CHARACTERS OF ORGANISMS. 20 



1. Structural Characters of Organisms. The minute 

 structure of living beings as shown by the microscope no doubt 

 helps to distinguish the textures of organisms from inorganic 

 structures. Although organic textures are found to differ very 

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

 namely, that at the earliest period of their existence they consist 

 of a minute mass of a substance called Protoplasm, which is known 

 as a cell. In plants this cellular structure remains obvious in all 

 stages of development of the 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 cell- 

 ular structure can be recognized. In the less developed members 

 of the animal kingdom, and during the initial stages in the exist- 

 ence 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 variously modified in form and function, and 

 the protoplasm manufactures 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 dis- 

 persed 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 

 inorganic matter. No doubt their chemical construction com- 

 monly 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 commonly 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 elements known to chemists take part 

 in making up the tissues of animals. The majority of these are 



