32 Studies in the Theory of Descent. 



the basis of the theory, but can scarcely otherwise 

 be understood. 



If it be Lsked what significance attaches to the 

 duplication of the winter form, it may be answered 

 that the species was already dimorphic at the time 

 when it appeared in only one annual generation. 

 Still, this explanation may be objected to, since a 

 dimorphism of this kind is not at present known, 

 though indeed some species exhibit a sexual 

 dimorphism, 18 in which one sex (as, for instance, 

 the case of the female Papilio Turnus) appears 

 in two forms of colouring, but not a dimorphism, 

 as is here the case, displayed by both sexes. 19 

 Another suggestion, therefore, may perhaps be 

 offered. 



In A. Levana we saw that reversion occurred 

 in very different degrees with different individuals, 

 seldom attaining to the true Levana form, and 



18 [For other remarkable cases of sexual dimorphism (not 

 antigeny in the sense used by Mr. S. H. Scudder, Proc. Amer. 

 Acad., vol. xii. 1877, pp. 150 158) see Wallace "On the 

 Phenomena of Variation and Geographical Distribution, as 

 illustrated by the Papilionidse of the Malayan Region," Trans. 

 Linn. Soc., vol. xxv. 1865, pp. 5 10. R.M.] 



19 [Eng. ed. Dimorphism of this kind has since been made 

 known : the North American Limenitis Artemis and L. Proser- 

 pina are not two species, as was formerly believed, but only 

 one. Edwards bred both forms from eggs of Proserpina. 

 Both are single-brooded, and both have males and females. 

 The two forms fly together, but L. Artemis is much more 

 widely distributed, and more abundant than L. Proserpina. 

 See " Butterflies of North America," vol. ii.] 



