584 DISHARMONIES AND OTHER SHADOWS 



which has swallowed it ; in its searchings for food and shelter 

 it may discover what is to it simply a new world on or 

 beneath the surface of another organism. It is not another 

 organism to them as it clearly appears to us; it cannot be 

 separated off from other areas of safety and abundance 

 which other struggling organisms may secure. 



It is exceedingly difficult to draw a dividing line between 

 some parasites which are of some slight use to their hosts, 

 e.g., the beautiful Infusorians in the stomach of some herbi- 

 vores like horse and cow, which seem to help in breaking 

 down the food, and certain symbions or commensals which 

 are on the whole useful, but levy a slight tax. Some ecto- 

 parasites behave as if it was their duty in life to keep the 

 surface of their host's body clean. All the three modes of 

 life are to be looked at as expressions of the widespread 

 tendency in Animate Nature to establish inter-relations be- 

 tween organisms, to link lives together, to weave a web 

 of life. It may be occasionally repulsive, but it is to be 

 considered broadly as a part of a complex external systema- 

 tisation or correlation that has been evolved in the course 

 of ages and is of great importance in the process of Natural 

 Selection. 



It must not be forgotten that parasites occasionally play 

 a part as eliminative agents, and may work towards conserva- 

 tion as well as wastefully. They may weed out the weakly 

 members of a stock. They may put a useful check on abrupt 

 changes of distribution. Another exonerating fact is that 

 in a number of cases, e.g., among Crustaceans, the parasitism 

 is connected with the continuance of the race, and is altru- 

 istic as much as egoistic, for it is confined to the mother-ani- 

 mals, who seek a safe place in which to bring forth young. 



(4) It must be admitted that there is an occasional hint 



