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- 



