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 metabolism 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 causative 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. 
