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 Ave 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 versa ? 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, — 



