86 ANIMAL LIFE AND HUMAN PROGRESS 



of the numbers of insectivorous birds and lizards has mean 

 an increase of injurious insects, and so the influences spread 

 like surface circles on a quiet stretch of the river. 



2. Sometimes man's operations should be called encourag- 

 ing certain forms of life, rather than directly extending their 

 range. Thus, by considerable carelessness in the disposal of 

 refuse or in neglecting to nip an evil in the bud, he has en- 

 couraged the multiplication of rats, which now cost Britain 

 several million pounds every year. In some other countries 

 it is worse ; thus around one sugar-factory in Java between 

 nine and twelve thousand rats were killed every day for 

 several years. It is not only their destructiveness that is to 

 be deplored, but there is serious risk involved in the. fact that 

 the rat harbours the Nematode parasite, Trichinella spimlis, 

 which causes the disease of trichinosis in pig and in man, and 

 in the fact that the rat-flea harbours the bacillus of bubonic 

 plague. It has been said that plague is distinctly less frequent 

 in those Indian villages that have plenty of cats. For the cat 

 kills the rat. 



Professor Fraser Harris has suggested that the emerods 

 or haemorrhoids mentioned in the Book of Samuel (1 Samuel 

 vi. 5) were the buboes or swellings of plague. Now this 

 scourge is due to a bacillus, which also occurs in rats, mice 

 and marmots, and man is usually infected by being bitten by 

 a flea which has been feeding on a plague-stricken rodent. 

 Professor Fraser Harris asks whether we may find in the five 

 golden mice made on the recommendation of the Philistine 

 soothsayers any glimpse of the fact that the over-running of 

 the land with mice was not unconnected with the plague of 

 emerods. He notes that " no rats, no plague " is an old 

 saying amongst the people of India. 



3. In yet other cases it must be admitted that man's 

 disturbance of the balance has followed as the almost inevitable 

 consequence of a laudable achievement. The potato-beetle 

 or Colorado beetle (Doryphom decemlineata) was a native of 

 the Central West of North America, where it fed on the deadly 

 nightshade and was kept in check by natural enemies. The 

 introduction of the potato plant (an ally of the nightshade) 



