On the Seasonal Dimorphism of Butterflies. 43 



constant crossing of individuals which have be- 

 come changed in different degrees. Here also 

 the theory is fully in accord with facts. 



In A. Levana the Levana form is decidedly 

 more constant than \\\& Prorsa form. The first is, 

 to a slight extent, sexually dimorphic, the female 

 being light and the male dark-coloured. If we take 

 into consideration this difference between the sexes, 

 which also occurs to a still smaller extent in the 

 Prorsa form, the foregoing statement will be found 

 correct, viz. that the Levana form varies but little, 

 and in all cases considerably less than the Prorsa 

 form, in which the greatest differences occur in 

 the yellow stripes and in the disappearance of the 

 black spots on the white band of the hind wing, 

 these black spots being persistent Levana mark- 

 ings. It is, in fact, difficult to find two perfectly 

 similar individuals of the Prorsa form. It must, 

 moreover, be considered that the Levana marking, 

 being the more complicated, would the more 

 readily show variation. Precisely the same thing 

 occurs in Pieris Napi, in which also the var. 

 sEstiva is considerably more variable than the 

 var. Vernalis. From the behaviour of the var. 

 Bryoni&y on the other hand, which I regard as the 

 parent-form, one might be tempted to raise an 

 objection to the theory ; for this form is well 

 known to be extraordinarily variable in colour 

 and marking, both in the Alps and Jura, where it 

 is met with at the greatest altitudes. According 

 to the theory, Bryonicz should be less variable 



