326 Mr. Hi. Seeley on the Avian Affinities of Pterodactyles. 
to correspond in length in an animal where one was used for 
flying and the others for clinging. But they do correspond ; 
and therefore I conclude that they were not used in dissimilar, 
but in the same function. And hence, as the only way in which 
they could have been equally used was in walking, it follows 
that the Pterodactyle was quadruped. This, as we shall see, is 
a consideration of theoretical value, as bearing on their position 
in the animal kingdom, and will help to show their affinity to 
birds in a. direction which removes them far from reptiles. 
It is well known that many bones of most birds are filled with 
air, and that, as a principle, the more the motion of the animal, 
the greater is the number of bones filled with air. This air is 
received from the air sacs, which receive it from the lungs and 
return it through the lungs again. Thus there is in birds a 
sort of supplemental lung-system, which circulates air through 
the body. Nothing of this sort is observed in reptiles, even the 
lungs with them being generally in.a very rudimentary con- 
dition, while in birds the respiratory system is more perfect and 
complex than in any of the other Vertebrata, and, as a result, the 
temperature of the blood is hotter. 
Now in Pterodactyles the bone-walls are all very thin, the 
bones being hollow and showing pneumatic apertures, which are 
large, precisely as in birds of great flight. The fact that the 
bones are supplied with air necessitates an elaborate system of 
air-sacs to furnish the supply ; and the existence of these air- 
sacs speaks incontestably to bronchial tubes opening on the sur- 
face of the lungs to supply them, and to the existence of lungs 
essentially like those of birds. But the circulation of this air 
through the body was seen in birds to have relation to rapid 
motion through the air, which necessarily would produce more 
rapid respiration. But rapid respiration only means more rapid 
oxidation of the blood, and conversion of the purple cruorine into 
scarlet cruorine—that is, the conversion of venous blood into 
arterial blood. And if venous blood is rapidly converted into 
arterial blood, there must be rapid circulation. Now rapid cir- 
culation cannot take place without a heart with two auricles and 
two ventricles ; therefore I conclude that Pterodactyles had the 
heart like that of birds and mammals: and hence it follows that 
they must have had hot blood. But it has been seen that the 
Pterodactyles were quadrupedal; and hence it may fairly be 
concluded that they could not have moved so much through the 
air as birds, and therefore the stimulus to active respiration 
could not have been so great ; and yet they possess to perfection 
the elaborate respiratory system of the most active birds; and 
so it follows that their circulatory and respiratory organs were 
not less developed than in birds, but rather more developed. 
