68 PRACTICAL ZOOLOGY. 



taken (or cell starvation follows), and into this surround- 

 ing liquid the cell throws its waste products. 



In consequence of the complexity of structure, a compli- 

 cated set of organs is required to circulate this liquid to 

 all parts of the body, so that each cell may be supplied, 

 and to bring air into contact with this current in one part 

 of its course, where oxygen is absorbed for all the cells. 



It now becomes evident why one can hold his breath 

 longer after taking several deep breaths, and why distress 

 for breath continues for some time after one has stopped 

 at the end of a foot-race. 



The differentiation of cells in the formation of the vari- 

 ous tissues of the body may be compared with the division 

 of labor in a community. In a community of savages 

 there is little division of labor ; each one, or at least each 

 family, gets and prepares his own food, makes his own 

 clothing, builds his own dwelling (if he has one), and in 

 general supplies all his own wants ; in rude communities 

 of more enlightened people the same is very nearly the 

 case. But as time goes on, it is found more advantageous 

 to divide the labor, each devoting himself to one special 

 kind of work ; thus each acquires skill in his special line, 

 better articles are produced, and time is saved for all; so, 

 in time, come tailors and butchers, teachers and tanners. 



But just in proportion as each individual devotes him- 

 self exclusively to one pursuit, he neglects the others, and 

 consequently grows unable to do well in them ; that is, by 

 becoming especially fit for one kind of labor, he propor- 

 tionally unfits himself for others. The grocer does not 

 make his own boots, clothes, hat, house, nor wagon. 



In the human body, also, the work is better done by 

 having a special set of cells devoted to a given kind of 



