STRUCTURE AND HABITS OF ARCHiEOPTERYX. 281 



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 

 backwardly-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. — Arclmopteryx, 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 prolonged 

 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. 269), and consequent relatively small size of the 

 great pectoral muscle, indicate a deficiency of propulsive 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 tight 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 



