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



So far as South Africa is concerned, I am strongly of 

 opinion that the exact converse is the case, and on writing to 

 Mr. de Niceville he tells me that he is of entirely the same 

 opinion as regards Indian butterflies, and cites Mr. Doherty's 

 experiment in which, by the application of humidity, he 

 produced the wet-season form of Melanitis leda, L., during 

 the dry season. Unfortunately the few simple experiments 

 which I have been able to make as yet are not of a 

 sufficiently conclusive character to adduce here ; but so far as 

 they show anything they tend to support the humidity theory, 

 and they also negative the supposition that heat — at least 

 dry heat — tends to produce the summer form in this country. 

 This state of affairs is not so contradictory as it might seem 

 at first sight; for, in seeking for the climatic cause of dimor- 

 phism, we should naturally look to the more unstable factor 

 in the climate of any given country. So far as temperature 

 is concerned, the annual range in Europe is considerable, 

 owing to the large land area; whereas S. Africa has a high 

 diurnal and low annual range. Taking Prof. Ferrel's cal- 

 culations, we find that the difference between the average 

 temperatures for January and July on 40° N. lat. is 33°, and 

 on 50° N. lat. it is 45°; whereas the greatest difference in 

 the Southern Hemisphere is 15° between 20° and 30° S. lat., 

 and diminishes to the north and south of these parallels. 

 The annual range of humidity exhibits an exact converse to 

 this, being comparatively small in Western Europe; whereas 

 in S.E. Africa, as is well known, the contrast is most marked 

 in this respect. There is therefore strong prim a facie evidence 

 that humidity and not temperature is the exciting agent of 

 dimorphism in this part of the world. 



In the cases of what he terms " adaptive seasonal dimor- 

 phism," of which P. stsamus is an evident example, Prof. 

 Weiemann has come to the conclusion that an " alternation 

 of ihe two forms is provided by nature, apart from the 

 influences of temperature" ; also " that temperature in general 

 does not here take part as the actual cause, but that it is a 

 question of a process of selection which goes on independently 

 of the temperature, and gradually alters some of the ids." 

 This does not appear to be an altogether satisfactory con- 

 clusion to my mind, for if the alternation in these cases were 

 due entirely to selection apart from climatic causes, it seems 

 evident that artificial temperatures could not in any way 

 influence the appearance of either form. But Prof. Weismann 

 has himself abundantly proved that artificial temperatures 

 have a very potent influence in the case of Vanessa prorsa- 



