294 Proceedings of the Royal Physical Society. 



the possibility of bridging over the gap by any biological 

 processes whatever. This is the problem we have now to 

 consider. 



During the aeons that have rolled past since life began on 

 the globe, we have seen how myriads of generations of 

 animals came into existence, flourished for a time, propa- 

 gated their kind, and then vanished for ever. Glimpses of 

 these evanescent life-panoramas show streams of varied 

 beings, arranged in groups, and each group possessing 

 characters admirably adapted for making the best use of 

 certain conditions in their environments. They were pro- 

 vided with special means for attack and defence, and 

 for procuring the necessities of life. Some birds and 

 insects had their bodies transformed into flying machines, 

 not, however, always on the same plan. For example, the 

 Pterodactyle had its fore-limbs modified so as to be used, 

 partly as prehensile organs, and partly as wings. It had 

 three of its fingers on each limb free and furnished 

 with claws, while the fourth was enormously elongated, in 

 order to support the outer edge of a sail-like membrane 

 which, on the inner side, was attached to the arm, body, 

 hind limb, and tail. On the other hand, the Archoeopteryx, 

 though differing very much from the birds of the present 

 day, had large feathers on its wings and tail, and its feet 

 were similar to those of modern perching birds. Again, 

 some air-breathers reverted to an aquatic life, as the 

 whales and seals ; and their limbs, which had been previ- 

 ously adapted for locomotion on land, were converted into 

 paddle-like flappers for propulsion in water. But, with 

 respect to the air-breathing organ, a remarkable fact is to be 

 noted. Although these animals were actually descendants 

 from aquatic ancestors possessed of gills, it was entirely 

 beyond the power of nature to retrace her own evolutionary 

 steps and restore them their lost gills. And hence, these 

 animals have had ever since to come to the surface at stated 

 intervals to breathe. Animals when they get on definite 

 lines have not the power of reconsidering their position. 

 They are impelled, as it were by a vis a tergo, to pursue their 

 course on the selected path, subject only to modifications 



