CHAPTER VI. 



Alternating Generations in Oak Gall-Flies. The Relation- 

 ship OF THE PaRTHENOGENETIC TO THE SeXUAL GENERATION. 

 How IS THIS GeNERATION-CyCLE TO BE EXPLAINED ? 



It only remains, in conclusion, to review generally the 

 subject of alternation of generations in oak gall-flies, 

 but I would premise that I have chosen the term 

 ' alternating generations ' ^ without intending in any way 

 to prejudge the question. It only indicates the posses- 

 sion of cyclical propagation ; while the different terms 

 used with regard to this form of propagation, such as 

 alternating generation, heterogenesis, metagenesis, are, 

 although all closely related, each used in a different 

 sense. Lubbock requires, as a necessary condition of 

 alternating generation, that one generation should pro- 

 pagate itself by budding, like Aphides ; but with gall-flies 

 propagation by budding does not take place. Although 

 parthenogenesis and budding may not differ in principle, 

 yet there is this important distinction between them, that 

 in the former, embryonic development runs its entire 

 course outside, and in the latter, inside the ovarium* 

 Among gall-flies both generations develop in exactly 

 the same manner. For this reason I cannot agree with 



^ Generationswechsel. 



