268 PROGRESS OF SCIENCE IN THE CENTURY. 



Almost throughout the century there has been 

 continuous inquiry into the nature and origin of 

 coal; much has been done in the recognition of the 

 flower less plants (especially club-mosses) which 

 gave rise to it; experimental work has shown the 

 probability of its formation under water, under 

 great pressure, and in warm conditions ; but there is 

 still no unanimity in answering the question whether 

 coal was formed in the site where the plants that 

 formed it grew, or whether the material was flooded 

 off from the old forests and deposited elsewhere. 



Animals. — The influence of animal life upon the 

 earth is also manifold. On the one hand, we see 

 destructive agencies, — the boring sponge Cliona 

 tunnelling through and through the oyster shell and 

 tending to reduce it to sand, the Pholads and many 

 other borers helping to break dov>^n the most solid 

 sea-shore rocks, the crayfish and their enemies the 

 watervoles uniting to make the river-banks collapse, 

 the beavers cutting down trees, building dams, dig- 

 ging canals, and changing the aspect of even large 

 tracts of country, and so on through a long list. 



On the other hand, we see conservative agen- 

 cies, — the formation of great shell-beds, the accumu- 

 lation of calcareous and siliceous ooze in the great 

 abysses of the oceans, and most strikingly the rise 

 of coral-reefs, such as the great barrier reef of Aus- 

 tralia which is over 1000 miles in length. 



That there are great limestone beds which have 

 been formed by the remains of marine animals is 

 an obvious fact. They are often so thoroughly 

 penetrated by recognisable shells of nummulites, 

 coral, sea-lilies and molluscs, that he who runs 

 may read their origin. In other cases, however, there 

 are no big remains which the eye recognises at 



