2 



to belong to two entirely distinct species. This twofold aspect of 

 the same species is what is known as dimorphism, and inasmuch as 

 in the case that we have been considering the successive changes in 

 aspect bear an evident relation to the successive changes of season, 

 the particular kind of dimorphism here exhibited is known as 

 • Seasonal Dimorphism." 



The case of Araschuia Icrana and prorsa is perhaps the longest- 

 known instance of seasonal dimorphism among butterflies ; but it 

 by no means stands by itself. When our attention is once drawn to 

 the existence of this phenomenon, we can trace the presence of a 

 similar dimorphism in many other species; though it would be diffi- 

 cult to find, among European butterflies at least, so well-marked a 

 case as that which we have just had under notice. But we have not 

 far to go before meeting with another instance of seasonal dimor- 

 phism among butterflies with which everyone in this country is 

 familiar. The green-veined white butterfly {Ganoris napi), one of 

 the smaller species of common garden white, occurs in two forms 

 which, though very far indeed from being so diverse as Araschnia 

 levana and prorsa, are yet quite easily distinguishable from each 

 other. The specimens that fly in the spring have a grey tip to the 

 forewing and the green veining of the underside strongly marked. 

 From the eggs laid by this spring brood there results about July a 

 brood in which the tip of the forewing is nearly black, while the 

 green veining is much fainter and in some cases has almost disap- 

 peared. The relation of these differences with the change of season 

 is as well-marked in this case as in the former ; and although so 

 very much slighter in degree, the dimorphism of the green-veined 

 white is evidently similar in kind to that of Arau-hnia lerana. There 

 are many other instances among the butterflies and moths of this 

 country where more than one brood is produced in the course of the 

 year; and if these instances are carefully examined, it will often be 

 found that there is a constant difl'erence in aspect between the 

 broods, corresponding to the period in the annual cycle to which 

 they belong. The difference may be slight or considerable ; but in 

 no case is it so conspicuous as in the butterfly with which we 

 began. 



It is now natural to ask whether this phenomenon of seasonal 

 dimorphism is peculiar to temperate regions such as our own country 

 and continental Europe, or whether an analogous state of things 

 prevails in the tropics, where seasonal conditions difi'er widely from 

 our own. Only a few years ago this question would have met with 

 an uncertain answer ; now, however, we are able to make a very 

 positive statement on the subject. 



In many parts of Africa there occurs a salmon-red or brick-red 

 butterfly with black borders and spots, known as Precis octavia, and 

 in the southern and eastern parts of its range as Precis natalensis. 

 This handsome insect is to be met with chiefly in the wet season. 

 In the same parts of Africa is found, during the dry months of the 



