1 84 NATURAL SCIENCE. March, 



know of no rational argument in favour of the view that the animal 

 did live largely upon the ground. For quadrupedal locomotion in 

 trees the animal is admirably adapted. Long, flexible digits, provided 

 with claws on all four limbs, fit it at least as perfectly to arboreal 

 quadrupedalism as Galeopithecus or Petaurus or any other " flying " 

 mammal ; and I think, from the greater length and flexibility of those 

 digits, fit it even more perfectly for such a habit than even these 

 mammals, and incomparably more so than the bats with their back- 

 wardly-directed hind limbs, which, while they serve to enable the 

 animal to hold on to the tree or other body, are so modified for the 

 support of the wing as to be of little use for quadrupedal locomotion. 



As to flight. — Avchceoptevyx, though less well fitted for prolonged 

 flight than most modern birds, was certainly capable of flight. As 

 some have maintained that it was well fitted for powerful and pro- 

 longed flight, I will mention the features pointing to an opposite 

 conclusion. The absence of a pectoral crest on the humerus, the 

 small size of the sternum {see p. 115), and consequent relatively 

 small size of the great pectoral muscle, indicate a deficiency of pro- 

 pulsive and sustaining power in flight. The absence of pneumatic 

 foramina and consequent absence of air-cavities in the wing-bones 

 show that rapid to and fro {i.e., up and down) movement of the wing 

 would involve a larger amount of exertion than in a wing with hollow 

 bones as in most modern birds. It is not so much the extra weight 

 that would be impedimental as the extra inertia. The narrowness of 

 the body and smallness of the sternum indicate that the air-cavities 

 of the abdomen, even if present, were smaller and less effective for 

 respiration than in modern birds. I may be permitted here to make 

 a remark as to the use of abdominal air-sacs in birds, or I may be 

 suspected of having fallen into an old error of supposing that they 

 materially diminish the weight of the bird. Flight involves, perhaps, 

 a greater, i.e., more rapid, consumption of energy than any other 

 form of locomotion, and all powerful fliers, whether birds or insects, 

 are provided with respiratory organs of enormous effectiveness. In 

 birds, instead of air being only pumped into the bronchial tubes 

 and the rest being left to diffusion, the air is drawn right through the 

 lungs into the abdominal and other air-sacs, so that the highly 

 vascular lung with its venous blood supply is brought into more 

 direct relation with the " tidal air " than in mammals, while the 

 " residual air," which is not changed at each double respiratory 

 movement, rests, not, as in mammals, in the lung itself, or at least not 

 chiefly so, but in the air-sacs outside the lungs. The respiratory 

 organ proper is thus brought into direct relation with air capable of 

 far more rapid renewal than in the mammalian lung. In other 

 words, birds breathe the tidal air, while mammals breathe the 

 residual air. Avchceoptevyx, however, shows no sign of the possession 

 of large air-sacs, or of that large expanded sternum which, in modern 

 birds of flight, insures the rapid change of the air by the same 



