186 ON DEATHS IN THE SOCIETY'S GARDENS. 



many difficulties, and will be undertaken later. In mammals, 

 liowever, tlie comparison is much easier, and the preliminary 

 survey which has been carried out shows that in Oarnivora the 

 thyroid is from two to three times as large as in Ungidata of 

 corresponding body-weight. The key to the problem is probably 

 furnished by Marine and Lenhart's work on thyroid enlargement 

 in tiout kept in hatcheries. These authors have shown that the 

 goitrous condition, which so frequently supervenes, is due to the 

 diet of horseflesh with which the fish are fed and that it can be 

 obviated and even cured l)y supplying a ration of sea-fish. Their 

 investigations have shown that the constituent deficient in the 

 horseflesh diet is iodine. The necessity for fixing the inadequate 

 amounts- present in flesh is met by a hypertrophy of the gland. 

 "When adequate amounts of iodine are supplied either in the form 

 of sea-fish or soluble iodides, the enlar£;ement subsides. 



