Page:The Zoologist, 4th series, vol 6 (1902).djvu/85

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we are told that the number of birds on the British list amounts to 415 species, most of us are at first inclined to treat this statement cum grano salis, as though it were a fabrication. On further consideration, however, we shall see that after all this is quite possible—that is to say, if we assume that the 415 birds are not all with us at the same time.

Allowing, then, that there are two well-defined seasons in the year—summer and winter—and also that there are two primary classes of birds on the list—resident and migratory—we see our difficulties somewhat disappear. Observing further that the latter of these two classes may be subdivided into two further classes—summer and winter migrants—we then see that it is comparatively easy to account for the apparently large number of so-called British Birds. This last phrase is used advisedly, for it must be patent to all that some of our British Birds have very little title to the name.

And thus we find there is a fourth class, beside the resident, the summer, and winter migrants, namely, the accidental or occasional straggler to our islands. Owing to many causes—such as migration, gales, and probably, to a greater extent than is generally imagined, to the escape of rare captive birds—our list of accidental visitors is a long one.

To this last class might be added a fifth—the extinct class; but as only one species that has lived within the memory of man in England has so disappeared, we may, I think, omit this class; for it only includes the now world-famous Great Auk, which became extinct about two hundred years ago.

Having reduced our list of birds to four—or, if preferred, five—great classes as regards occurrence, and using the same terms for their degree of frequency, we may, without going into too close details, examine the matter more carefully with respect to families and genera.

In our examination we will, for sake of uniformity, invariably follow the order of classification and nomenclature of Seebohm.