142 THE MAIN CURRENTS OF ZOOLOGY 



diseases spoken of above are all produced by animal 

 organisms. On the other hand, the bacteria whose 

 pathogenic qualities have been known so long are 

 plants. Thus, typhoid fever, tuberculosis, diphtheria, 

 etc., are produced by plant parasites and malaria, 

 sleeping sickness, and probably yellow fever are 

 produced by minute animal organisms. 



Among other diseases transmitted by insects may 

 be mentioned the bubonic plague, by the bite of lice 

 of rats; many diseases of cattle, by bites of flies, 

 ticks, etc. These insects pass along the disease 

 germs which they have derived by blood sucking 

 from other animals. 



