.2 . Studies in the Theory of Descent. 



may arouse the horror of the philologist, but, as it 

 is as concise and intelligible as possible, I propose 

 to retain it in the present work. 



The species of Arascknia through which the 

 discovery of seasonal dimorphism was made, 

 formerly bore the two specific names A. Levana 

 and A. Prorsa. The latter is the summer and the 

 former the winter form, the difference between the 

 two being, to the uninitiated, so great that it is diffi- 

 cult to believe in their relationship. A. Levana 

 (Figs, i and 2, Plate I.) is of a golden brown 

 colour with black spots and dashes, while A. 

 Prorsa (Figs. 5 and 6, Plate I.) is deep black 

 with a broad white interrupted band across both 

 wings. Notwithstanding this difference, it is 

 an undoubted fact that both forms are merely the 

 winter and summer generations of the same species. 

 I have myself frequently bred the variety Prorsa 

 from the eggs of Levana, and vice versd. 



Since the discovery of this last fact a consider- 

 able number of similar cases have been established. 

 Thus P. C. Zeller 1 showed, by experiments made 

 under confinement, that two butterflies belonging to 

 the family of the * Blues/ differing greatly in colour 

 and marking, and especially in size, which had 



1 " Uber die Artrechte des Polyommatus Amyntas und Poly- 

 sperchon" Stett. ent. Zeit. 1849. Vol. x. p. 177 182. [In 

 Kirb/s " Synonymic Catalogue of Diurnal Lepidoptera " 

 Plebeius Amyntas is given as a synonym and P. Polysperchon as 

 a var. of P. Argiades Pall. R.M.] 



