It is true that a few island forms may not come very exactly under this definition, but in these cases the differences are such as are obviously parallel to those obtaining in non-isolated areas on the mainland. Where evolution and isolation have evolved forms which are definitely divided from all others by some characteristic which is not one merely of degree, I have treated them as distinct species.
In India we are constantly meeting with the most intricate cases of subspecific variation, and a study of birds which admits the recognition of these geographical races and the wisdom of naming them affords infinitely greater interest both to the field and to the scientific worker than does the easier method of lumping them all together. For instance, to take two of our most common birds, the Indian House-Crow and the Red-vented Bulbul. Two species of the former and many of the latter have been recognized and given specific names, although the differences between them are in no way specific and are not any greater than the differences which exist in many other forms which have been left undivided.
The second point to which reference must be made is the unfortunate necessity which has arisen for very numerous corrections in Oates' nomenclature. Such corrections cannot but be a source of some difficulty to the older race of field naturalists, and students who have learnt these names will now have to learn those which replace them. The younger generation will, however, have the satisfaction of knowing that they are learning names which, with few exceptions, will be permanent; for, with strict adherence to the laws of priority, a time will soon come when we shall really have arrived at the bed-rock of nomenclatorial research. It should be mentioned here that I have had the unstinted help of Mr. Tom Iredale in this particular branch of the work, and his unrivalled knowledge of bibliography and nomenclature has been of inestimable help to me.
Another difference between this and the preceding edition will also be noted. With the approval of the editor, Sir Arthur E. Shipley, the synonymy has been reduced to references to the original description and to the Blanford and Gates' edition of this work, in the former case the type-locality being given in brackets after the reference. The saving of space thus obtained and the use of briefer descriptions has given additional room for
