218 ON THE ECONOMICAL USES OF FISHES. 



poisonous qualities of certain fishes appear to be 

 induced periodically, and are probably connected 

 with their kind of food at the time, although the 

 causes on which these anomalous properties depend 

 are at present wholly unknown, notwithstanding the 

 many h}"potheses which have at different times been 

 proposed for their explanation. The most probable 

 of these, and the one best sustained by facts, ascribed 

 the developement of the poison in question to an 

 impregnation with copper, but this is now considered 

 as untenable. Not less so is that theory which 

 traced the poisonous effects to the process of putre- 

 faction, for, however fresh the fish may be, fatal 

 consequences have resulted from eating of them. In 

 the West Indies, the most poisonous fish, and the 

 one of which the deleterious properties have been 

 investigated with most success, is a kind of herring, 

 the yellow-billed sprat, Clupea thyrsa^ which, though 

 at times considered as excellent food, and much 

 esteemed by the negroes, yet is at certain periods, 

 and when taken in certain situations, so poisonous, 

 that a single mouthful, though immediately ejected, 

 has been kno^ATi to cause death *. Several AYest 

 Indian fishes become poisonous in the same way, 

 and among others the baracouda, Verca major ^ which, 

 however, is supposed to owe its poisonous properties 

 to the yellow-billed sprat, upon which it sometimes 

 feeds. When fishes of doubtful excellence .as food 

 are taken within the tropics, it is customary to boil 

 them along with a silver coin, and if the silver be 

 * Dr, Ferguson, Edin. Med. and Surg. Journal. 



