4IO GENERAL PRINCIPLES 



838. Adaptations with regard to light are much more gen- 

 eral and important among plants than among animals. As 

 touching plants the matter has already been fully discussed. 

 Animals are much less dependent on light, and adaptation with 

 regard to light affects chiefly the eyes. Animals adapted to 

 absolute darkness, such as the fishes, salamanders and cray- 

 fishes of caves have usually httle or no pigment in the skin. 

 The most striking pecuharity of these animals is that they are 

 bUnd and the eyes are usually very rudimentary. The tactile 

 organs, however, are better developed than are those of the 

 normal type. This is especially true of the antennae of the 

 crayfishes. Blind fishes are found in caves in many parts of 

 the world, and they ''belong to many different famihes, but are 

 always related (similar) to the forms living in nearby streams." 

 This fact is strong evidence that the blind forms have descended 

 from the ancestors of those which now live in the surface 

 streams. 



839. Adaptations to Changes of Temperature. — All plants 

 and most animals are directly dependent on the temperature 

 of the surrounding medium, so that growth and other vital 

 processes proceed more or less rapidly in accordance with the 

 changes in temperature of the surrounding water or air. The 

 time required for a frog to develop from the egg, for example, 

 may vary from seventy days at a temperature of 60° F. to two 

 hundred and thirty days at 51° F. The temperature of sea 

 water seldom passes the limits within which vital processes 

 are possible. The temperature of the air varies much more 

 widely, and consequently terrestrial organisms present several 

 types of special adaptations with regard to temperature. All 

 the vital activities of all terrestrial plants and animals, except 

 birds and mammals, are suspended when the temperature falls 

 to or below freezing. Insects, Amphibia and Reptiles usually 

 hide in sheltered recesses at such times and remain dormant until 



