X Introduction. 



had demonstrated the existence of cycHcal propagation, 

 many curious explanations were offered, in order to 

 account for the lengthened interval that elapses between 

 the death of one generation and the appearance of the 

 next. The currant-gall, for example, appears on the 

 male catkins of the oak in May, the fly quits the gall 

 in June, lives for a few days only, and nothing more is 

 seen until the male catkin appears again next May. 

 What had become of the eggs in this long interval ? 

 Spontaneous generation of the insect, within a gall that 

 had no external opening, had its advocates. Later it 

 was believed that a form of metempsychosis took place, 

 and galls were among the stepping-stones in the path of 

 organic evolution, by which the vegetable passed into 

 the animal soul. By some it was supposed that the 

 eggs, found in the fly in June, reached the ground, 

 whence they were drawn up, mingled with the sap, and 

 arrested next spring in the leaves or flowering 

 catkins, there to form the currant-galls again. Dr. Adler, 

 by proving the existence of cyclical propagation, has 

 shown that the interval between the. appearance of 

 the currant gall-fly in one spring, and its reappearance 

 in the next, is occupied by another agamous generation. 

 But while he showed that this rule holds good for the 

 majority of species, he has also demonstrated that, in 

 some at least, no sexual generation now exists. 



Pliny ^ knew that a fly was often produced in a gall, 

 but he did not associate it with the cause of gall-growth ; 

 on the contrary, he thought galls grew in a night, like 

 fungi. Many early observers, however, considered them 

 as insect productions and were aware that a variety of 

 insects emerged from them ; but the attention which 

 some of these authors bestowed upon this subject was 



^ Pliny, Nat. Hist. xvi. 9, lo ; xxiv. 5. 



