INTRODUCTION, 
Tue task of compiling a list of the known species of Birds becomes 
very difficult when the Passeriformes are approached, and I have 
found an extraordinary increase in the number of the Muscicapide 
and Pycnonotide, since I wrote the fourth and sixth volumes of the 
‘Catalogue of Birds’ in 1879 and 1881. 
Apart from the work involved in picking up the threads since the 
‘Catalogue’ was finished, the question of Classification presents grave 
difficulties in the case of the Passeriformes. A good deal of sterling 
work has been done during recent years, but we are still far from 
a satisfactory system as regards the Passerine birds. Although the 
late Mr. Seebohm afterwards admitted that he ought to have included 
Pratincola, Cossypha, etc. in the fifth volume of the ‘ Catalogue,’ that 
does not account for all the difficulties of the question; but I have, of 
course, not enumerated such genera as these among the Flycatchers. 
There remain, however, many doubtful forms in the Muscicapide, and I 
have followed Mr. Eugene Oates in some of the modifications which he 
has introduced in his excellent volumes of ‘Aves’ in the ‘ Fauna of 
British India.’ Genera like Cryptolopha, Parisoma, Polioptila, etc. may 
just as well be Sylviide as Sertcornis, Acanthiza, and Hypotais; and it 
seems extremely difficult to draw a line between the Flycatchers and the 
Warblers. In face of the uncertainty which surrounds the classifica- 
tion of the Old-World Passeres, I have adhered, in the present volume, 
mainly to the order of Vol. IV. of the ‘ Catalogue.’ 
Proofs of this volume have been sent to many of my colleagues 
abroad, and the assistance they have rendered me requires my sincere 
recognition, I have tried everywhere to acknowledge the original 
