THE FACTORS. 413 



drought may kill it either by drying up its medium or by 

 giving it a medium inadequately aerated. Thus each organ- 

 ism, adjusted to a certain average in the actions of its 

 inorganic environment, or rather, we should say, adjusted to 

 certain moderate deviations from this average, is destroyed 

 by extreme deviations. So, too, is it with the environ- 



ing organic agencies. Among plants, only the parasitic kinds 

 and those united by symbiosis (as well as a few innocent 

 "lodgers") depend for their individual preservation on the 

 presence of certain other organisms (though the presence of 

 certain other organisms is needful to most plants for the pre- 

 servation of the race by aiding fertilization). Here, for the 

 continuance of individual life, particular organisms must 

 be absent or not very numerous beasts that browse, cater- 

 pillars that devour leaves, aphides that suck juices. Among 

 animals, however, the maintenance of the functional balance 

 is both positively and negatively dependent on the amounts 

 of surrounding organic agents. There must be an accessible 

 sufficiency of the plants or animals serving for food; and of 

 organisms that are predatory or parasitic or otherwise detri- 

 mental, the number must not pass a certain limit. 



This dependence of the moving equilibrium in every indi- 

 vidual organism on an adjustment of its forces to the forces 

 of the environment, and the overthrow of this equilibrium 

 by failure of the adjustment, is comprehensive of all cases. 

 At first sight it does not seem to include what we call natural 

 death; but only death by violence, or starvation, or cold, or 

 drought. But in reality natural death, no less than every 

 other kind of death, is caused by the failure to meet some 

 outer action by a proportionate inner action. The apparent 

 difference is due to the fact that in old age, when the 

 quantity of force evolved in the organism gradually dimi- 

 nishes, the momentum of the functions becomes step by step 

 less, and the variations of the external forces relatively 

 greater; until there finally comes an occasion when some 

 quite moderate deviation from that average to which the 



