BY THE INTRODUCTION OF PARASITES. 371 
On a point of detail, it is difficult to agree with the authors in their inclusion 
of birds in catastrophic checks. There is good reason to think that birds are a 
“ facultative ” check, in that they eat abundant insects, and so tend to check only 
those species that become abundant ; their effect increases with the increase of 
the insect, and they are, in the best sense, facultative.* How far this applies in 
the case of the two species of moths under consideration and the American birds 
the writer does not know, but on general grounds he differs from the authors on 
this point. . 
But despite all criticism, the fact remains that this huge experiment is a work 
of the highest importance, and in many of the details other workers will find 
much information of the greatest value. 
H. MaAxwewti LErFroy. 
* [One of the most striking cases of birds operating as a ‘“‘ facultative’ control is to be found 
in the so-called “lesser locust-birds,” the wattled starling (Dilophus carunculatus), of South 
Africa. Mr. C. P. Lounsbury appears to be of opinion that these birds constitute the most 
effective natural check upon the plague locusts of that country.—Kp. | 
