THE MUTUA^ RELATIONS OF ORGANISMS 233 



some of them are beneficial to our life, and therefore are not 

 true parasites, according to the definition given above. Between 

 these two extremes are many intermediate grades. As a rule, 

 parasitism injures the host, and indeed, strictly speaking, para- 

 sitism is a term that should only be used when one animal or 

 plant feeds upon another, to the distinct detriment of the latter. 

 Upon the Parasite. The effect of parasitism upon the 

 parasite itself is no less profound than its effect upon the host, 

 but it is of a totally different nature. The general effect of 

 parasitism is to cause degradation of the parasite. It is a 

 general law of living nature that any organs which are not used, 

 inevitably begin to degenerate. If an animal becomes a parasite 

 upon another, it shows a general tendency to lose many of 

 its original characters. For example, the tapeworm has become 

 parasitic in the intestines of animals. Here it finds its food 

 already digested by the digestive juices of the host; it has thus 

 no need of a mouth, of digestive organs, or of any power of 

 motion; and, in conformity with the above law of nature, 

 having no need of these functions, it has lost them. The tape- 

 worm has thus become degraded to a very simple organism, 

 without digestive organs and with all of its systems of organs 

 reduced to the lowest possible condition. Thus, parasites, 

 depending as they do upon their host for their nourishment, 

 lose their power of independence and become degraded. This 

 is a biological law of great significance, the law that failure to 

 use any function results in its loss, running through the whole 

 scale of nature. It is exemplified in the human race in numer- 

 ous aspects of civilized life, where one class of people depends 

 upon another. In our highly organized cities this principle of 

 loss of power as a result of disuse is as well illustrated as it is 

 among animals, since in the city individuals are so mutually 

 dependent that each one has practically lost his ability to live 

 by himself unaided by others. The principle of the loss of 

 function by disuse is one of the most fundamental and sig- 

 nificant of the laws of nature. 



