INTRODUCTION. 5 



each person learns to do one thing well, all together work 

 economically for the common good, time is saved, and 

 better goods are produced. In the body there is a division 

 of labor similar to that of a community. Each organ has 

 its own work to do, and all work together for the common 

 welfare. The cells of each tissue have certain properties 

 and peculiarities of form differing from the form and 

 properties of the cells of any other tissue. While the 

 general structure of all cells is essentially the same, and 

 while they all have certain properties in common, each has 

 some one kind of work that it can do well, and to which 

 work it devotes itself. The nerve cells receive impressions 

 from the outer world, carry nervous impulses, and control 

 the various activities of the body. The muscle cells have 

 as their work the production of motion. All the cells 

 must take food for themselves and grow. Each has a 

 birth, life, and death, as each individual in a community 

 of men ; and as the community endures, while the indi- 

 vidual members are continually changing, so, in the body, 

 while the form remains about the same from year to year, 

 the cells are continually changing, some dying, and others 

 taking their places. . 



In an animal of a single cell, like the ameba, the one 

 cell must do everything for itself. The higher animals all 

 begin their individual life as an egg, which is, in fact, a 

 single minute cell. This grows and divides, forming two 

 cells. By repeated division there accumulates a mass 

 of cells. These take on the arrangement peculiar to the 

 kind of animal from which the egg came. But as the cells 

 increase in number one group of cells takes up one part 

 of the work of .the body, other cells another part of the 

 work, and so on. 



In studying history (sociology) we have to deal with the 



