ADAPTATION 



213 



both this type and distinctly adaptive characters, both of which 

 must be explained by any theory of evolution if the theory is to 

 be adequate. 



We cannot, however, avoid the belief that there is stimulus and 

 response in all organic conditions. Non-adaptive characters 

 therefore serve to emphasize the importance of the internal environ- 

 ment, in response to which parts of an organism may attain even 

 harmful development with regard to external conditions. But 

 even in these cases, inherent powers of development can be real- 

 ized only if the proper conditions surround the part. 



The chisel-like incisors of rodents are an evidence of these rela- 

 tionships. Ordinarily those of the upper and lower jaws are ex- 

 actly opposed to each other 

 so that they wear away equally 

 and maintain a constant 

 length and position. Several 

 cases are on record of the 

 serious effects of loss of one 

 incisor; one of these is illus- 

 trated in Figure 123. The 

 lower incisor in this case was 

 broken and the opposed upper 

 incisor, continuing its normal 

 growth without an}^ com- 

 pensating wear, finally penetrated the brain and caused the death 

 of the animal. This case involves definitely adaptive structures 

 but the fate of the animal was due entirely to the power of growth 

 inherent in its own body and to the removal of an influence 

 normally supplied by its body. The abnormal growth of the 

 upper incisor was in no way adaptive, but it was due to definite 

 responses no less than truly adaptive structures. 



Adaptation to the Physical Environment. Such adaptation is 

 closely linked with the three major habitats, water, earth and air. 

 An organism may be adapted to two or to all three, but in any 

 case it shows its fitness b}^ structural characters. The loon, for 

 example, is highly developed for aquatic life. Its feet are effective 

 for swimming, but they also enable it to move about on land 

 as a terrestrial organism. Like most birds, it is also able to 

 fly, and the volant adaptations of birds surpass those of all other 

 animals. 



Fui. 124. — Gonionemus, a hydrozoan 

 jelly-fish. (From Hegner, after Har- 

 gitt.) 



