1.52 ZOOLOGY 



first of all a difference of size nor a difference in the number 

 of parts, — just as a large population or numerous cities are 

 not the primarjr characteristics of a highly civilized state. But 

 just as a complex civilization is one in which each of the 

 different citizens has his own special task to perform for the 

 commonwealth, so a highly developed organism is one in which 

 each different organ has its special role to play. The worm 

 Nereis' has more segments to the body than the craj'fish.but 

 these segments are very nearly alike — the fins esi^ecially are 

 quite similar. In the crayfish, on the other hand, the append- 

 ages are dissimilar. Each jjair has a special function to perform 

 and is specially adapted, often complexly fashioned, to meet 

 this need. What is true of the appendages is likewise true, to 

 an equal degree, of the internal organs. The internal organs 

 of Nereis are repeated in each segment ; but in the crayfish 

 the egg ducts lie in one segment, the heart in another part of the 

 body, and so on. Some of the segments have given up one or 

 more functions to perfect a single one in which it has specialized. 

 There has been a division of labor between the tlifferent parts 

 of the body, and in consequence a greater perfection in the 

 performance of each function. More perfect fulfilment of 

 function is the result of physiological division of labor, just 

 as a higher civilization is the result of indi\'idual division 

 of labor. 



1 Compare Figure IGl. 



