X PREFACE. 



of such animals as are to be met with in their own country, 

 it must surely include all those which have been known to 

 occur in it hitherto. Neither is the reason obvious why the 

 characters, or even a detailed description, of such species, 

 should be suppressed. If it be quite certain that they have 

 appeared in one instance, it cannot be deemed improbable, 

 clearly not impossible, that they may occur in other in- 

 stances*, and should this prove to be the case, it is very 

 desirable that the student should have the means of iden- 

 tifying them. 



A line of quite as much importance, in the opinion 

 of the author, as that on which Dr. Fleming has insisted, 

 and one more easily drawn, though never yet attempted 

 to be drawn with accuracy -f, is that between species, the 

 occurrence of which in this country at one time or another 

 there is no ground for questioning, and others whose claims 

 to be considered as British have not yet been made substan- 

 tially good. In distinguishing between these two classes, 

 the author has generally been guided, at least in the case 

 of Birds, by the fact of the existence or not of British- 

 killed specimens in any known collection, or by the com- 

 parative recency of the occurrence of any species, and the 

 circumstances under which that occurrence has been an- 

 nounced. Species which are not to be found in any of 

 our Museums, for which no authority is known, or whose 

 claims rest on statements made many years back, at a time 

 when specific differences were but little attended to, he has 

 no hesitation in saying, ought to have a separate place 



Several cases might be mentioned of species, which only a few years back 

 had occurred but in a single instance, and which have since been met with 

 more than once. 



f This is said with reference only to the Vertebrate Division of the British 

 Fauna. Mr. Stephens lias taken great pains to draw the line spoken of in the 

 rase of our native Insects. 



