54 GENERAL ANATOMICAL CHARACTERS 
adaptation to its special function is not the only cause of the 
particular form or structure of an organ, but that this form, having 
in all probability been arrived at by the successive and gradual 
modification of some other different form from which it is now to a 
greater or less degree removed, has other factors besides use to be 
taken into account. In no case is this principle so well seen as in 
that of the organs of digestion. These may be considered as 
machines which have to operate upon alimentary substances in very 
different conditions of mechanical and chemical combination, and to 
reduce them in every case to the same or precisely similar 
materials ; and we might well imagine that the apparatus required 
to produce flesh and blood out of coarse fibrous vegetable substances 
would be different from that which had to produce exactly the 
same results out of ready-made flesh or blood ; and in a very broad 
sense we find that this is so. Thus, if we take a large number of 
carnivorous animals, belonging to different fundamental types, and 
a large number of herbivorous animals, and strike a kind of average 
of each, we shall find that there is, pervading the first group, a 
general style, if we may use the expression, of the alimentary organs, 
different from that of the others. That is to say, there is a specially 
carnivorous and a specially herbivorous modification of these parts. 
But, if function were the only element which has guided such 
modification, it might be inferred that, as one form must be supposed 
to be best adapted in its relation to a particular kind of diet, that 
form would be found in all the animals consuming such diet. But 
this is far from being the case. Thus the Horse and the Ox, for 
instance—two animals whose food in the natural state is precisely 
similar—are most different as regards the structure of their ali- 
mentary canal, and the processes involved in the preparation of that 
food. Again, the Seal and the Porpoise, both purely fish-eaters, 
which seize, swallow, and digest precisely the same kind of prey, in 
precisely the same manner, have a totally different arrangement of the 
alimentary canal. If the Seal’s stomach is adapted in the best conceiv- 
able manner for the purpose it has to fulfil, why is not the Porpoise’s 
stomach an exact facsimile of it, and vice versd ? We can only answer 
that the Seal and Porpoise belong to different natural groups of 
animals, formed either on different primitive types, or descended 
from differently constructed ancestors. On this principle only can 
we account for the fact that, whereas, owing to the comparatively 
small variety of the different alimentary substances met with in 
nature, few modifications would appear necessary in the organs of 
digestion, there is really endless variety in the parts devoted to 
this purpose. 
- Mouth.—The digestive apparatus of mammals, as in other ver- 
tebrates, consists mainly of a tube with an aperture placed at or 
near either extremity of the body,—the oral and the anal orifice, — 
