RESPONSES OF POIKILOTHERMS TO VARIATIONS 23 



that of its environment it will radiate energy more than 

 it absorbs from it, but if it be at a lower temperature, 

 it will absorb more than it will radiate. The total emis- 

 sive and absorptive power of a body is directly propor- 

 tional to the area of the surface. A perfectly black body 

 will absorb all radiant energy that falls on it ; and, since 

 a body cannot absorb more than is incident on it, the 

 black body is considered as unity. There are no animals 

 which have a skin that is perfectly black and conse- 

 quently any animal can absorb only a part of the radiant 

 energy incident. Likewise there are no perfectly white 

 animals and therefore all must absorb some radiant 

 energy. However, there are a few animals, such as the 

 marine Coelenterata, which are such good transmitters 

 of visible light that the radiant energy absorbed by them 

 is negligible. Buxton (1923) gives an admirable account 

 of the color of desert animals and emphasizes the im- 

 portance of color as an adaptive feature to physical 

 rather than organic environmental conditions. Desert 

 reptiles with a scaly integument reflect most of the light 

 that falls upon them and thereby resist to a large extent 

 the radiant energy of the sunlight. 



Pigmentation plays an important part in the gain and 

 loss of heat in animals. For example a turtle, with its 

 highly pigmented shell, may, when basking in the sun, 

 have a body temperature higher than that of the. atmos- 

 phere about it. The junior writer has performed aa ex- 

 periment which demonstrates this. A turtle was placed 

 on a table in the laboratory beneath a source of radiant 

 energy (an infra-red generator) and its resulting body 

 temperatures are shown in Fig. 2. It will be observed 

 that, while the atmosphere was not warmed appreciably, 

 the temperature of the turtle rose constantly to a point 

 higher than that at which a turtle could live. It appears. 



