VERTEBRATE PALEONTOLOGY IN 1912 13 



How, then, were these giants capable of flight ? One sug- 

 gestion is that the attraction of gravity, owing to the diameter 

 of the earth having been greater, was less in past epochs than 

 at the present day. But an increase in the earth's radius of 

 some 60 miles, which is the maximum that could be allowed, 

 would cause but slight diminution in the pull of gravity. On 

 the other hand, an increase in atmospheric pressure would have 

 much more effect on the flying capacity of animals. Suppose, 

 for example, an animal flying by wing-beats (and it is certain 

 that pterodactyles did not glide from trees or cliffs in aeroplane- 

 fashion), in which the wing-expanse was double that of the 

 largest modern birds. From the formula given above it will 

 be evident that under existing conditions such an animal would 

 require, per unit of weight, a power equal to that of our largest 

 birds multiplied by the square root of four (in other words, 

 doubled), which would manifestly be impossible to realise. But 

 if the atmospheric pressure were four times as great as at 

 present, flight would be possible with the power diminished 

 by one-half. And, as a matter of fact, the necessary power, 

 per unit of weight, being doubled in one way and halved in 

 another, would remain the same and be no greater than in the case 

 of existing birds. Accordingly, an augmentation in atmospheric 

 pressure in the proportion of one to four would compensate a 

 similar increase in the size of the animal. So that we have the 

 general rule that all increase in the size of the animal would 

 be compensated by a proportional augmentation of pressure. 

 Thus in the case of the largest known pterodactyles, of which 

 the wing-expanse was about double that of the biggest living 

 birds, the impossibility of flight on account of their size would 

 be annulled by a double atmospheric pressure. If the tempera- 

 ture were higher than at the present day there would be a 

 further slight increase in the pressure. The fact, then, that 

 giant reptiles which could not fly under present conditions did 

 do during the Cretaceous, coupled with the similar case pre- 

 sented by the giant dragon-flies of the Coal period, leads the 

 authors to regard (so far as conclusions of this kind have any 

 value and always bearing in mind the possibility that nature 

 may have utilised means of which we have no cognisance) an 

 increased atmospheric pressure during geological time as the 

 most plausible and probable explanation of the problem. 



During the past few years the dinosaurian quarries of Tenda- 



