EVOLUTION 



1061 



is often used in a less general way for a particular adjustment of 

 structure or function that meets a special need. Thus the chamaeleon 

 has a rapidly protrusible club-shaped viscid tongue, as long as the 

 body in front of the tail, well suited for the capture of insects. It 

 has a prehensile tail, movable in an unusual way — dorso-ventrally, 

 £ind thus well-suited for gripping the branches. Its hand is split 

 into two groups of digits, three to the inside and two to the outside, 

 again well-suited for taking a firm hold of a branch. The foot is 

 similarly split, but it illustrates an interesting reciprocity of adjust- 

 ment in having two toes to the inside and three to the outside. The 

 pigment-cells or chromatophores in the skin are quick to expand 



Fig. 181. 



Regeneration in a Starfish. A separated arm (A), showing the ambulacral 

 groove (G) is regrowing the four missing arms (R). This is popularly- 

 called a "comet" form of starfish. 



or contract, thus altering the chamaeleon 's colour, sometimes at 

 least protectively — another adaptation. These are instances of the 

 many particular adaptations with which the chamaeleon 's body is 

 equipped, and in which it differs from other lizards, which have 

 other adaptations of their own. So it is convenient to distinguish 

 such special adaptations from more general phj^siological characters, 

 such as cold-bloodedness, or from structural peculiarities, such as 

 the laterally-compressed skull, to which no particular adaptive 

 significance can be assigned. From unicellular organisms onwards 

 there is a general adaptiveness, so fundamentally characteristic; 

 and we see also the resulting general adaptations common to large 

 groups of organisms, which are distinguishable from special 

 adaptations, such as are illustrated by the chamaeleon 's tongue, 

 tail, etc. 



