ANIMALS INTRODUCED FOR SAKE OF UTILITY 259 



Muskelunges brought to the lake to exterminate the Carp 

 have themselves fallen victims and have disappeared, and 

 the Carp still flourish. 



For utilitarian purposes the gigantic Australian "Murray 

 Co&" (Otigorusmacquariensis) and, about 1883, the Canadian 

 Black Bass have been introduced into English rivers, but 

 the experiments have met with no success. 



THE MEDICINAL LEECH (Hirudo medicinalis} 



The universal use of the Leech for blood-letting in the 

 old days led to its abundant appearance in our own country. 

 Sir J. Dalzell illustrated it in his Powers of the Creator 

 (1853) from a Loch Leven specimen, and there are other 

 records of its presence, for it is probable that many a doctor 

 dropped his Leeches into a convenient pool, where he could 

 readily find them again. But the Medicinal Leech has failed 

 to gain a firm foothold. Its gradual elimination from use in 

 medicine led to its disappearance, for the stock is no longer 

 replenished and the pools where once it was common know 

 it no more. 



COUNTER-PESTS 



I cannot close this section without mentioning a new de- 

 velopment of scientific research, which promises on account 

 of their utility to add many insects to old faunas. The 

 typical results of this fresh method, however, are to be seen 

 in America and on the continent of Europe rather than in 

 this country. I refer to the introduction of insect counter- 

 pests insects which at one stage or other feed upon some 

 pest of cultivated crops. America has found special need 

 for such destroyers. Records show that 407 introductions 

 of foreign plants were made to the United States of 

 America during the first three months of 1913, and the im- 

 portations of field and garden plants from other countries 

 exceed a thousand a year. With this great annual inflow of 

 plants come their native pests, and these, unwittingly ad- 

 mitted, have spread, in some cases with a rapidity beyond 

 belief, since the new land may harbour no destroyer bird 

 or insect such as kept the pest in check in its old country. 

 This factor in the success of many accidentally introduced 



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