PREFACE ix 



Thus the peculiar features of a gall may predicate con- 

 clusively in many cases the insect which has caused it, 

 whilst in many others they may imply with equal clear- 

 ness that of the plant attacked, and the special tissue of 

 the plant which has been implicated. The aphis galls 

 on the Spruce Fir are especially instructive in this 

 respect. 



It is worthy of remark that it is possible that, after 

 all, the production of local warmth is the immediate 

 efficient in the causation of insect galls. Although it is 

 only conceivable as being very small in amount, there is 

 no doubt that a development of eggs, as of all other 

 local processes attended by growth, is productive of 

 local warmth. It may not be easily appreciable, and 

 may be impossible of measurement, but it must be 

 recognized as a constant condition. 



It gives me particular pleasure to recommend a work 

 on the subject of galls from the pen of my friend 

 Mr. Swanton, because I know that not only has he 

 been for many years a zealous collector of facts respect- 

 ing them, but that I know that he is well informed as 

 to the special branches of natural history which are 

 involved in their study. He is no specialist restricted 

 to one branch of natural history, but a specialist alike 

 in botany, entomology, and general biological science. 



JONATHAN HUTCHINSON 



Haslemere, 191 2 



