20 DOCTRINE OF EVOLUTION 



in the different consistency of the substances secreted 

 by the cells to lie between them; skin cells are soft- 

 walled masses lying close together; even blood is a 

 tissue, although it is fluid and its cells are the corpuscles 

 which float freely in a liquid serum. Thus an organism 

 proves to be a complex mechanism composed of cells 

 as structural units, just as a building is ultimately a 

 collection of bricks and girders and bolts, related to 

 one another in definite ways. 



Our analysis reveals the living creature in an entirely 

 new light, not only as a machinelike structure whose 

 parts are marvelously formed and coordinated in 

 material respects, but also as one whose activities 

 of workings are ultimately cellular in origin. Struc- 

 ture and function are inseparable, and if an animal 

 or a plant is an aggregate of cells, then its whole varied 

 life must be the sum total of the lives of its constituent 

 cells. Should these units be subtracted from an ani- 

 mal, one by one, there would be no material organism 

 left when the last cells had been disassociated, and there 

 would be no organic activity remaining when the last 

 individual cell-life was destroyed. All the various 

 things we do in the performance of our daily tasks are 

 done by the combined action of our muscle and nerve 

 and other tissue cells ; our fife is all of their lives, and 

 nothing more. The cell, then, is the physiological or 

 functional unit, as truly as it is the material element of 

 the organic world. Being combined with countless 

 others, speciahzed in various ways, relations are estab- 

 lished which are like those exhibited by the human be- 

 ings constituting a nation. In this case the life of the 

 community consists of the activities of the diverse 



