36 Mr. G. A. K. Marshall on Seasonal Dimorphism 



parficulaily Insects and birds, tend to develop brilliant 

 colours whenever they are aV)le to do so with impunity, and 

 it is therefore quite conceivable that a strong-flying and 

 wary insect like this should be able to indulge this tendency 

 without unduly endangering its chances in the struggle for 

 existence. This is more especially probable during the wet 

 or summer season, when the lizards would be able to tind a 

 considerable amount of other food in the shape of small 

 Coleoptera, Diptera, &c., and would therefore be less inclined 

 to attempt the more difficult feat of stalking butterflies. But 

 as the cold dry season advances the vast majority of these 

 small fry disappear, either dying off or retiring to winter- 

 quarters, and, as a consequence, those butterflies which 

 remain on the wing during that time — and the majority of 

 species in this country do so — would suffer far more per- 

 secution at the hands of their various foes, and would thereby 

 be compelled to adopt protective measures in the way of 

 procryptic colouring and greater wariness. This, in the case 

 of o.-natalensis, would have resulted in the dry form sesamus ; 

 although why the salmon-red upperside should be changed to 

 blue is not altogether evident, except perhaps tliat tiie latter 

 colour is more in keeping witli tlie shady haunts so often 

 frequented by that form. It is instructive to note that the 

 typical form of octavia, which occurs in the forest-clad regions 

 of West Africa, has not been able to develop the brilliant 

 hues of its eastern subspecies, being smaller and much more 

 dully coloured ; this being probably due to the keener 

 struggle for existence in that country. At the same time its 

 representative dry form amestris, Diury, shows a slightly 

 less divergence from it than does sesamus from natale/isisy 

 which might perhaps be attributed to the less marked contrast 

 in the general conditions of the environment at the two 

 seasons. 



It has been pointed out by Mr. L. de Nicdville, and also 

 by Prof. Weismann, that both forms of any dimorphic species 

 must be adaptive, otherwise the non-adaptive form would be 

 gradually supplanted by its more favoured relative. While 

 fully accepting the general principle of this argument, it does 

 not seem to me necessary that we should have to assign some 

 specially adaptive character to each form, exclusive of that 

 general adaptation to the surrounding conditions of life 

 without which no species could exist. For there is nothing 

 to show that the development of a dry-season phase is not a 

 geologically recent event, and the process of elimination of 

 the non-adaptive wet form may be going on slowly but 

 surely under our very eyes. Mr. Distant's observations of 



