150 Alternating Generations in Oak Gail-Flies. 



Lichtenstein, who so ably investigated the Phylloxera, 

 in assuming that the agamous generation of gall-flies 

 holds a subordinate position to the sexual generation, 

 in the way that the budding generations of Phylloxera 

 and Aphides do to their winged and sexual generations. 



The question of the mutual relationship of the two 

 generations to each other, is of fundamental importance 

 in the inquiry into the origin of alternation of genera- 

 tions. It is necessary therefore next to consider par- 

 thenogenesis, in so far as it has a bearing on alternation 

 of generations. 



When I first discovered the alternation of generations 

 in gall-flies, I believed in the existence of a definite law, 

 in accordance with which the gall-flies of one generation 

 invariably propagated themselves parthenogenetically, 

 and those of the next generation always sexually. But 

 more extended observation convinced me that there was 

 no such universal law. I soon found that some species 

 continue to propagate themselves by an annual parthe- 

 nogenesis. This new light led me to investigate more 

 fully parthenogenesis as it exists in other families of 

 Hymenoptera, and I will state briefly the result in so 

 far as of interest to the preceding question. 



Parthenogenesis has frequently been observed among 

 saw-flies. Professor v. Siebold showed, by accurate 

 observations made on Nematus ventricosus, that parthe- 

 nogenesis very frequently occurs, although an equal 

 number of males and females exists in this species. 

 I have myself closely investigated another species, 

 Nematus Vallisnierii. 



In the autumn of 1876, I collected a large quantity of 



