90 CONDITIONS AFFECTING THE 



fresh water ; the station of a marine fish is in the sea, and a 

 marine animal may have a station higher or deeper. So 

 again with land animals: the differences in their stations 

 are those of different soils and neighbourhoods ; some being 

 best adapted to a calcareous, and others to an arenaceous 

 soil. The third condition of existence is FOOD, by which I 

 mean food in the broadest sense, the supply of the materials 

 necessary to the existence of an organic being ; in the case 

 of a plant the inorganic matters, such as carbonic acid, 

 water, ammonia, and the earthy salts or salines ; in the 

 case of the animal the inorganic and organic matters, 

 which we have seen they require ; then these are all, at 

 least .the two first, what we may call the inorganic or 

 physical conditions of existence. Food takes a mid-place, 

 and then come the organic conditions ; by which I mean 

 the conditions which depend upon the state of the rest of 

 the organic creation, upon the number and kind of living 

 beings, with which an animal is surrounded. You may 

 class these under two heads : there are organic beings, 

 which operate as opponents, and there are organic beings 

 which operate as helpers to any given organic creature. 

 The opponents may be of two kinds : there are the indirect 

 opponents, which are what we may call rivals ; and there 

 are the direct opponents, those which strive to destroy the 

 creature ; and these we call enemies. By rivals I mean, 

 of course, in the case of plants, those which require for their 

 support the same kind of soil and station, and, among 

 animals, those which require the same kind of station, or 

 food, or climate ; those are the indirect opponents the 

 direct opponents are, of course, those which prey upon an 

 animal or vegetable. The helpers may also be regarded 

 as direct and indirect : in the Case of a carnivorous animal, 

 for example, a particular herbaceous plant may in multi- 

 plying be an indirect helper, by enabling the herbivora on 

 which the carnivore preys to get more food, and thus to 

 nourish the carnivore more abundantly ; the direct helper 

 may be best illustrated by reference to some parasitic 

 creature, such as the tape-worm. The tape-worm exists in 

 the human intestines, so that the fewer there are of men 

 the fewer there will be of tape-worms, other things being 

 alike. It is a humiliating reflection, perhaps, that we may 

 be classed as direct helpers to the tape-worm, but the fact 

 o : we can all sec that if there were no men there would 

 Jbe no tape-worms. 



