99 
of two species of the genus Rana. 
observed in nature : the urinary organs of the turtle and 
tortoise, seem to be a connecting link between those of the 
animals in question, and those of serpents and lizards. 
Perhaps additional facts are not required to prove, that 
the secretion of the kidneys of animals depends more on the 
intimate and invisible structure of these organs, than on the 
kind of food the animals consume ; were such facts wanting, 
there would be no difficulty in furnishing them. How dif- 
ferent is the urine of the brown-toad and that of any species 
of small lizards ! yet flies are the favourite and common diet 
of both animals. Other remarkable instances might be men- 
tioned, of similarity of diet and difference of urinary secretion ; 
and, vice versa, instances might be afforded of difference of 
diet and similarity of urine : I will mention one only ; it is that 
of parrots and snakes ; their urine, as I have found, being 
much the same, consisting chiefly of uric acid, though their 
diet is altogether different, the birds feeding entirely on vege- 
table matter, and the reptiles entirely on animal matter. But 
let me not be supposed to maintain that the urinary secretion 
depends entirely on the organ, quite independent of the nature 
of the food or of the blood, from which the elements of the 
urine are derived. It appears to be pretty satisfactorily proved, 
that, cceteris paribus, there is a certain relation between the 
nature of the food and of the urine. Whilst this has been 
generally admitted, the relation between the organ and the 
secretion has been less insisted on, though perhaps not less 
curious and deserving of attention. 
I have hitherto made no allusion to the difference of opinion 
amongst comparative anatomists, on the subject of the uri- 
nary organs of the frog, whether it has, or has not, an urinary 
