COMMENSALISM AND PARASITISM 349 



in color and shape so as to resemble some part of the envi- 

 ronment, and thus the animal may be unperceived by its 

 enemies. Many insects have lost their wings through this 

 cause. 



279. Immediate causes of degeneration. When we say 

 that a parasitic or quiescent mode of life leads to or causes 

 degeneration, we have explained the stimulus or the ulti- 

 mate cause of degenerative changes, but we have not 

 shown just how parasitism or quiescence actually produces 

 these changes. Degeneration or the atrophy and disap- 

 pearance of organs or parts of a body is often said to be 

 due to disuse. That is, the disuse of a part is believed by 

 many naturalists to be the sufficient cause for its gradual 

 dwindling and final loss. That disuse can so affect parts 

 of a body during the lifetime of an individual is true. A 

 muscle unused becomes soft and flabby and small. Whether 

 the effects of such disuse can be inherited, however, is open 

 to serious doubt. If not, some other immediate cause, or 

 some other cause along with disuse, must be found. Such 

 a cause must be sought for in the action of natural selec- 

 tion, preserving the advantages of simplicity of structure 

 where action is not required. 



