IN Tit OD UCTION. 7 



and peculiarities of form differing from the form and proper- 

 ties of the cells of any other tissue. The general structure of 

 all cells is essentially the same, just as all men are alike in 

 their plan of structure. But one man learns to do one kind 

 of work, and sometimes can hardly do anything else. Often 

 the nature of his work can be judged from his appearance. 

 With the cells, while they all have the same essential struc- 

 ture, and 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 one 

 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 com- 

 munity endures, while the individual members who compose 

 it are continually changing, so, in the body, while the form 

 remains about the same from year to year, the cells are con- 

 tinually changing, some dying, and others taking their places. 



In an animal made of a single cell, like the Ameba, the one 

 cell must do everything for itself. The higher animals all be- 

 gin 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. An individual liv- 

 ing alone (Robinson Crusoe) had to do everything for himself. 

 Just as soon as this " community of one " became a commu- 

 nity of two by the accession of Friday, there sprang up a 

 division of labor. 



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



