136 PHYSIOLOGY. 



as to have lost apparently nothing [by digestion], only having yielded 

 [some small proportion in] a solution. This food yields nourishment 

 chiefly by expression ; and least so in those animals that have a 

 stomach of but one cavity. In order that more of this juice might be 

 expressed in some animals, nature has furnished four stomachs [or 

 cavities]. But even this does not dissolve it entirely ; for we find the 

 dung of such animals a good deal of the colour of the food ; for example, 

 a cow's dung shall be green when she lives on grass. 



The instinctive principle of food in animals is a curious fact : a stork 

 which swallows birds, mice, rats, frogs, &c, will not swallow a toad. 

 He takes it up in his bill, and after nibbling it, as if to kill it, he lets it 

 fall ; and this he will do several times, and at last leaves it. The sea- 

 gull, which will eat the same food as the stork, will not touch a toad. 

 Animals do not seem to distinguish food accurately by the eye ; they seem 

 to be only directed to it, and give a kind of general guess ; but they are 

 obliged to have recourse, for further particulars, to the other senses. 

 Quadrupeds commonly, if not always, have recourse to the nose ; there- 

 fore they seldom take into their mouth what is not fit for them to eat : 

 but the bird seems to have but little smell ; and therefore the nostrils 

 are at some distance from the end of the bill 1 ; and when they are 

 directed to food, [the fitness of which] they are not certain of, they 

 take it into the mouth, and, if it be unfit for food, they throw, it out 

 again 2 . 



We do not distinguish things at once by the senses : the simple 

 sensation does not in all cases inform the mind with the true idea of 

 the thing represented : the mind is often obliged to inquire how it is 

 that a hollow of any particular shape, and a round of the same shape 

 (a concave and a convex surface), give the same shades and will appear 

 concave or convex according as the mind is most susceptible of the 

 idea, or has been accustomed most to conceive it to be. But a little 

 reasoning upon the collateral circumstances attending the sensation will 

 determine the simple impression in the mind to be what it really is. It 

 is often extremely puzzling in paintings or drawings to make out what 

 the painter means ; for, if the corresponding relative parts are not 

 attended to, the reasoning faculty has not its materials or data to direct 

 it, so as to be able to draw its inference, or make its conclusions. This 

 must be always the case with things, the knowledge of which is ac- 



1 [Except in Apteryx, where, from the terminal position of the nostrils, the smell 

 would seem to be a much used and important sense.] 



- [This I have observed in the ostrich, where a piece of leather was repeatedly 

 picked up, tested by frequent nips with the mandibles, and then rejected.] 



