12 



Sexual Dimorphism. 



By Dr. F. A. Dixey, M.A., F.R.S., F.^.S.—Eead November dt/i, 



1920. 



Those of us who are familiar with the oak woods of the south- 

 eastern parts of Great Britain, the counties for instance of Essex 

 and Sufltblk, have very likely had opportunities of watching the 

 splendid " Purple Emperor " butterfly, as it glides about among the 

 breezy tree-tops and settles from time to time upon a lofty twig, the 

 glossy metallic purple of its wings flashing and glowing in the sum- 

 mer sunshine. We may also, though this is less, likely, have 

 encountered, in the same places a somewhat larger butterfly with 

 similar markings, but with a ground-colour of sober brown ; the 

 brilliant purple showing no trace of its presence. The difference 

 between these two forms of butterfl}^ is one of sex ; the one with 

 wings of glowing purple being the male, and the larger, quieter- 

 coloured insect, the female. We have here, then, dimorphism of 

 another kind than that with which we dealt on a former occasion, 

 the two different aspects being borne, not by separate generations 

 according to season, but by separate individuals of the same 

 generation. 



There are many more mstances of this sexual dimorphism 

 among our common butterflies ; the female, for instance, of the 

 well-known "'Orange-tip," is entirely destitute of the bright patch 

 of colour that ornaments the wings of the male. In many species 

 of blue butterfly the two sexes are so dissimilar in aspect, that they 

 might easily be taken for different species. 



On the other hand, there are plenty of cases where the sexes 

 hardly differ. So far as the wing-patterns are concerned, the sexes 

 of the tortoiseshells, the peacock, the painted lady, and the red 

 admiral are nearly identical. The same may be said of the swal- 

 lowtail, the wood white, the pale clouded-yellow, the small heath, 

 and several others. In another set of instances, the sexes although 

 very similar are yet distmguishable at a glance. Hence we see that, 

 as in the case of seasonal dimorphism, every kind of transition 

 exists between a difference so slight as to be scarcely distinguishable, 

 and a divergence so great that without direct proof no one would 

 believe that the two forms could belong to the same species. 



As in the dimorphism which we were considering when I last 

 addressed you, the question at once suggests itself, can we do any- 

 thing towards explainmg this curious phenomenon of sexual diver- 

 gence ? Must we regard it as an ultimate fact of nature, or can we 



