PARASITISM AND DEGENERATION 367 



the wings. Insects that live in caves are mostly blind; they 

 have lost the eyes, whose function could not be exercised in the 

 darkness of the cave. Certain island-inhabiting insects have 

 lost their wings, flight being attended with too much danger. 

 The strong sea breezes may at any time carry a flying insect 

 off the small island to sea. Probably only those which do not 

 fly much survive, and so by natural selection wingless breeds 

 or species are produced. Finally, we may mention the great 

 modifications of structure, often resulting in the loss of certain 

 organs, which take place to produce protective resemblances 

 (see Chapter XIX). In such cases the body may be modified 

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

 ment, and thus the animal may be unperceived by its enemies. 

 Many insects have lost their wings through this cause. 



When we say that a parasitic or quiescent mode of life leads 

 to or causes degeneration, we have explained the stimulus or 

 the ultimate reason for the degenerative changes, but we have 

 not shown just how parasitism or quiescence actually produces 

 these changes. Degeneration or the atrophy and disappear- 

 ance of organs or parts of a body is often said to be due to dis- 

 use. That is, the disuse of a part is believed by many natural- 

 ists to be the sufficient cause for its gradual dwindling and final 

 loss. That disuse can so affect parts of a body during the life- 

 time 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 in- 

 heritance must be assumed if disuse is to account for the gradual 

 growing less and final disappearance of an organ in the course 

 of many generations. Some naturalists believe that the results 

 of such disuse can be inherited, but as yet such belief rests on 

 no certain knowledge. If characters acquired during the life- 

 time of the individual are subject to inheritance, disuse alone 

 may explain degeneration. If not, some other immediate 

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



We are accustomed, perhaps, to think of degeneration as 

 necessarily implying a disadvantage in life. A degenerate 

 animal is considered to be not the equal of a nondegenerate 

 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 active, keen-sighted, 

 highly organized nondegenerate in free competition. But free 



