Inter-Relations of Living Creatures 655 



2. Man has sometimes brought about very unfortunate re- 

 sults by encouraging, rather than directly extending the range of 

 certain living creatures. Thus by careless and unthrifty ways of 

 dealing with refuse and crumbs of all sorts, he has encouraged the 

 multiplication of rats to an extent that is ominous, to say the least. 

 Many millions of brown rats are at present being supported in 

 Britain alone, and the damage is not confined to the destruction 

 of all sorts of stores ; they foul much more than they eat. Further- 

 more, they harbour a dangerous human parasite, Trichinella spir- 

 alis, which has the pig as its second host, intermediate between rat 

 and man. Again, the microbe (Bacillus pestis) of the bubonic 

 plague of India, once known as "the black death" in Britain, is 

 more or less at home in the rat, and is imbibed by the rat-flea along 

 with its meals of rat's blood. From a dead rat the fleas jump 

 away, and an infected flea may inoculate a man with the bacillus 

 of the plague. Nothing could better illustrate the intricacy of in- 

 ter-relations, and the instance becomes still more picturesque when 

 we notice the report that plague is distinctly less frequent in those 

 Indian villages that abound in cats. For the more cats the 

 fewer rats, and the fewer rats the fewer rat-fleas, and the less 

 plague. 



3. In many cases man's disturbance of the Balance of Nature 

 is just the tax that has to be paid on a laudable achievement. The 

 potato-beetle, or Colorado beetle (Doryplwra decemlineata) , 

 which has occasionally shown face in Britain, used to be restricted 

 to the central west of North America, where it fed on the deadly 

 nightshade, and did not multiply greatly, being kept in check by 

 natural enemies. But the introduction of the potato plant (a 

 relative of the deadly nightshade) and the great extension of 

 potato-fields year after year gave the beetle a chance for prolific 

 multiplication. It spread gradually eastward, and its enemies 

 could not any longer keep it in check. Year after year it con- 

 tinued its eastward march until it reached the Atlantic seaboard. 

 Many counteractive methods have been tried, but the Colorado 



