. The natural history of birds . mammalia, because, with the exceptionof those mammalia which have flying membranes,the motion of fore-legs is crosswise to that of the bat tribe there is a slight approximation to 58 WINGLESS BIRDS. this species of insertion of the humerus in the largeclavicles and blade-bones. In those mammalia whichuse the fore-legs for progressive motion only, thereare no clavicles, and bears and other hugging animalshave them cartilaginous at both extremities. Sothat the absence of clavicles indicates motion inthe direction of the mesial plane only ; and theirprese
. The natural history of birds . mammalia, because, with the exceptionof those mammalia which have flying membranes,the motion of fore-legs is crosswise to that of the bat tribe there is a slight approximation to 58 WINGLESS BIRDS. this species of insertion of the humerus in the largeclavicles and blade-bones. In those mammalia whichuse the fore-legs for progressive motion only, thereare no clavicles, and bears and other hugging animalshave them cartilaginous at both extremities. Sothat the absence of clavicles indicates motion inthe direction of the mesial plane only ; and theirpresence, motion across that plane. Some of the birds which can fly, often use the half-expanded wings to assist in balancing them whenthey run ; and this habit is most frequent with thosethat have the legs long. Some of the birds whichcannot fly, have rudimental wings, which they appearto use for the same purposes. The only bird whichhas the wings so perfectly rudimental and concealedwithin the integuments as not to be of any use in. , is the Apteryx: and it has the legs veryshort and stout, and, though its habits are not known,it is probably not much of a runner. Those short-winged birds which dive under water,whether they tread the water itself by means ofwebbed feet, as is the case with the di%ing ducks,and divers properly so called, or tread the bottoms THE FEET. 59 of the shallows with feet that have the toes free, asis the case with the dipper, use the wings in thewater. But wings are not very efficient instrumentsof progressive motion either on the ground or in thewater, though they are of advantage in balancing onland, and in ascending or descending in the wings, whether perfect or imperfect, are formedand articulated on the same general plan ; and thoughthey admit of a little inclination to the front or therear, their principal motions are always across theaxis of the body. From what was formerly said of wings acting moreefficiently in an atmosp
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