364 What are Animals and Plants ? 



IV. In every animal and plant these four elements (oxy- 

 gen, hydrogen, carbon, and nitrogen) unite to form a special 

 substance known as protoplasm, of which every living organ- 

 ism is at first entirely composed, while the Avhole inorganic 

 world is destitute of such material. 



This curious substance, while living, has six very remark- 

 able powers : — 



(1.) A power of internal circulation, or of the movement 

 of various parts of its substance within the whole, unlike any- 

 thing in the inorganic world. 



(2.) A power of contraction and expansion under condi- 

 tions different from those which contract and expand inor- 

 ganic substances. 



(3.) A power of performing chemical changes and evolving 

 heat more gently and continuously than in the combustion of 

 inorganic bodies. 



(4.) A power of converting other adjacent substances into 

 material Uke itself — into its own substance. 



(5.) A power of forming from its own substance substances 

 both different from its own and from substances adjacent to 

 it. Thus it is that since every living creature consists at 

 first entirely of protoplasm, every other kind of substance 

 found in every animal or plant comes from protoplasm and is 

 formed by its agency. 



(6.) A power of exchanging gases with its environment — 

 notably of absorbing oxygen and giving out carbonic acid. 



These exclusively vital powers of living particles of pro- 

 toplasm give to each whole organism of which they form a 

 part certain further characters by which they aU differ from 

 the inorganic world. Thus : — 



y. Every living creature, whether plant or animal, affects 

 that interchange of gases just mentioned (absorbing oxygen 

 and giving out carbonic acid), that is to say, it respires or 

 breathes — whatever other changes it may affect. 



