136 Mr. G. A. K. Marshall on Diaposcmatism, vnth reference 



the brighter under-side colours of the wet phases of these 

 species has been more recently evolved from the older 

 cryptic pattern of the dry phase. On general grounds 

 such a conclusion seems very difficult to accept. There 

 can be no question that for butterflies which are on the 

 wing throughout the year in South Africa the dry season 

 is the time of greatest stress and danger. And from this 

 it can only be concluded that that phase which alone is 

 able to subsist under such conditions must present a 

 higher degree of protective efficiency, either in colour or 

 habits, than does the wet phase. If then it be supposed 

 that this cryptic coloration, or some modification of it, 

 was originally common to both seasons, it is difficult to 

 understand why it should subsequently have been elimin- 

 ated for tiie purpose of establisliing a scheme of colouring 

 having a lower protective value. For if this cryptic under- 

 side afforded an efficient protection from the greater risks 

 of the dry season, it must have been amply sufficient to 

 meet the requirements of the species during the less 

 dangerous summer months. A comparison of various 

 details of the patterns also seems to me to point strongly 

 to the opposite conclusion, namely, that the dry phase is 

 a more recent development of the Avet in response to a 

 greater need for protection ; while in the case of sesamns, 

 there is important experimental evidence in the same 

 sense. In these experiments, which are as yet unpub- 

 lished, I found that by the application of moisture I could 

 convert the dry phase into the wet ; but I could not con- 

 vert the wet into the dr}^, either by dryness or cold. If 

 we accept Weissman's arguments in respect to Araschnia 

 levcma, this evidence must be taken as indicating that the 

 wet form is phylogenetically older than the dry. Thus 

 the suggestion that the wet phase represents an unpalat- 

 able and warningly-coloured form, which has been evolved 

 from the cryptic dry phase, finds no support. On the 

 contrary, the evidence points to the conclusion that the 

 cryjDtic dry phase is a later development in response to a 

 greater need for protection, and also shows that there is 

 no adequate reason why we should postulate distastefulness 

 in the wet phase of this genus. 



Finally, we may turn to the evidence yielded by the 

 various experiments which are detailed in the earlier 

 pages of the same paper. In the course of these experi- 

 ments examples of five species of Precis (including 



