THE STAGE SETTING 



II 



Animals are said to be warm-blooded or cold-blooded. The foriiK r 

 term means that they have a constant body temi)erature {honwio- 

 thermal), while the latter means that the body temperature varies 

 with the external temperature {poikilothcnnal) . Frogs can often 

 be frozen stiff and, when thawed out gradually, will live. This is 

 true of many animals and is an undoubted adaptation which enables 

 them to withstand great cold. Homoiothermal animals, however, 

 are more or less independent of the external temperature because 

 their internal body heat remains at a constant temperature regard- 

 less of outside fluctuations. 



Animals are divided into two groups depending on whether they 

 can easily stand changes in external temperature, some being 

 restricted to a relatively narrow range of temperature changes {steno- 

 thermaV), while others have not only the ability to withstand a large 

 range of temperature, but also may become acclimated to new tem- 

 perature ranges if they are changed gradually from one environment 

 to another {eur y thermal) . A classic series of experiments by Dallinger 

 with protozoans showed that he could change their li\'ing conditions 

 from 15.6° to 70° C. without having the animals die. It is this ability 

 that gives us the plant and animal populations in some hot springs. 



Light as a Factor 



Light is a form of radiant energy. Passed througli a prism it is 

 broken up into the primary colors of the spectrum, each of which has 



Left : A nasturtium plant exposed to ordinary Kreeniu...se lif^hl sin.e ^.vnuuMum. 

 Right: Same plant exposed to onr-sideddlunnnal.on for .SIX hours. 



