394 GENERAL PRINCIPLES 



"fire flies." Here the males are always good fliers, and in some 

 species the females are also, but in other species the females 

 have rudimentary wings or the wings are entirely wanting. 



798. Among plants the rudimentary leaves and other organs 

 to which frequent reference has been made are further examples 

 of the principle under discussion. 



799. In the first series of examples cited in this section we 

 saw how organs adapted for such diverse purposes as swimming, 

 walking, flying and writing may be constructed on a common 



FIG. 246. Hibernia defoliaria. The female is wingless. See preceding 

 figure. A, Male; B, female. Xi 1/2. 



plan. No plausible explanation for this remarkable fact has 

 ever been offered except that of a common origin. If these 

 animals had a common pentadactyl ancestor the present 

 diversity as to the condition of their appendages is the result 

 of modification in different directions as a result of different 

 conditions of environment. 



800. The vestigeal organs also can only be accounted for 

 on the supposition that the ancestral forms possessed the organs 

 in a functional condition, and that changed conditions, involv- 

 ing a disuse of the organs, resulted in their degeneration. This 



