228 ECONOMIC ZOOLOGY 



and mice; Sciuridae, the squirrels; Leporidae, the rabbits and hares, 

 Castoridae, the beavers; Geomyidae, the pocket gophers; and Ccendidae 

 (Hystricidae) the porcupines. 



This largest of the orders of mammals is also one of the most im- 

 portant, economically, since it is one of the most destructive of all 

 the animal groups. It is the negative importance, then, that will 

 be chiefly considered here, though some of the species have some posi- 

 tive value to man, as already noted in connection with the fur- 

 bearing animals. 



Murida. Rats and Mice. The largest of the above -mentioned fami- 

 lies of rodents, both as to numbers of species and of individuals, and by 

 far the most important economically is the family Murida. Of the 

 Muridae the most important genus is Rattus, including the only too 

 familiar rats, whose damage to man each year in the United States 

 has recently been estimated by the Biological Survey to be at least $200,- 

 000,000, or the value of the labor of an army of 200,000 men. Not 

 only does the rat cause this enormous monetary loss, but it is the 

 known disseminator of the bubonic plague and some less serious diseases, 

 and is supposed to be the carrier of several others; so that a prominent 

 medical man recently stated that he considered the rat the worst dis- 

 ease carrier we know. 



The plague was known long before the beginning of the Christian 

 Era, and on several occasions, in Europe, under the name of the " black 

 death" it carried off people by the millions. In 1907 the deaths from 

 plague in India were estimated at 2,000,000. Since rats, as is well 

 known, are common on board of ships, it is very easy to see how the plague 

 has been carried from infected ports to all parts of the world. When it 

 became known that the plague is a disease of rats, carried, on the death 

 of the rat, to man by the rat flea, the problem of plague control resolved 

 itself into the problem of rat extermination. How well this may be 

 done by a vigorous, cooperative effort is illustrated by the successful 

 campaign waged, some years ago in San Francisco and surrounding cities. 

 Incidentally the problem was complicated there by the discovery that 

 the rat fleas were also carried by the ground squirrels, so widely dis- 

 tributed in that part of the country. The methods of extermination 

 of both rats and squirrels will be discussed later. 



Three kinds of rats are found in the United States and are gener- 

 ally distributed throughout most parts of the civilized world. Their 



