PERPETUATION OF LIVING BEINGS. 121 



■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 herbivora 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 



