THE DESTRUCTIVE AGENCIES OF NATURE. 163 



The consequences of the great destruction of 

 species at the glacial period, and the profound 

 modification of the surviving species would 

 be permanent. Many of the checks, direct 

 and indirect, which formerly regulated the 

 numbers and modification of the survivors 

 would be removed, and these would assume a 

 more prominent place in Nature than was 

 possible before, and would increase largely in 

 numbers, and would then come into competition 

 with each other, and thus be subjected, already 

 greatly modified and improved, to the powerful 

 action of Natural Selection, which would in- 

 tensify and permanently fix the changes which 

 they had already undergone. No more active 

 agency for the rapid development of higher 

 organic life than the Glacial Period could well 

 be imagined. 



A part analogous to that of the Glacial Period 

 among animals and plants is played by eras of 

 pestilence among man. Pestilence was one of 

 the agencies which was employed to remove the 

 rotting carcase of the Eoman Empire, to allow 

 of the development of the nations of Modern 

 Europe. The most terrible of all pestilences in 

 modern time was the Black Death, which swept 



