1 88 THE KINGDOM OF MAN 



physical world, which he has ventured to wrest from the 

 old rule of natural selection and adaptation. 



The adjustment of all living things to their proper 

 environment is one of great delicacy and often of sur- 

 prising limitation. In no living things is this more 

 remarkable than in parasites. The relation of a parasite 

 to the "host " or " hosts " in which it can flourish (often 

 the host is only one special species or even variety of plant 

 or animal) is illustrated by the more familiar restriction 

 of certain plants to a particular soil. Thus the Cornish 

 heath only grows on soil overlying the chemically pecu- 

 liar serpentine rocks of Cornwall. The two common 

 parasitic tape-worms of man pass their early life the one 

 in the pig and the other in bovine animals. But that 

 which requires the pig as its first host (Tcenia solium) 

 cannot use a bovine animal as a substitute ; nor can the 

 other (Tcenia mediocanellata) exist in a pig. Yet the 

 difference of porcine and bovine flesh and juices is not a 

 very patent one ; it is one of small variations in highly 

 complex organic chemical substances. A big earth-worm- 

 like stomach-worm flourishes in man, and another kind 

 similar to it in the horse. But that frequenting man 

 cannot exist in the horse, nor that of the horse in man. 

 Simpler parasites, such as are the moulds, bacteria, and 

 again the blood-parasites, trypanosoma, etc., exhibit ab- 

 solute restrictions as to the hosts in which they can or 

 can not flourish without showing specific changes in their 

 vital processes. Being far simpler in structure than the 

 parasitic worms, they have less " mechanism " at their dis- 

 posal for bringing about adjustment to varied conditions 

 of life. The microscopic parasites do not submit to 

 alterations in the chemical character of their surround- 

 ings without themselves reacting and showing changed 

 chemical activities. A change of soil (that is to say gf 



