THE WEB OF LIFE 301 



Looked at broadly, parasitism is a way out of the struggle 

 for existence. Just as some animals have betaken them- 

 selves underground or into caves or down to the great 

 abysses, so others have become parasitic. It implies an 

 abandonment of direct competition, and its occurrence at 

 almost every level among backboneless animals shows that 

 it has been frequently resorted to, and with great success, 

 in many cases, as regards self-preservation and increase 

 in numbers. It should be noted that in many parasitic 

 types, e.g. among Crustaceans and Insects, only the females 

 have adopted the habit, doubtless in relation to egg-laying 

 and the protection of the offspring. 



A thoroughgoing parasite, such as a tapeworm, is very 

 effectively adapted to the conditions of its life. It is 

 safe from all enemies (unless perhaps the practitioner with 

 his vermifuge) ; it floats in a plethora of food, which it 

 can absorb by the whole surface of its tape-like body ; it 

 can live and thrive with a minimum of oxygen, and it has a 

 mysterious ' anti-body ' which preserves it from being 

 digested by its host ; it has muscular adhesive suckers 

 and, it may be, attaching hooks, so that it is safely fixed 

 to the wall of the intestine ; it lives in warmth and comfort 

 without any expensive sense-organs to keep, with a low type 

 of nervous system a life of dull sentience. It has 

 attained to what economists have called ' complete 

 material well-being '. 



The other side of it is, of course, degeneration. The 

 tapeworm has a lowly developed nervous system, no sense- 

 organs, slowly contracting smooth muscles, and so on. 

 Only its reproductive system is highly developed, and even 

 there a hint of degeneracy may be found in the self- 

 fertilization that often occurs. For some of the tape- 



