66 : ANIMAL LIFE 
The locomotive is an inorganic machine; the animal is 
an organic machine. There is a great and real difference 
between an organism, a living animal, and a locomotive, an 
inorganic structure. But for a good understanding of the 
relation between function and structure, and of the com- 
position of the body of the complex animals, the compari- 
son of the animal and locomotive is very instructive. 
41. The specialization of organs.—The organ for the per- 
formance of some definite function in one of the higher 
animals may be very complex. The corresponding organ 
in one of the lower animals for the performance of the 
same function may be comparatively simple. For example, 
the organ for the digestion of food is, in the case of the 
polyp, a simple cylindrical cavity in the body into which 
food enters through a large opening at the apical or free 
end of the body. The digestive organ of a cow is a long 
coiled tube, comprising many regions of distinct structural 
and physiological character and altogether extremely com- 
plicated. An organ in simple or primitive condition is 
said to be generalized ; in complex or highly modified con- 
dition it is said to be specialized. That is, an organ may 
be modified and complexly developed to perform its func- 
tion in a special way, in a way differing in many particu- 
lars from the way the corresponding organ in some other 
animal performs the same general function. The speciali- 
zation of organs, or their modification to perform their 
functions in special ways, is what makes animal bodies 
complex, for specialization is almost always in the line of 
complexity. Later we shall see more clearly how specializa- 
tion is brought about. For the present we may study 
one of the more important organs of the animal body for 
the sake of having concrete examples of some of the gen- 
eral statements made in this discussion of function and 
structure. 
42. The alimentary canal.— The organ which has to do 
with the taking and digesting of food is called the ali- 
