THE DAWN OF LIFE 



189 



thickness, and extend over vast areas, Eozoon may 

 be supposed to have been as efficient a world-builder 

 as the Stromatoporae of the Silurian and Devonian, 

 the Globigerinae and their allies in the chalk, or the 

 Nummulites and Miliolites in the Eocene. It is a 

 remarkable illustration of the constancy of natural 

 causes and of the persistence of animal types, that 

 these humble Protozoans, which began to secrete 

 calcareous matter in the Laurentian period, have 

 been continuing their work in the ocean through 

 all the geological ages, and are still busy in ac- 

 cumulating those chalky muds with which recent 

 dredging operations in the deep sea have made us 

 so familiar. 



