68 ANIMAL ECOLOGY 



part of the coast in tropical regions, there is a definite niche 

 filled by animals which browse upon the corals, just as herbi- 

 vorous mammals browse upon vegetation on land. There are 

 enormous numbers of holothurians or sea-cucumbers which 

 feed entirely in this way. Darwin ^o gives a very good descrip- 

 tion of this niche. Speaking also of Cocos-Keeling Island, he 

 says : 



" The number of species of Holothuria, and of the in- 

 dividuals which swarm on every part of these coral-reefs, is 

 extraordinarily great ; and many ship-loads are annually 

 freighted, as is well known, for China with the trepang, which 

 is a species of this genus. The amount of coral yearly con- 

 sumed, and ground down into the finest mud, by these several 

 creatures, and probably by many other kinds, must be immense. 

 These facts are, however, of more importance in another point 

 of view, as showing us that there are living checks to the growth 

 of coral-reefs, and that the almost universal law of ' consume 

 and be consumed,' holds good even with the polypifers forming 

 those massive bulwarks, which are able to withstand the force 

 of the open ocean." 



This passage, besides showing that the coral-eating niche 

 has a geological significance, illustrates the wide grasp of 

 ecological principles possessed by Darwin, a fact which con- 

 tinually strikes the reader of his works. We have now said 

 enough to show what is meant by an ecological niche, and how 

 the study of these niches helps us to see the fundamental 

 similarity between many animal communities which may 

 appear very different superficially. The niche of an animal 

 may to some extent be defined by its numbers. This leads us 

 on to the last subject of this chapter, 



The Pyramid of Numbers 



21. *' One hill cannot shelter two tigers." In other and 

 less interesting words, many carnivorous animals, especially 

 at or near the end of a food-chain, have some system of terri- 

 tories, whereby it is arranged that each individual, or pair, or 

 family, has an area of country sufficiently large to supply its 

 food requirements. Hawks divide up the country in this way, 



