xxxviii Introduction. 



seem quite clear, unless they can be swallowed and 

 voided undigested by a temporary host. 



It is remarkable that characters, closely resembling 

 those acquired by fruits, should have been evolved from 

 a totally different cause. In the case of fruits these 

 characters have been of service in securing the dis- 

 tribution of the seeds ; in the case of galls, in securing 

 the safety of the larvae ; but in both cases it has been 

 their fitness that has brought them into existence. 



Darwin and all writers before him held that the force 

 calling out gall formation was due to a chemical 

 secretion injected by the gall-mother. Malpighi con- 

 sidered that it acted as a ferment on juices existing in 

 the plant ; and this was the view of Reaumur, but he 

 added to it the thermal effect of the ^%%y and the nature 

 and character of the wound, which varies according to the 

 shape of the ovipositor of each species. Dr. Derham 

 thought the formation was ^ partly due to the act of the 

 plant, and partly to some virulency in the juice or ^^^^ or 

 both, reposited on the vegetable by the parent animal ; 

 and just as this virulency is various according to the dif- 

 ference of its animal, so is the form and texture of the gall 

 excited thereby.' Darwin speaks of galls as produced ' by 

 a minute atom of the poison of a gall-insect,' and com- 

 pares them to the specific local processes of zymotic 

 diseases. Sir James Paget, writing in 1880, said that 'the 

 most reasonable, if not the only reasonable theory, is 

 that each insect infects or inoculates the leaf or other 

 structure of the chosen plant with a poison peculiar to 

 itself.' This may be taken as the view accepted by 

 scientists^ until in the following pages Dr. Adler showed 

 conclusively that there was no foundation for supposing 



^ See ' Galls,' Encyclop. Britann. ed. 9, where the same view is 

 expressed. 



