CARCINOMA OF THE THYROID IN SALMONOID FISHES. 433 



the unused food. The material of the trough, though water-soaked, affords no perma- 

 nent lodgment for gross particles of organic matter. 



Since thyroid carcinoma in fishes is rare in nature and common in domestication it 

 would seem that some or all of the conditions of domestication — artificial propagation 

 of fish — either by their own direct action cause the disease, predispose to it, or carry 

 its cause to the fishes. That domestication itself, viewed as a single complex whole, 

 is per se the cause of thyroid disease, is a priori highly improbable, and is negatived 

 by the occurrence of thyroid tumors in wild fish. This occurrence, though sporadic 

 and rare, is now established beyond dispute and is a fact of great significance. Domesti- 

 cation predisposes to the disease and carries the causative agent to the fishes, and some 

 one or very few of the constituent factors or conditions of domestication is probably 

 of chief importance in this action. From a study of the conditions at such a station as 

 Craig Brook, while the disease is spontaneous and endemic under the routine of fish 

 culture, it is difficult to fix accurately upon any one of these conditions to the exclusion 

 of others as the prime essential. Feeding is probably more important than any other 

 one factor. The removal of feeding or change to natural food tends toward the recovery 

 of affected fishes, and prevents or delays the initial process. The inevitable reduction 

 in the available unit of water supply per fish and the crowding of fish together in small 

 areas are no doubt factors of next importance. We do not believe that the soluble 

 products of metaboUsm and of the disintegration of organic matter left from feeding 

 or otherwise play any important part in the origin or progress of this disease, unless 

 as creating more favorable conditions for the development of a causadve agent. An 

 unfavorable general hygiene of the fish predisposes to the disease, domestication itself 

 as usually practiced involving a more or less unfavorable hygiene, and in this way 

 these soluble products may enter somewhat into the problem of causation. 



