136 The Study of Animal Life part h 



Since the ultimate analysis of the objective side of life 

 seems to show that life is to be pictured as matter in an 

 unstable and constantly altering condition, it will be of 

 interest to find the conditions that determine which of the 

 elements are to take part in it. 



It seems that matter in order to enter into life must be — 



(1) Common, (2) mobile, that is capable of easily 

 entering into solution or becoming gaseous, and (3) capable 

 of forming many combinations with other elements. 



Nine .of the elements fulfil the first condition ; a tenth, 

 nitrogen, is the chief constituent of the atmosphere ; 

 while hydrogen is present everywhere in water. Why do 

 not aluminium and silicon take their share in life ? Because 

 they do not fulfil conditions (2) and (3). Their oxides are 

 quarLz and aluminia, two of the hardest substances known. 

 Emery and ruby are two forms of aluminia ; while the oxide 

 of carbon, the source of all the carbon used in life, is a gas. 



Carbon, which'takes so great a part in life processes that 

 the chemistry of organic substances is commonly spoken of 

 as the chemistry of the compounds of carbon, fulfils all three 

 requirements in an eminent degree: For although in its 

 pure form a solid, and sometimes a very hard substance, yet 

 it readily forms an oxide which is present in the atmosphere, 

 and, as we know, serves as one of the chief foods of plants. 

 Its power of entering into combination with other elements 

 is practically infinite. Nitrogen, although by itself an inert 

 form of matter, is able to combine with carbon compounds 

 and add fresh complexity. 



It is easy to see why water is so important in life. It 

 dissolves the other substances, and so allows them to come 

 into closer contact, and to change in position more easily, 

 than if they were solid. So the first stuff that was complex 

 and unstable enough to be properly described as living was 

 almost certainly formed in water, long ago, when the condi- 

 tions of greater heat, and consequently greater mobility of 

 all substances, made chemical changes more active. 



The importance of the solvent power of water in a com- 

 plex organism is obvious when we think of the blood, the 

 great food stream and drain. It is shown in an interesting 



