13 



the members of the same brood may show great individual differ- 

 ences in their response to what are to all appearance identical con- 

 ditions. Thus Mr. Marshall on one occasion reared under precisely 

 similar conditions the larvae resulting from two eggs laid on the 

 same day by the same parent butterfly. One of these emerged as 

 Precis natolensis and the other as Precis sesanuis. The second fact 

 is that, as we have seen, there is much difference between one species 

 and another in the period of immature life at which the application 

 of the requisite stimulus is most effective. In some cases, as has 

 been conclusively shown by Mr. Mernfield, the application of 

 artificial warmth during even the last few days of the pupal period 

 may exert a considerable effect upon the seasonal colouring. Now 

 it has been observed by more than one naturalist that it is in 

 generally hot and dry localities, such as Aden, that the various 

 seasonal forms are most often to be seen on the wing together. 

 Such regions are liable to occasional heavy rainfalls of a temporary 

 character, and it is perhaps possible that these sudden interruptions 

 of the general conditions of aridity may last long enough to 

 cause those species that happen to be in the specially susceptible 

 stage to assume the wet-season appearance, though their actual 

 emergence in the perfect condition may take place during the usual 

 drought. Other species, not in the sensitive period of their life- 

 history, may be unaffected by the temporary moisture, and may 

 show on emergence nothing but the aspect which is ordinarily 

 associated with the dry season. These considerations are doubtless 

 not suflBcient to account for all the apparent anomalies that occur, 

 but they go some way towards suggesting an explanation that may 

 apply to many of them. 



We may now put together what we have learned on the subject 

 of Seasonal Dimorphism in Butterflies. 



(1) We have seen that one and the same kind of butterfly may 

 appear under aspects so diverse that they have been considered to 

 belong to entirely different species. 



(2) These different phases of the same insect are often found to 

 appear each at its own time of the year. Thus, in thD case of 

 Araschnia levana, the levana form appears in the spring, the jirorsa 

 form in the summer. 



(3) Many species of butterfly which produce more than one gener- 

 ation in the course of the year show this kind of seasonal dimor- 

 phism. The degree in which the difference is manifested varies 

 greatly from species to species, every transition being found between 

 a difference so slight as to be scarcely noticeable, and a diversity so 

 extreme that nothing short of the breeding of one form from the 

 other could be accepted as sufficient evidence of their specific 

 identity. 



(4) Seasonal dimorphism is found not only in the butterflies of 

 temperate regions, where the most obvious difference between the 

 seasons is one of temperature, but also in those of the tropics, where 



