210 



THE CHAIN OF LIFE. 



further been supposed that geological changes in the southern 

 and northern hemispheres may have alternated with each other, 

 so that there may be in the former Cretaceous beds in which 

 the remains of ancestors of the Eocene mammals may be 



V »^ " 



Fig. i68.— Jaw, and enlarged molar of P/tascolot/wn'upn Bucklandi. Stonesfield 



slate. England.— After Phillips. 



found. But we do not as yet know of such deposits. We 

 may be content, therefore, to suppose that at the close of the 

 Cretaceous there was established somewhere a sort of Eden 

 for the first placental mammals, in which they were introduced 

 and could live unharmed by the decaying monsters of the 

 reptilian age, until the time came when they could increase and 



Fig. 169. — Plagiaulax Becklesii. Taw, and pre-molar enlarged, showing flat 



surface, with ridges. — Purbeck. 



multiply and replenish the earth. The nearest approach to 

 such a centre of mammalian life is perhaps to be found in those 

 great American lake basins embedded in the mountains of 

 the West, which have been so well described by Hayden and 



