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 improperly 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 distinction, 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 existing 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.
