EVOLUTION 



1065 



Static and Kinetic. — Among structural adaptations a first 

 distinction may be drawn between the static or passive and the 

 kinetic or active. The internal structure of a long bone — take the 

 thigh-bone — is arranged in a manner that arouses the admiration 

 of the constructive engineer: its struts and stays of bone are dis- 

 posed so that they meet the most frequent strains to which the leg 

 is exposed. This is a static or passive adaptation. 



The massive encasement of a tortoise (carapace and plastron) 



Fig. 184. 



Protective Resemblance in Kallima Butterfly. From a specimen. K, the 

 butterfly seated among the leaves, which it so closely resembles in its 

 shape and markings. 



built on the principle of an arch strengthened by the addition of 

 a transverse base, has an extraordinarily intricate mingling of 

 (a) true endoskeleton (e.g. vertebrae and ribs), {b) sub-cutaneous 

 skeleton (the bulk of the plastron), (c) cutaneous skeleton (the 

 vertebral and costal scutes), and (d) the ectodermic skeleton of horny 

 scales. We mention this fourfold detail because it illustrates the 

 manifoldness of what is, after all, just a rigid box enclosing the 

 animal body. The tortoise is almost invulnerable ; and it can retract 

 head, tail, and limbs within the shelter. A very effective adaptation, 

 yet wholly passive; a fortress, in fact. 



