I 8 ZOOLOGY 



showing the effect of some of the more important stimuli on protoplasm (as light, 

 heat, electricity). What of the external conditions are so important as to merit 

 the term "primary conditions of life"? Why may protoplasm be described as 

 chemically unstable? Compare oxidation in the protoplasm with oxidation in 

 ordinary combustion. 



30. Summary. i. Scientists are not agreed whether life 

 is merely the action of the ordinary chemical and physical 

 forces in connection with a peculiar substance, or represents 

 these, guided by a type of energy of a higher order. 



2. Protoplasm, a chemical mixture of exceeding complexity 

 and instability, is the "physical basis of life." Differences 

 in various living things are probably due to differences in the 

 chemical and physical structure of the protoplasm of which 

 they are composed. 



3. Owing to the unstable character of the protoplasm it is 

 readily acted upon and changed by external forces; and the 

 various parts of the protoplasm act on each other in such a 

 way as to produce a display of energy. The agents are called 

 stimuli. Protoplasm responds to stimuli because of its irri- 

 tability and contractility. These latter powers belong natively 

 to protoplasm because of its physical and chemical composition. 



4. Protoplasmic matter and the materials which are de- 

 stroyed in the production of energy are alike produced by the 

 assimilation of food substances into new protoplasm. This 

 is a most fundamental quality. 



5. Growth is increase of mass, following the formation of 

 new substance by assimilation. The mere absorption of water 

 also results in growth. Growth leads naturally to reproduction. 



6. Oxygen is one of the chief agents by which the unstable 

 compounds in the protoplasm are made to release their energy. 

 The breaking down of these compounds leaves unused mate- 

 rials which must be excreted. Respiration, which is a term 

 applied to the using of oxygen and the elimination of carbon 

 dioxid, and excretion are thus seen to be protoplasmic func- 

 tions immediately connected with its activity. 



