448 METAZOAN PHYLA 



has played the largest part in the development of human civilization. 

 Mammals have also been the pets and associates of man since early in 

 his history. Many mammalian products have been important articles 

 of commerce, including musk, which is the product of certain glands 

 of ruminants; ivory, which is supplied mainly by the tusks of walruses 

 and elephants; oil, which has been secured from the fat of sperm whales; 

 and ambergris, a product of the intestinal canal of whales used as a 

 base in the manufacture of fine perfumes. Formerly whalebone was 

 an important article of commerce but its value has diminished in recent 



years. 



Not only are many mammals valuable to man, but the group also 

 includes some which are decidedly injurious. Among these are rodents, 

 which destroy crops in the fields or commit ravages about houses and 

 outbuildings. The rat is injurious not only for this reason but because 

 it is also a carrier and distributor of the germs of disease. Various small 

 mammals are now known to harbor the bubonic plague, tularemia, and 

 Rocky Mountain spotted fever. In some countries, particularly tropical 

 ones, wild mammals are a menace to the lives of people, and everywhere 

 carnivorous mammals are a constant threat to the safety of cattle and 

 other domesticated animals. 



