CHAPTER LI 

 UNITY OF LIFE 



312. The multiplicity of living forms. When we recall the 

 plants and animals that are familiar to us, at least by sight or 

 by name, we must be impressed by the great variety of forms 

 in which life is to be found. There are probably over a million 

 different kinds of animals and over a million different species 

 of plants. Yet throughout all this variety we find the common 

 facts of life. 



A survey of what being alive means, from a biological point 

 of view, will show us how much alike these varied beings 

 really are. We may compare a one-celled organism, a common 

 plant such as a daisy, and the human body. 



313. Unity in nutrition. In the one-celled animal there is 

 nutrition. This involves the taking in of food, its chemical 

 transformation, and the ultimate assimilation of its usable parts. 



In the daisy there is nutrition. This involves the absorption 

 of carbon dioxid by the cells of the leaves ; it involves further 

 the chemical transformation of the material received, and the 

 ultimate assimilation of some of the product. 



In man there is nutrition. This involves the taking of 

 foreign material into the body, its chemical and physical trans- 

 formation, and the ultimate assimilation of a part of the intake. 



But whereas, in the case of the ameba, the absorption, trans- 

 formation, and assimilation are all carried on by a single cell, 

 the corresponding operations in the daisy and in man involve 

 the work of millions of cells. 



In the matter of nutrition the ameba acts as a unit. The 

 daisy, although made up of many organs and many cells, and 

 the human body, although made up of still more organs and 



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