xviii Introditction. 



the perfect state. Certain Phylloxeridae reproduce 

 themselves by subterranean budding, without ever arriv- 

 ing at the winged and sexual forms : he regarded this 

 as analogous to the multiplication of plants by suckers, 

 without the intervention of seed. He compared the 

 life-history of gall-flies with that of Phylloxera, and gave 

 it as his opinion that, among the gall-flies, the individuals 

 of Neuroterus lenticularis had been mistaken for females, 

 because they possessed an ovipositor and eggs, that, 

 properly speaking, they had no sex, but were only larval 

 forms of Spathegaster baccarum, their so-called eggs 

 being gemmations. 



It is difficult to regard species which have a perfect 

 female form, and ovaries filled with perfect eggs, as 

 being perpetually in the larval state, because they are 

 wholly agamous and have no males. In Rhodites rosae, 

 as has been said, the few males found are functionless, 

 and are disappearing; it seems therefore more in 

 accordance with facts to speak of the individuals of 

 agamous species as females, the males of which have 

 ceased to appear. Although the Aphides are scarcely 

 comparable to the Cynipidae, since they possess a form 

 which is asexual, they may nevertheless be arranged, 

 like the Cynipidae, in alternate generations, without 

 having recourse to the larval theory. 



The importance of sexual reproduction has been 

 greatly exaggerated, and there seems no good ground 

 for assuming that a generation ought to extend from 

 one fecundated egg to another. In Cynips Kollari, and 

 many others, no sexual generation is known ; the flies 

 all possess perfect female forms, but they are agamous 

 and have the power of reproducing themselves by 

 parthenogenetic eggs. At the same time it is difficult 

 to believe that the agamous can be the primitive 



