4 Studies in the Theory of Descent. 



perceptible differences, to divergence to the extent 

 of varieties, and finally to that of species. 



Nor are the instances of lesser differences be- 

 tween the two generations very numerous. Among 

 the European diurnal Lepidoptera I know of 

 about twelve cases, although closer observation in 

 this direction may possibly lead to further dis- 

 coveries. 3 Seasonal dimorphism occurs also in 

 moths, although I am not in a position to make a 

 more precise statement on this subject, 4 as my own 

 observations refer only to butterflies. 



That other orders of insects do not present the 

 same phenomenon depends essentially upon the 

 fact that most of them produce only one genera- 

 tion in the year ; but amongst the remaining orders 

 there occur indeed changes of form which, although 



8 [Eng. ed. W. H. Edwards has since pointed out several 

 beautiful cases of seasonal dimorphism in America. Thus 

 Plebeius Pseudargiolus is the summer form of P. Violacea, and 

 Phyciodes Tharos the summer form of P. Marcia. See 

 Edwards' " Butterflies of North America," 1868-79.] 



4 [Eng. ed. I learn by a written communication from Dr. 

 Speyer that two Geometrae, Selenia Tetralunaria and S. Illu- 

 naria Hub., are seasonally dimorphic. In both species the 

 winter form is much larger and darker.] [Selenia Lunaria, 

 S. Illustraria, and some species of Ephyra (E. Pundaria and 

 E. Omicronarid] are likewise seasonally dimorphic. For 

 remarks on the case of S. Illustraria see Dr. Knaggs in Ent. 

 Mo. Mag., vol. hi. p. 238, and p. 256. Some observations 

 on E. Punctaria were communicated to the Entomological 

 Society of London by Professor Westwood in 1877, on the 

 authority of Mr. B. G. Cole. See Proc. Ent. Soc. 1877, 

 pp. vi, vii. R.M.] 



