40 ELEMENTARY ZOOLOGY. 



alive. If it is proper to speak of any particular part of 

 the substance being alive, it could only be the proteids. 

 It may be, however, that life is rather the result of the 

 intimate relation and interaction of these various sub- 

 stances than the property of any single one of them. 



It is thus to be seen that protoplasm is complex in its 

 make-up, both chemically and physically. 



53. The Properties of Protoplasm. It is believed 

 that the peculiar activities of living objects arise out 

 of the activities of the cells, of which the organism is 

 composed ; the activities of the cell arise from the activity 

 of the protoplasm which is its essential portion; the 

 special activity of the protoplasm springs from the prop- 

 erties of protoplasm; and the properties of protoplasm 

 are in turn dependent on the chemical and physical 

 constitution of protoplasm detailed in the preceding 

 section. It is believed, therefore, that the differences 

 between organisms, and between the powers of the various 

 cells of the same organism, are due to differences in their 

 different protoplasm physical and chemical differences 

 that we cannot detect by the microscope or by chemical 

 analysis. 



Because the living substance (protoplasm) is so com- 

 plex, chemically and physically, it is unstable and subject 

 to continual change. While this is true in some degree 

 of all substances, it is markedly true of protoplasm. It 

 is so unstable that the very effort to analyze it destroys 

 the particular organization and power that we call life, and 

 decay ensues at once. 



This instability of protoplasm gives rise to a most 

 interesting and important property w^hich we call irri- 

 tability. By irritability is meant the fact that protoplasm 



