MUTUAL RELATIONSHIP OF ANIMALS. n 



peak can be built up to the 25-fathom line by a rain of 

 foraminiferan shells, assisted in many cases by deep-sea 

 corals, and that when once a coral colony is established 

 on the summit, it can progress seawards on its own detritus 

 broken off and rolled down the slope. It may be taken for 

 granted that a coral colony growing in moderate depths will 

 reach the surface as a cup or small atoll, by the ordinary 

 laws regulating the growth of a sedentary organism. 



Many marine organisms thus play an important part in 

 nature's economy by the formation of chalks and lime- 

 stones. Others constitute powerful destructive agencies. 

 As examples we may cite the boring Mollusca which 

 tunnel through wood or rocks. 



When we .turn to terrestrial organisms, we find that 

 their efforts are quite as effective in modifying the surface 

 of the land, though usually they act indirectly through the 

 plant kingdom. 



Earthworms have been shown to have an important 

 function in burrowing through the earth and passing it 

 through them. They are nature's ploughs, and are cease- 

 lessly employed in bringing fresh soil to the surface, as can 

 be easily observed in an unrolled tennis-court. The lob- 

 worm {Arenicola) performs much the same function on the 

 seashore. 



Insects, birds and mammals act on the physical 

 world mainly through plants. Birds are great distri- 

 butors of plant seeds, and thus conduce to supplying 

 oceanic islands and other districts with plants, which them- 

 selves alter the physical constitution of the islands. Grazing 

 cattle may denude a well-wooded district of its trees by 

 feeding on the young shoots, and the loss of forests may 

 alter the rainfall and other physical conditions. It has 

 been suggested that the Pampas of Argentina have thus 

 lost their primeval forests. 



2. Organic Relations. — No organism can live with- 

 out having some action and reaction upon other organisms. 

 Animals, as we have seen, are either plant-eaters (herbivora) 

 or animal-eaters (carnivora). This connection, as regards 

 food, often leads to more permanent connection which is 

 known by different terms according to its intimacy. 



