EDITOR'S PREFACE 



While pursuing the study of galls as a branch of 

 comparative pathology, I was fortunate enough to 

 become acquainted with Dr. Adler's monograph of 

 Alternating generations in oak gall-flies. 



The originality of the matter, and the important light 

 which it threw on some of the great biological problems 

 of the day, induced me to ask his permission to publish 

 it in an English dress. I have to thank him for the 

 generous manner in which he placed his work, and the 

 beautiful drawings which accompanied it, at my disposal. 

 Unfortunately the stones from which the illustrations 

 were printed had been broken up, but I venture to 

 think that they have been very faithfully reproduced. 



In the Introduction, and especially in the description 

 of Cynips Kollari in the Appendix, I have used freely 

 the writings of Dr. Beyerinck and Professor Mayr. 

 I have added an analytical table of galls, a short 

 bibliography, and a list of the Cynipidae. I have 

 enclosed my own notes to the text in brackets, and 

 in these I have given the synonyms of each species, 

 its popular name, the inquilines and parasites which 



