THE CYCLE OF LIFE 441 



anything like thoroughness, and (2) that the process of 

 reproduction tends to become physiologically exhausting, 

 especially to the female sex. It is a noteworthy fact, 

 however, that in wild nature, the usual termination of hfe 

 is violent. Most animals die before their time, devoured 

 by their fellows, killed ofE by some environmental vicissi- 

 tude, or starved by a seasonal disappearance of their food. 

 Very few cases of microbic death are known among wild 

 animals, and it is possible that all such cases are due to 

 some human interference. Sir Ray Lankester cites the 

 case of a sandhopper which sufiers from a bacterial 

 epidemic, but admits that this may be quite ' unnatural '. 

 In regard to the legions of parasites with which animals 

 are infested, it has to be recognized that these are rarely 

 fatal. It would be almost a contradiction in terms that 

 they should be, for it is not advantageous to a parasite 

 to kill its host. Parasites are destructive when they are 

 transported into hosts which are not physiologically accus- 

 tomed to them, which have altered their geographical 

 distribution, and thus become susceptible to novelintruders. 

 Then we hear of plagues and decimation, but in most cases 

 parasitism is an old-estabhshed, going concern. There 

 rise in the mind cases hke those of Ichneumon-flies, which 

 lay their eggs in caterpillars and the hke, and there the 

 fatahty is well known. The Ichneumon-grubs hatched 

 in the caterpillar, proceed to devour their temporary host. 

 But this is not an ordinary type of parasitism. 



On the whole, therefore, we are led to agree with the 

 general conclusion, which many naturalists have reached, 

 that in a state of Nature, most animals die a violent death 

 before they have nearly reached the end of their tether. 

 And this is one of the reasons why hfe in Nature is so 



