THE DAWN OF LIFE 129 



now brought down to the Palaeozoic, we may hope to trace it to 

 the Primordial, and thus to bring it still nearer to Eozoon in time. 

 Though Eozoon was probably not the only animal of the 

 Laurentian seas, yet it was in all likelihood the most con- 

 spicuous and important as a collector of calcareous matter, 

 filling the same place afterwards occupied by the reef-building 

 corals. Though probably less efficient than these as a con- 

 structor of solid limestones, from its less permanent and con- 

 tinuous growth, it formed wide floors and patches on the 

 sea bottom, and when these were broken up, vast quantities of 

 limestone were formed from their debris. It must also be borne 

 in mind that Eozoon was not everywhere infiltrated with ser- 

 pentine or other silicious minerals ; quantities of its substance 

 were merely filled with carbonate of lime, resembling the 

 chamber wall so closely that it is nearly impossible to make out 

 the difference, and thus is likely to pass altogether unobserved 

 by collectors, and to baffle even the microscopist. Although, 

 therefore, the layers which contain well characterised Eozoon 

 are few and far between, there is reason to believe that in the 

 composition of the limestones of the Laurentian it bore no 

 small part, and as these limestones are some of them several 

 hundred feet in 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 Globi- 

 gerinse 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 accumulating those chalky muds with 

 which recent dredging operations in the deep sea have made 

 us so familiar. (See Note appended.) 



All this seems sufficiently reasonable, more especially since 



