lately discovered on the West Coast of Greenland. 523 
genus intermediate between Larus and Sterna. If every diversity 
_ of character is to become the foundation of a new genus, we shall 
have as many genera as there are species ; and when multiplied to 
excess, genera will cease to afford the facility of investigation to the 
examiner of species which they were designed to give. Between 
each individual species there will always be some difference of 
character beyond that of colour of plumage, which may be made 
to constitute what with some modern ornithologists is so impro- 
perly called a generic distinction. In the present instance the 
forkedness of the tail is very remarkable; but if the want of an 
even termination of the tail-feathers is to constitute a generic di- 
stinction, then the Larus tridactylus must be removed from its 
present place and become a new genus; for the tail of that bird, 
though slightly forked, is certainly uneven, and more distinctly so 
in the younger birds. 
Since I received the specimens which I have described above, 
I have obtained one from Hudson's Bay of another black-headed 
Gull bearing a strong resemblance to the Larus Sabini, except 
that it has an even tail, and is without the dark collar round its 
neck. It is certainly an undescribed species, and must be placed 
in the genus Larus, although its bill approaches more to that of 
a Tern than that of any other Gull with which I am acquainted. 
Such instances, I think, satisfactorily prove that, if every exist- 
ing species were before us in one view, we should find individuals 
so approaching each other in particular points, as to make it very 
difficult to determine where to place the boundaries of genera. 
VOL. XII. 3v XXXHE Re. 
