Appendix. 155 



from the annual cycle, with respect to causes and effects. 

 An investigation of this kind would be of the greatest 

 importance, not only for seasonal dimorphism, but also 

 for the elucidation of questions of a much more general 

 nature, and would be a most satisfactory problem for the 

 scientific entomologist. I may here be permitted to 

 develope in a purely theoretical manner the principles in 

 accordance with which such an investigation should be 

 made : 



On the change in the number of generations of the annual 

 cycle. A change in the number of generations which a 

 species produces annually must be sought chiefly in 

 changes of climate, and therefore in a lengthening or 

 shortening of the period of warmth, or in an increase or 

 diminution of warmth within this period ; or, finally, in 

 both changes conjointly. The last case would be of the 

 most frequent occurrence, since a lengthening of the 

 period of warmth is, as a rule, correlated with an eleva- 

 tion of the mean temperature of this period, and vice 

 versa. Of other complications I can here perceive the 

 following : 



Climatic changes may be active or passive, i. e. y they 

 occur by a change of climate or by a migration and ex- 

 tension of the species over new districts having another 

 climate. 



By a lengthening of the summer, as I shall designate 

 the shorter portion of the whole annual period of warmth, 

 the last generation of the year would be advanced fur- 

 ther in its development than before ; if, for instance, it 

 formerly hibernated in the pupal state, it would now 

 pass the winter in the imago stage. Should a further 

 lengthening of the summer occur, the butterflies might 

 emerge soon enough to lay eggs in the autumn, and by 

 a still greater lengthening the eggs also might hatch, the 

 larvae grow up and hibernate as pupae. In this manner 

 we should have a new generation interpolated, owing to 



