CHARACTERS OF ORGANISMS. 21 



helps to distinguish the textures of organisms from inorganic 

 structures. Although organic textures are found to differ very 

 widely in their characters, they are all related in one respect, 

 namely, that at some period of their existence they consisted of 

 cells, i.e., minute masses of a substance called Protoplasm. In 

 plants this cellular structure remains obvious in nearly all parts 

 of the adult organism, no matter how much the texture may be 

 modified by adaptation to the requirements of any given duty or 

 function. If we examine with the microscope the leaves, bark, 

 wood, or pith of a plant, in all of them a cellular structure can be 

 recognized. In the less developed members of the animal king- 

 dom, and during the initial stages in the existence of the highest 

 animals, the textures are composed exclusively of aggregations of 

 living cell-elements. We shall shortly see that in the more fully 

 developed condition of the higher animals, the cells become vari- 

 ously modified in form and function, and the protoplasm manufac- 

 tures various structures adapted to the performance of the diverse 

 functions of the different parts. In all organic textures which can 

 be said to be living, cells are dispersed in greater or less number 

 throughout them, and regulate their nutrition and repair. 



2. Chemical Composition. There are no characters in 

 the chemical composition of the textures of organic beings which 

 can be said to be absolutely distinctive or to separate them from in- 

 organic matter. No doubt their chemical construction commonly 

 exhibits certain peculiarities, not seen in dead matter, which may 

 be taken as characteristic, but living textures only differ in the 

 general plan of arrangement and composition from that most com- 

 monly met with in the construction of inorganic materials. 



In the first place, the great majority of the chemical elements 

 which we know of, take no share in the formation of living 

 creatures, and are never found to enter their composition. Prac- 

 tically only fifteen of the sixty-three elements known to chemists 

 take part in making up the tissues of animals. The majority of 

 these are only present in very small quantity and with no great 

 constancy. On the other hand, there are four elements, namely, 

 carbon, oxygen, hydrogen, and nitrogen, which are found with 



