SOME MIXED RELATIONSHIPS. 



81 



this case a kind of mutual and alternating exchange of 

 parentage. The Biorhiza gives origin to the Teras, 

 and the latter, in its turn, develops the Biorhiza. To 

 use a very old simile, the offspring never resemble 

 their parents, but their grand- 

 parents. It is a simple 

 truism, then, to say that, 

 somehow or other, the gall- 

 fly relationship have become 

 of a character certainly 

 " mixed " in their type. 



One explanation, at least, 

 of this curious interchange of 

 personalities not unknown, 

 by the way, in other insects 

 and in other groups of ani- 

 mals is perhaps more easily 

 found than might at first 

 sight be supposed. The his- 

 tory of our gall-flies is one 

 in which apparently a youth- 

 ful form of a species has 

 acquired a wonderful power 

 of producing young. We 

 have to wait for the adult 

 stage of things, as a rule, 

 before the animal illustrates 

 the power of like begetting 

 like. Now and then, how- 

 ever, we do find that the 

 young form has acquired a 

 power of producing eggs, and 

 of giving origin to new beings, as if it had attained 

 its mature state. 



Fig. 22. Galls on oak-leaf: 

 : spangles" and "button galls." 



