kiwi-kiwi (Fig. 4) of New Zealand, a near relative of the ostriches and running-birds in general, represents a still more degraded condition of 
Fig. 4. the organs of flight, for the wing is reduced in size to an extraordinary degree, and exists in a highly abortive condition; while only one complete finger is represented in the hand—other birds, as a rule, possessing three modified fingers. The logger-headed duck of South America has wings so reduced that it can but "flap along the surface of the water," a condition of matters closely imitated among ourselves by the Aylesbury duck—although, indeed, the young ducks are able to fly. The wing of the penguin (Fig. 5) is a mere scaly appendage utterly useless for flight, but useful as a veritable fin, enabling it to swim under water with great facility; and of the auk's wing the same remark holds good. In the
Fig. 5.
birds, then, there is ample evidence of deterioration of organs in the rudimentary nature of the wings of many species. How these conditions have been brought about is not difficult to explain in most instances. In New Zealand, where we find a singular absence of quadrupeds, wingless birds—many being extinct—of which the apteryx is a good example, take the place of the four-footed population. In view of an immunity from the attack of other animals, the ground-feeding habits of these birds would become more and more strongly settled as their special way of life; and, in the pursuit of such habits, the wings,
