struggle for existence. The risks to which the eggs are exposed are indeed enormous. Many fail of fertilization; many are devoured or otherwise destroyed. Nor do the dangers to which the salmon are subject end with their hatching. Even if the growing fish escape the voracity of the trout in their natal streams, a host of enemies await them in the ocean. If the number of salmon from year to year remains fairly constant, it is evident that the chances for any one egg surviving to become a mature fish, to replace one of its parents and in turn have offspring, would be represented by two, while the chances of destruction would equal nearly the total number of eggs produced by the female, which amount to many thousand. Yet compared to the dangers which a deep-sea fish like the cod must escape, the hatching and early life of the salmon in the lakes and streams probably offer great advantages. Were the salmon to remain in fresh water, however, where the food supply is limited, their numbers would be at least as limited as are the trout. The same explanation probably holds good for their fasting in fresh water. Were they to feed during the spawning season they would leave nothing for the newly hatched fish,—and indeed the spawn of the preceding year and their own eggs would form, as in the case of other fish, a large part of their diet.
In the salmon nature offers to science, on a scale far exceeding the resources of any laboratory, an experiment in the metabolism of hunger—a demonstration that the energy liberated within the animal body comes not directly from the combustion of the carbonaceous substances of the food, as the energy of a steam engine from its fuel, but from the breaking down of the tissues themselves. Furthermore the conditions are reduced to their simplest terms. In the warm-blooded animals the maintenance of a temperature many degrees above their surroundings, necessitates a continual drain on the potential energy stored in their tissues. In the cold-blooded salmon, on the other hand, lying quietly, for weeks or months together, between the stones on the bottom of the Rhine, the processes of oxidation sink to a minimum, involving little else than the movements of the gills and the beating of the heart. Thus the energy latent in the fats, carbohydrates and albuminous substances, which the fish brings from the sea, is utilized almost wholly in the contractions of the muscles; and within the limits of the dynamic efficiency of these tissues this force is expended in the mechanical work of swimming. For physiologists the especial importance of this experiment lies in its bearing on the recent revival, in modified form, of the theory of Liebig, that the heat of the body is maintained by a combustion of the fats, but that the albuminous substances or proteids are the source of muscular work. Unfortunately the data upon this subject collected by Miescher are only partially available. The conclusions, however, at which he arrived, are sup-
