On the whole, this book is confined to species actually known to be extinct, but a few are included of which a small number is still known to exist, because firstly there seems no doubt that they will vanish soon, and secondly, as in the case of _Notornis_, it was necessary to clear up certain misconceptions and contradictory statements. In the case of a few species believed to be quite extinct, it is possible that some individuals may still exist in little known parts of their range, while on the other hand it is more than likely that several of the species referred to in my lecture (Proc. Orn. Congress pp. 191-207, 1907) as threatened with destruction, have already ceased to live. This may also be the case with some birds not alluded to at all. In several instances I have treated of extinct flightless species under genera including existing species capable of flight. This may appear to be inconsistent, seeing that I maintain _Notornis_ separate from _Porphyrio_, but, while not considering flightlessness in itself a generic character, the great development of the wing-coverts and the modification of the toes appear of sufficient generic value in this case. I know that several of the most eminent ornithologists of the day, among them Dr. Sharpe, differ from me, and are convinced that the loss of the power of flight is so profound a modification, that it is imperative that we should treat it as sufficient for generic distinction. While agreeing that many genera are founded on much less striking modifications, I cannot concur in this opinion, for, unless the loss of the power of flight is also accompanied by other changes, in some cases it is difficult to find at first sight even specific differences other than the aborted wings. The cause of recent extinction among birds is in most cases due directly or indirectly to man, but we also have instances of birds becoming extinct for no apparent reason whatever.