198 ANIMAL LIFE 



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. 



104. 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. Such inheritance must be assumed if 

 disuse is to account for the gradual growing less and final 

 disappearance of an organ in the course of many genera- 

 tions. Some naturalists believe that the results of such 

 disuse can be inherited, but as yet such belief rests on no 

 certain knowledge. If characters assumed during the life- 

 time of the individual are subject to inheritance, disuse 

 alone may explain degeneration. If not, some other imme- 

 diate cause, or some other cause along with disuse, must 

 be found. Such a cause must be sought for in the action of 

 natural selection, preserving the advantages of simplicity of 

 structure where action is not required. 



105. Advantages and disadvantages of parasitism and de- 

 generation. We are accustomed, perhaps, to think of degen- 

 eration as necessarily implying a disadvantage in life. A 

 degenerate animal is considered to be not the equal of a non- 

 degenerate animal, and this would be true if both kinds of 

 animals had to face the same conditions of life. The blind, 

 footless, simple, degenerate animal could not cope with the 



