INTRODUCTION. 15 



while dispersing are much more likely to remain in touch, mate, and 

 multiply. 



By methods based upon the first idea, and by the subsequent modi- 

 fication of the second idea, some of the most important natural ene- 

 mies of both species have been established in the United States to a 

 certainty. It has been found with several species that they could not 

 be recovered until after three years had elapsed from the time of the 

 original colonization; hence it follows with a reasonable certainty 

 that other species which have not been recovered will ultimately be 

 recovered as a result of colonization one, two, and three, and even 

 perhaps four years ago. It is deemed, however, at this time that 

 nearly as much has been accomplished as can be accomplished by the 

 earlier methods, and subsequent efforts will be devoted to a more 

 specific attempt to import the species still lacking, several of which 

 are known in their original homes to be of very great importance. As 

 will be pointed out elsewhere, attempts will also be made to import the 

 species which, while of lesser importance at home, may here fill in 

 gaps and may possibly multiply to an unprecedented extent in the 

 face of new conditions and a superabundance of host material. 



The work has been going on since 1905. Nothing has been pub- 

 lished concerning its progress except the short accounts in the annual 

 reports of the writer submitted each year to the Secretary of Agri- 

 culture, and except a bulletin on the general subject prepared by the 

 junior author and published by the State forester of Massachusetts. 

 It is hoped that the present account will be deemed a satisfactory 

 reply to all expressed desire for information as to progress. 



The joint authorship of the bulletin is deemed desirable by both 

 authors, but the writer takes it upon himself to sign this introduction 

 for the explicit purpose of stating in his own way the conditions under 

 which it has been prepared. The work from the beginning has been 

 under the direct supervision of the writer, and he is therefore to be 

 held responsible for any failures in the speedy accomplishment of 

 results, but the greatest credit in bringing about the results which 

 have been accomplished, he wishes frankly to state, belongs to Mr. 

 Fiske. Following the breakdown in health of Mr. E. S. G. Titus in 

 the spring of 1907, as is shown in the bulletin, Mr. Fiske was stationed 

 at the parasite laboratory and has since been given every freedom in 

 the conduct of its affairs. Nearly every suggestion which he has 

 made, while it has been fully discussed by the two of Us, has been 

 adopted. The ingenuity which he has displayed in matters of method 

 and the broad grasp which he has shown of the whole phenomena of 

 parasitism in insects, together with his competent and practical 

 grouping of his ideas, deserve every praise. Such portions of the 

 bulletin as were dictated by the writer have received the editorial 

 criticism of the junior author, and the portions prepared by the latter 



