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 aud 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 
tu 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 
