260 PROCEEDINGS OF THE ACADEMY OF [1895. 



watchfulness, if any existed, of the small- brained laud giants. The 

 reptilian stream of life, in short, may have been thus assailed with in- 

 creasing pertinacity and intelligence at its source, the eggs devoured, the 

 young perhaps destroyed, and the numbers of these lords of land and 

 sea rapidly reduced. As Achilles had his only vulnerable spot in 

 the heel, these giant reptiles had theirs in the nest. The egg-destroy- 

 ing mammals had a double advantage. Laying no eggs themselves, 

 and caring for their young, they could only be destroyed when in the 

 mature stage, while their assault upon their foes was by the safer and 

 more effective process of devouring them in the egg — a method which 

 may well have caused rapid reduction in numbers and final extinc- 

 tion. The reptilian forms which continued to exist were likely to be 

 the smaller and more prolific ones, and perhaps those which had 

 developed somewhat efficacious methods of caring for their eggs — 

 methods which may have continued to improve as the mental 

 acuteness of their foes increased. 



The ground once cleared by the disappearance of the larger 

 reptiles, the subsequent rapid development of the mammalia is 

 readily comprehensible. They now became the dominant class, and 

 in all ages of geological history each new dominant class has ex- 

 panded rapidly in numbers, in variety of species, and in life of 

 individuals. They were no longer forced to glean after the harvest 

 of powerful competitors, but had the earth's stores of food for their 

 own, and developed accordingly, the remaining reptiles becoming in 

 their turn the gleaners after the harvest. Not only the land, but 

 the ocean, had lost its masters and become an open field for compe- 

 tition. Its reptilian dynasties, impregnable by direct assault, and 

 having no powerful enemies in their liquid domain, had yielded to 

 indirect attack, and mammalian life quickly overflowed into this 

 great reservoir of food in the form of seals, cetacea, and other air 

 breathing swimmers. The axe had been laid to the root of monster 

 reptilian life, and a new race of lords of the earth succeeded. 



To come now to the second instance of extinction alluded to, that 

 of the Post- pliocene, or Recent period, it is one that is, in some of its 

 features, very difficult of explanation. The only general cause that 

 has been adduced for it, that of the intense chill and deep snow-fall 

 ■of the Glacial Age, in all probability had much to do with it, though 

 certainly not all. As regards the mammoth and the woolly rhinoceros 

 of Europe and Asia, there seems much reason to believe that they 



