24 



ADVANCED PHYSIOLOGY 



own work well because he has only one thing to do. The 

 life of a city is of much higher grade than that of a pioneer 

 family; its population has many more luxuries and ac- 

 complishes much more, all because of this division of labor. 

 The people become more and more 

 dependent upon each other, but 

 for just that reason they are better 

 served. 



So it is among organisms. Where one 

 cell does everything, the life is simple 

 and on a very low scale. Each cell can 

 feed itself and perform all the necessary 

 functions, but the whole life is only one 

 of growth and reproduction. As the 

 cells become more abundant, they also 

 become unlike. Each takes upon itself 

 certain duties, each contributes to the 

 good of the other cells, and each receives 

 aid from the others. In Figure 17, for 

 example, we have a simple animal in 

 which two kinds of cells are shown. 

 Those in the center take care of the 

 digestion of food, while those on the 

 outside protect the animal from ex- 

 ternal injuries. The entire organism 

 is thus much better served than it 

 would be if each cell had all the 

 varied duties to perform. The life of 

 any animal is the sum of the lives 

 of its cells, and with many kinds of 

 cells all working together for a common good, a higher 

 grade of activity is produced than with each working for 

 itself alone. Division of labor goes hand in hand with a 

 rise in the scale of accomplishment and results in a superior 

 type of life. 



B 



FIG. 17. A MULTICEL- 



LULAR ANIMAL (HY- 

 DRA) 



Showing its body to be 

 made of many cells that 

 are not all alike, B.base 

 for attachment ; M, 

 mouth; DC, digestive 

 cavity. 



