ECONOMIC ZOOLOGY 503 



scavengers, attacking and destroying carrion and other decaying 

 organic matter. 



506. Animals Directly Injurious to Man. When man first 

 appeared on the earth many mammals now extinct, much larger 

 and fiercer than those of the present time, were abundant. Un- 

 questionably these were much more of a menace to him then 

 than the predaceous animals are now. These animals, except 

 in a few poorly inhabited parts of the world, are now practically 

 negligible. A few poisonous snakes, some sharks and crocodiles, 

 a few members of the cat and dog families and a few species of 

 bear, almost extinct, about exhaust the list of animals really 

 dangerous because of size or ferocity. But in place of these there 

 are now numerous species that are no less dangerous to him 

 because of diseases which they bring to man directly or indirectly. 

 Reference has been made to Protozoa which produce malaria 

 and yellow fever, and to the intestinal and other parasites that 

 belong to the unsegmented worms and produce all sorts of dis- 

 comfort, inefficiency, and disease among men. The mosquitoes 

 and flies and other insects that spread these diseases are just as 

 important to us as the germs themselves. The bubonic plague 

 is believed to be a disease of rats and other rodents carried to 

 man from the rat by fleas. All these temporary external para- 

 sites that go from animal to animal, as lice, fleas, bedbugs, 

 mosquitoes, etc., are especially favorably situated to be the 

 carrier of disease. Cattle are subject to tuberculosis; and many 

 investigators believe that this is the same as the human disease, 

 or at least that it is intercommunicable in man and cattle. 



507. Animals Hurtful to our Animal Friends. Our domestic 

 animals are apparently almost as open to diseases as man 

 himself. Hence it is that there are many animal diseases, caused 

 by the various parasites of blood and organs. The Texas fever 

 of cattle; the haemoglobinuria of cattle and sheep; pebrine, a 

 disease of the silkworm; the sleeping sickness of Africa; etc., 

 are due to protozoa, and are carried by ticks, flies, and the like. 

 The liver-rot and staggers of sheep, and the various tape- worm 

 and hook-worm diseases of hogs, dogs, cattle, and man are 

 caused bv the unsegmented worms. The "bot" diseases of 



