MUTUAL RELATIONSHIP OF ANIMALS. 73 
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 
cn 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 d/o/lusca 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 (Avenicola) 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. 
