THE LIVING MATERIAL OF THE BODY 23 



constantly remember that it is really the cells which are the 

 active, living parts. The combined lives of all these millions 

 of cells make the life of the whole, much as the combined lives 

 of the persons within a city make up its life. 



UNICELLULAR AND MULTICELLULAR ANIMALS 



As we have seen, some animals are composed of a single 

 cell. But this cell is able to carry on all the functions of life : 

 it feeds, digests, respires, moves, multiplies and performs all 

 the necessary duties of complete, individual life. In our own 

 bodies there are many cells, but each is not capable of carrying 

 on all the functions of life, and if separated from the others, 

 would die. Each is able to do primarily only one thing; 

 hence each is dependent upon the others. 



It may be asked why we should have so many kinds of 

 cells in our bodies, and why with us, too, one kind of cell 

 could not serve all purposes. The answer is easy to give. 

 A hermit can himself do everything needful to support 

 his life: he can prepare his own, food, make his own clothes 

 and build his own shelter. But he can do this only be- 

 cause he lives very simply. When a family lives alone on 

 the frontier the members divide the work among themselves, 

 the husband doing the work out of doors, the wife that 

 indoors and the children contributing their different shares. 

 When several families come together, it will be found that 

 some members of the community are more skillful in 

 building houses, others in making shoes, others in dress- 

 making, still others in cooking and so on; so the people 

 agree to divide their tasks and share the results of their work. 

 In this way they may have better houses, better shoes, better 

 clothing and better food than before, because each man does 

 what he can do best. As the community grows this divi- 

 sion of labor becomes extended until, in a large city, each 

 person does only a very small part of the work necessary 

 to supply him with the things he needs. But he can do his 



