318 ANIMAL BIOLOGY 



medium in which they live into alcohol and carbon dioxide. This 

 process of alcoholic fermentation may be approximately ex- 

 pressed by the formula: 



CeHiA (sugar) + Yeast = 2 C 2 H 5 OH (alcohol) + 2 C0 2 



The explanation is not far to seek. Deprived of an adequate sup- 

 ply of air, Yeasts resort to the energy released when, with the de- 

 composition of the sugar, the carbon and oxygen unite as C0 2 . 

 The formation of alcohol by the remnants of the sugar molecules 

 is, from the standpoint of the Yeasts, a mere incidental factor 

 which is, so to speak, unavoidable. On the other hand, from the 

 broad viewpoint, the waste products of the action of the Yeast 

 plants' enzymes represent an important phase in the general 

 simplification of organic compounds in nature. And Man turns 

 to account in numerous ways both products of the Yeasts' de- 

 structive powers — the alcohol and the carbon dioxide. 



Thus the Yeasts are practically independent of free oxygen and 

 in this they agree with many kinds of Bacteria as well as some 

 animals, chiefly parasitic worms, which are able to secure the 

 necessary energy by the rearrangement of the atoms within, or 

 the disruption of molecules containing oxygen. Indeed, certain 

 species of Bacteria not only do not need free oxygen at all, but are 

 killed when it is present in any considerable amount. All such 

 organisms are termed anaerobes. A common example is Clos- 

 tridium tetani which inhabits garden soil and street dust and pro- 

 duces tetanus, or lockjaw, in Man and certain domesticated ani- 

 mals when it gains entrance and develops in the tissues. 



Temperature. Although protoplasmic activity is restricted to 

 ranges of temperature which do not seriously interfere with the 

 chemico-physical processes involved, it is a commonplace that 

 various species are adapted to different degrees of temperature. 

 The great majority of organisms, however, find their optimum 

 temperature between 20° C. and 40° C, though species inhabiting 

 the polar and tropical regions show adaptations to the temperature 

 extremes of their surroundings. As a matter of fact, it is not possible 

 to state the upper and lower limits beyond which active life ceases, 

 but some Protozoa are known to multiply in the water of hot 

 springs, certainly at temperatures higher than 50° C, and others 

 in water until freezing actually occurs. 



Many of the Bacteria and Protozoa develop protective cover- 



