20 ZOOLOGY 



will soon put a stop to life. Such products are termed 

 excreta, and the process of getting rid of them is known as 

 excretion. The commonest substance which is excreted is 

 the gas known familiarly but incorrectly as carbonic acid. 

 This gas is very soluble in water, and as all living substance 

 is bathed in water or some other fluid (i.e. blood) largely 

 composed of water, it can be seen that ever opportunity is 

 given for the removal of this compound. Lastly, when an 

 animal takes in solid food and proceeds to digest it, there 

 often remains a residue which resists the digestive juice. 

 This residue is termed faeces, and the process of getting rid 

 of it is called defecation. 



Since oxygen can only be taken into the living substances 

 and the poisonous excreta got rid of by the process of diffusion, 

 it follows that living substance can never be accumulated in 

 large masses, but can only exist in the form of small granules, 

 or of thin plates presenting relatively large surfaces to a 

 circumambient fluid of some kind. By diffusion we mean the 

 process by which two liquids or two gases become commingled, 

 in consequence of the motion of the molecules of which they 

 are made up. In the case of liquids this is always a slow 

 process ; and a solid mass of protoplasm a quarter of an inch 

 in diameter, if such a thing existed, could no more be living 

 throughout its whole extent than a block of coal could be 

 burning throughout its whole extent. In both cases com- 

 bustion could only exist at the surface of the respective 

 masses. 



The task of finding out the physical and chemical structure 

 of living substance is one which ought properly to form part 

 of zoology, but one which, on account of its difficulty and the 

 amount of concentrated effort which it requires, has caused 

 the growth of a subsidiary science devoted to its solution alone. 

 This science is termed Biochemistry. We may briefly indi- 

 cate some of the points which the devotees of this young 

 science have already gained in their struggle with the mystery 

 of mysteries. Since life is a fire, and since this fire requires 

 the constant diffusion of oxygen into the living substance and 

 of carbonic acid out of it, living substance must be a fluid, since 

 only in fluids and gases can diffusion exist. All living substance 

 contains large quantities of water and all life is suspended by 



