6 PHYSIOLOGY 



naked protoplasm of the plasmodium of Myxomycetes, if placed on a piece 

 of wet blotting-paper, will crawl towards an infusion of dead leaves, or 

 away from a solution of quinine. It is the same property of adaptation, 

 the deciding factor in the struggle for existence, which impels the greatest 

 thinkers of our time to spend long years of toil in the invention of the means 

 for the offence and defence of their community, or for the protection of man- 

 kind against disease and death. The same law which determines the down- 

 ward growth of the root in plants is responsible for the existence to-day of 

 all the sciences of which mankind is proud. 



This " adjustment of internal to external relations " is possible, however, 

 only within strictly denned limits, limits which increase in extent with rise 

 in the type of organism, and in the complexity of its powers of reaction. 

 Some of these limiting conditions we shall have to study in the next chapter. 

 Among the chief of them are temperature, and the presence of food material 

 and of oxygen. At the present time the limits of temperature may be 

 placed between and 50 C. Many organisms, however, are killed by the 

 alteration of only a few degrees in the temperature of their environment. 

 Every shifting of a cold or warm current in the Atlantic, in consequence of 

 storms on the surface, leads to the destruction of myriads of fish and other 

 denizens of the sea. In the higher animals a greater stability in face of such 

 changes has been accomplished by the development of a heat-regulating 

 mechanism, so that, provided sufficient food is available, the temperature 

 of the body is maintained at a constant level, which represents the optimum 

 for the discharge of the normal functions of the constituent parts of the 

 body. The presence of food material in the environment of the living 

 organism is a necessary condition for its continued existence. In some cases, 

 and this we must assume to be the primitive condition, the food material 

 must be of a given character and form a constant constituent of the sur- 

 rounding medium. In the higher forms, the development of a complex 

 digestive system has enabled the organism to utilise many different kinds of 

 food, while the storage of any excess of food as reserve material, either in 

 the form of fats or carbohydrates, provides for a constant supply of food 

 to the constituent cells of the body, even when it is quite wanting in the 

 environment. Since plants depend for their food in the first place on the 

 carbohydrates produced within the chlorophyll corpuscles out of the 

 atmospheric carbon dioxide by the energy of the sun's rays, necessary 

 conditions for their existence will be sunlight and the presence of this 

 gas in the surrounding atmosphere. 



One other necessary condition for the existence of life is the presence of 

 water. Although this substance cannot furnish any energy to the complex 

 molecules of which the living matter is composed, it is an essential con- 

 stituent of all living matter, and takes part in all the changes which deter- 

 mine the transformations of matter and energy in the organism. 



This short summary of the chief characteristics of living beings would 

 be incomplete without the mention of what is perhaps their distinctive 

 feature, namely, organisation. Although little marked in the lowest members 



