SEXUAL DIMORPHISM IN BUTTERFLIES. 105 



is somewhat different from those that I have coinnicMted on 

 above, but I think they are equally explicable on much the 

 same lines. The physical characteristics o£ Aden and 

 Karachi are very similar, both being sea-ports on the borders 

 of desert hinterlands ; and both ;ire virtuully rainless reoions. 

 How, then, do we account for the simultaneous ap[)earance in 

 these places of many forms, that are seasonally dimor])hic in 

 other countries of their rjinge ? 



Both Aden and Karachi are much in the nature of calm 

 back-waters to the N.W. Monsoon current. They should 

 therefore afford a tempting dumping or resting ground for 

 stragglers of our migratory swarms, travelling from Africa 

 to the East. The first few generations of newly arrived 

 stock would retain in gradually diminishing degrees the 

 disposition to vary, which their progenitors have acquired in 

 the seasonally dimorphic regions of their origin. Again, the 

 proximity of the sea to places like Aden and Karachi, 

 modifies the physical (desert) conditions which obtain inland. 

 Sea mists might have considerable influence on the larvae and 

 pupae of recenily arrived immigrants whose constitutional 

 tendency to vary along certain lines is still active. These 

 tendencies would be stimulated or otherwise by their 

 immediate environment during the larval and pupal stages. 

 If much exposed to mists rising from the sea, a larger 

 proportion of wet and intermediate phases might be evolved. 

 Per contra, if sheltered from these influences by intervening- 

 highlands, a larger proportion of dry forms might result. 



In hazarding these somewhat speculative deductions, I 

 know I am treading on dangerous ground, and I have as my 

 sole justification the hope of stimulating interest and further 

 enquiry into these abstruse problems of Nature. There are 

 probably many diverse influences at work besides those I have 

 touched upon to account for these irregularities. Mimetic 

 adaptation, sexual selection, heredity, and many other factors 

 may have their influence in the complex scheme of Nature 

 wdiich produces so many beautiful forms to delight the eye 

 and perplex the mind. ^ , .-^ , ^ 



