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 
