were only twenty-five miles a day, the fish must make their way through two thousand miles of water. In fact, however, the Rhine is throughout much of its course a swift flowing river, leaving Switzerland at a speed of four or five miles an hour; and the spawning grounds of the salmon lie above the falls and rapids south of Basel. Indeed even these considerations can scarcely be taken as a measure of the physical exertions of the salmon, since the swifter the current the greater becomes their activity. The heaviest runs of the salmon occur during June and July, although many fish whose condition shows them fresh from the sea are taken at Basel even during January. All these fish remain near the head of the river through the spawning season, and in the following December unite in a headlong rush back to salt water. The average duration of their stay in the Rhine is therefore from six to nine months, while in some cases as many as fifteen months must elapse from the day the salmon enter the mouth of the river until their return to the sea.
In view of these facts it would seem almost beyond belief, had not Miescher established it by absolutely complete demonstration, that the salmon never feed in fresh water. From the day they leave salt water until they return to it they maintain an absolutely unbroken fast. Careful examination of more than three hundred fish caught at Basel at all periods of the year, and of many taken in the lower reaches of the river just after the fish had left the sea, showed that not only was the alimentary canal empty of all food material, but the digestive apparatus was in no condition to handle nutriment even if offered it. The gastric mucosa was in a more or less desquamated condition, and alkaline in reaction. The gall bladder was empty, and the pancreas shrunken. Only two exceptions were noted, and these more apparent than real. In the stomach of one fish was found a large winged insect—quite undigested; in another a minnow—only partially digested. Of the latter case, however, Miescher records that the fish was caught near Basel in January, long after the spawning season, and was so extremely emaciated as to suggest that it must have been prevented in some way from escaping back to the sea.
The development of the genitalia (or sexual glands), occurring almost wholly after the salmon have entered fresh water, becomes of the greatest interest in the light of these observations; for the material built up in these organs must be drawn from the other tissues of the animal itself. To the problems involved in these tissue changes, as well as to those resulting from the expenditure of energy by the fish in their long journey up stream, Miescher devoted himself especially. His examinations and analyses of the salmon caught in the lower reaches of the Rhine at all periods of the year, and of those taken at Basel from January to May, show that the fish on their way up stream
