THE LIVING ORGANISM 21 



human units that make it up. The farmer, the manu- 

 facturer, the soldier, clerk, and artisan do not all work 

 in the same way; they undertake one or another of 

 the economic tasks which they may be best fitted by 

 circumstances to perform. Their differentiation and 

 division of labor are identical with the diversity in 

 structure and in function as well, exhibited by the cells 

 of a living creature. We might speak of the several 

 states as so many organs of our own nation ; the com- 

 mercial or farming or manufacturing communities of 

 a state would be like the tissues forming an organ, 

 made up ultimately of human units, which, like cells, 

 are engaged in similar activities. As the individual 

 human Hves and the activities of differentiated eco- 

 nomic groups constitute the life of a nation and 

 national existence, so cell-Hves make the living of an 

 organism, and the expressions "division of labor" and 

 "differentiation" come to have a biological meaning 

 and application. 



The cell, then, is in all respects the very unit of the 

 organic world. Not only is it the ultimate structural 

 element of all the more familiar animals and plants 

 that we know, as the foregoing analysis demonstrates, 

 but, in the second place, the microscope reveals simple 

 httle organisms, like Amoeba, the yeast plant and bacteria, 

 which consist throughout their lives of just one cell 

 and nothing more. Still more wonderful is the fact that 

 the larger complex organisms actually begin existence 

 as single cells. In three ways, therefore, — the ana- 

 lytic, the comparative, and the developmental, — the 



