BY THE INTRODUCTION OF PARASITES. 371 



On a poiut 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 sj^ecies 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 diffei's 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. Maxwell Lefroy. 



■^ [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 (D/lopJmn caruncidatiat), of South 

 Africa. Mr. 0. P. Lounsljuiy appears to be of opinion that these birds constitute the most 

 effective natural check upon the plague locusts of that country. — Ed.] 



