254 



GENERAL BIOLOGY 



similar ones, are responses to external stimuli of 

 various sorts, although we have no knowledge of the 

 means by which the stimulus brings about its result- 

 ant reaction. Such reactions, however, differ in an 

 important way from those mentioned in the previous 

 section in that they are advantageous to the organism 

 exhibiting them, often even to 

 the extent of determining its 

 preservation from destruction. 

 A striking example of adaptive 

 response to environmental 

 change is found in the African 

 chameleon and in its Ameri- 

 can representative, the lizard 

 Anolis, of Florida and the 

 Southern States. This crea- 

 ture displays a wonderful 

 capacity for color change. 

 Normally bronze, it runs 

 through olive to pale green or 

 turquoise blue. This change 

 the is produced by the migration, 

 up and down through the 

 dermis, of black pigment cells. 

 Both heat and light are factors that bring about 

 these color changes. Just to what extent the color 

 changes of Anolis are advantageous to the lizards 

 in causing them to resemble their backgrounds is 

 a little difficult to estimate. There is not much 

 question, however, that in some other forms, in 

 which the changes are accomplished more slowly, 



FIG. 87. Anolis, 

 American chameleon. (Cole- 

 man.) 



