American Fisheries Society 385 



presented for years to come. They have not yet been defi- 

 nitely shown to apply to the thyroid tumors in fishes and 

 perhaps there is ground here for raising a question whether 

 cancer has been demonstrated in the thyroid tumor. More- 

 over, nobody can foretell what trend of cancer investigation 

 will in the end solve the problem, and it is not impossible 

 that the future may put the fish thyroid tumor outside the 

 true cancer as finally defined. Meantime we may insist 

 that there is only a presumption, though an overwhelming 

 one, in favor of the view that this tumor includes cancer. 

 This presumption is created by the broad facts in the case. 

 Cancer is found everywhere throughout the vertebrate king- 

 dom, among the cold-blooded as well as the warm-blooded 

 animals. There is a sort of specificity of certain kinds of 

 cancer for certain of the lower animals, as epithelioma of 

 the eye in cattle, cancer of the breast in mice, sarcoma in 

 rats, and round-celled sarcoma of the genitals of dogs. 

 These special occurrences of cancer in animals are doubtless 

 related to the species and its habits and mode of life. 

 Thyroid cancer in fishes, water animals with a specialized 

 and restricted mode of life, fits naturally into this series. 

 While cancer of other regions than the thyroid undoubtedly 

 occurs in fishes, the thyroid tumor is the most striking of 

 the neoplastic growths among fishes, and has a striking 

 resemblance to cancer. It does not so much matter that 

 this cancer is a terminal stage, the occasional climax of a 

 milder and commoner process. What is important is that 

 we have in fishes a process exactly analogous even in the 

 beginning to a serious human disease, and that this process 

 is linked with and leads on to another which is in the same 

 way analogous to a more dreaded disease of man derived 

 from unknown sources. 



The cause of thyroid tumor is unknown, as is the cause 

 of all tumors. One factor that is known in some cases of 

 malignant growth to play some part in causation is 

 mechanical injury or irritation. Applying this to fishes we 

 can find no injury preceding thyroid enlargement. De- 



