123 CONDITIONS AFFECTING THE 



■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 sur- 

 rounded. 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 oppo- 

 nents, 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 sup- 

 port 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 carnivo- 

 rous animal, for example, a particular herbaceous plant 

 may in multiplying be an indirect helper, by enabling 

 the herbivox-a on which the carnivore preys to get more 

 food, and thus to nourish the carnivore more abun- 

 dantly; 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 



