one time a part of the cryptic 

 sham by representing the midrib of a dead leaf, and at another time 

 is developed into tbe most prominent element in a violently con- 

 spicuous pattern. 



Now that we seem to have found a principle which helps us to 

 understand the seasonal changes in these particular butterflies, let 

 us see how far it can be made to apply to other cases of the same 

 phenomenon. There is an interesting genus named Iji/blia, which 

 for our present purpose may be looked upon as consisting of two 

 species — IJi/blia ilithijia, which is common to India and Africa, and 

 Bt/blia (jotzins, which is found in Africa only. Now each of these 

 butterflies occurs in two distinct phases, so distinct from one 

 another indeed that, just as in the case of Precis, they were at first 

 thought to be different species, and received separate specific names. 

 But from the ascertained facts of the times of their appearance, and 

 from the very frequent occurrence of the transitional forms, much 

 more common in these butterflies than in most species of /'(V'('/.s,itwas 

 soon suspected that the supposed distinct species were merely seasonal 

 phases of the same butterfly. All possible doubt on the subject was 

 set at rest by Mr. Marshall, who in each of these butterflies, Bi/blia 

 ilithijia and lii/blia (jntzins, actually bred specimens of one phase 

 from eggs laid by a parent belonging to the other phase, thus 

 supplying the most conclusive possible proof of specific identity. In 

 these Byblias, it is to the underside that we must look for the chief 

 differences between the wet and dry-season forms. In each case we 

 find that the wet-season phase has a variegated appearance, the 

 ground-colour being tawny and the region of the wings nearest the 

 body being furnished with an array of small black spots. In the dry 

 season the ground-colour of the forewings becomes darker, and the 

 hindwing puts on an entirely different appearance. The wing is 



