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. 1912 
