60 THE INVISIBLE WORLD 



cells of the complex organism as to make that or- 

 ganism complete in all its parts as a whole-handed 

 work. 



Here, too, is seen decided progress in the evolu- 

 tion of the vital relation. Every individual mi- 

 crobe in its colony enjoys a life peculiarly its own. 

 It is independent. Apart from its fellows, it lives 

 just as well. The individual cell, likewise, in the 

 great oak enjoys a life peculiarly its own; but its 

 life is also vitally related to the life of every other 

 cell in the great organism. 



Good to one cell is good to all. Food extracted 

 by the leaves of the topmost boughs from air, rain 

 and sunshine, finds its way down through and feeds 

 alike all the cells of the branches, stem and roots, 

 to the lowest rootlets. Conversely, food extracted 

 from the soil by the lowest rootlets finds its way 

 upward through, and feeds alike, all the cells of 

 the root, stem and branches to the topmost boughs. 



So injury to one cell is injury to all. Sever a 

 part, as a limb, from the tree, and it soon dies, and 

 leaves a wound to be healed on the parent stem. 

 Cut the bark to ^ the wood in a girdle round the 

 stem, and all the cells from the lowest rootlets to 

 the highest boughs feel the death stroke, and the 

 whole tree dies. 



So it is with the animal. The life of every 

 human being, like the life of every vegetable, be- 

 gins a single microscopic cell. This cell repro- 

 duces itself, and continues to multiply and differ- 

 entiate until thousands upon thousands of differ- 



