548 DR DAVY ON THE URINARY SECRETION OF FISHES. 



gen, as much as the latter is of carbonic acid. How strongly is this exemplified 

 in birds ; — of high temperature, consuming much atmospheric air, evolving much 

 carbonic acid, — their urinary secretion, also, is remarkably abundant, and abound- 

 ing in nitrogen.* And in other classes of animals, such as insects in their seve- 

 ral stages, such as serpents and lizards, and the hybernating ones of different 

 classes, whether active or torpid, a like accordance, though perhaps not so 

 strongly shown, is yet clearly observable. 



Reasoning hence, guided by analogy, might it not be expected that in the in- 

 stance of fishes, inasmuch as their temperature is low, and the quantity of car- 

 bonic acid evolved small, that their urinary secretion also would be small — pro- 

 portionally small ? And, granted that it is so, as the results of the experiments 

 described would seem to indicate, does it not lead to another conclusion, viz., 

 that subsisting, with few exceptions, exclusively on animal food, this their food, 

 under the influence of a high digestive power, is almost entirely assimilated, and 

 that no more is expended on the urinary secretion than is requisite to balance 

 the small amount consumed in carrying on the aerating process ? And if this be 

 admitted, does it not help to explain some of their peculiarities, — their remark- 

 able rapidity of growth when supplied with abundance of food, — their little 

 waste of substance when sparingly supplied, and their long endurance without 

 loss of life, under a total, or nearly total, privation of aliment ? 



The history of the salmon and its congeners, which of late years has been so 

 carefully and successfully studied, might be adduced in illustration, — exem- 

 plifying, 1st, The great activity and power of the organs carrying on the digestive 

 functions, — the stomach itself of the captured fish, with the parietes adjoining, 

 being found more or less dissolved by the action of the gastric juice in the short 

 space of a few hours, and in being always found empty in the migrating fish ; 

 Idly, The extraordinary increase in weight during the short sojourn of the young 

 salmon in the sea, when, without stint of food, it passes from the smolt stage of 

 growth to that of the grilse ; and, ddlt/, The comparatively very slow growth of 

 the young salmon in its parr stage, during the months of winter and early spring, 

 when its food is scarce. 



Lesketh How, Ambleside, Dec. 1, 1856. 



* I may mention as an instance the swallow, feeding like the trout, when the food of the latter 

 is chiefly insects, and, as regards the secretion in question, showing a remarkable difference. From 

 the nest of a pair I had an opportunity of observing, the young of which were only a few days old, 

 the droppings on a flag-stone beneath, in one day, were as many as forty-five; those collected and 

 dried thoroughly weighed 78-3 grains ; the following day, the droppings were seventy. They con- 

 sisted chiefly of lithate of ammonia with a little urea, and of the indigestible remains of insects, — the 

 urinous portion by far the largest. The excrement, it may be inferred, was chiefly from the young 

 birds, as the parent birds were almost constantly on the wing providing food. How large in quan- 

 tity was this excrement in comparison with the bulk of the birds ! I have found an old swallow to 

 weigh only about 300 grains, and when thoroughly dried no more than 105 grains, so that the 

 amount of excrement in two days exceeded considerably in weight one of the old bh'ds ! 



