BIOLOGY AND HUMAN WELFARE 



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there is any relation between the disease of the Rat and of Man, 

 and found that Man is infected with the plague bacillus, Bacillus 

 pestis, by being bitten by a Flea from an infected rat. Extermina- 

 tion of rats and fleas means the practical eradication of the disease, 

 but in California the Ground Squirrels have become infected with 

 the bacillus so the problem has become somewhat greater. Ru- 

 bonic plague is doomed, though it has already taken an incalculable 

 toll of human lives : even during the first four years of the present 

 century it destroyed about two million people in India. Still more 

 recently San Francisco has been fighting an outbreak of the 



Eggs Larva Pupa Adult 



Fig. 258. — Life history of the House Fly, Musca domestica. 



plague that not long ago would have been a national calamity; 

 but it was immediately stamped out with the loss of compara- 

 tively few lives. 



In brief, numerous parasitic Insects not only actually develop 

 at the expense of animal tissue, but others act as the transmitting 

 agents of Racteria, Protozoa, etc., which are the actual parasites, as 

 we have already seen in the case of malaria, yellow fever, and Afri- 

 can sleeping sickness. And last, but not least, we know that House 

 Flies, which we tolerate as uninvited guests at our tables, have 

 been shown to carry the germs of typhoid fever, tuberculosis, 

 dysentery, and several other scourges. (Figs. 224, 248, 258.) 



