The number and validity of recorded cases establish the fact, beyond dispute, that certain fish, especially the muscle, (_Mytilus Edulis_) and others of the shell tribe, have occasionally proved fatal to those who have eaten them; but it has been doubted whether such effects have arisen from a specific poison, or from the peculiar state of the stomach,[485] or idiosyncrasy of constitution, in the persons affected. In other words, ought we to consider the fish, so circumstanced, as an _absolute_ or _relative_ poison? Each of these theories has met with its advocates, and many striking facts and illustrations have been adduced in their support. The weight of authority, however, as well as of argument, strongly inclines in favour of the existence of a specific virus, generated under circumstances which we are at present unable to appreciate. At the same time, it would be vain to deny, that certain fishes are more obnoxious to the stomach of one individual than to that of another; there are, for instance, those persons who are disordered whenever they eat a muscle; others who are incapable of taking an oyster without considerable disturbance of the digestive functions. This is obviously _Idiosyncrasy_, and must not be confounded with those cases where a number of persons have been simultaneously affected from a particular food, which, on all former occasions, had been eaten by the same individuals with perfect security. We must, therefore, at the very outset of our inquiry, admit the occasional action of these articles of diet as _relative_ poisons; although it is evident to demonstration, that an _absolute_ virus is generated in particular fishes, by the operation of causes hitherto unknown.