ANIMAL INTER-RELATIONS 31 



the large margin necessary to support this super- 

 structure of predatory animals. At each stage the 

 animals are larger, they breed more slowly, and there 

 is less of the original plant-produced living matter 

 to be used in providing the margin. This pheno- 

 menon has been called the pyramid of numbers (Elton, 

 1927), and is an essential feature of the structure of 

 animal communities. It does not of course apply to 

 food-chains of parasites, which get smaller and smaller 

 with individuals more numerous at each stage (para- 

 site, hyperparasite, etc.). 



Just as man has been compelled to adopt methods 

 of artificial Hmitation of numbers in his population, 

 now that w^ars and famines and infant and adult 

 mortality from disease organisms no longer provide 

 a natural check, so animals which have no natural 

 enemies, or which are comparatively immune from 

 them, tend to adopt systems of limitation of numbers. 

 This forms an interesting subject which cannot be 

 more than mentioned in passing. Reference may be 

 made to the territory systems found among many 

 birds during the breeding season, e.g., in kingfishers, 

 hawks, warblers, etc. (Howard, 1920), among carni- 

 vorous animals such as tigers, badgers, and in certain 

 social insects (e.g. wood-ants, Elton, 1932). Territory 

 should be considered as a phenomenon which often 

 leads to limitation of population, but has not neces- 

 sarily been evolved solely as an adaptation for that 

 purpose. 



Much ecological work is being done now upon the 

 exact food-habits of animals, but a great deal more 

 needs to be done before complete preliminary maps 

 of the food-relations of animal communities can be 

 drawn up. It will therefore be realized that few 

 attempts have been as yet made to make such maps, 

 and that these are at best rough and very incomplete. 

 They do, however, enable the conclusions to be drawn 

 which have just been outlined — the ideas of the 

 complex food relations of animals among themselves 



