EVIDENCES OF EVOLUTION 



67 



with root processes and reproductive organs. Everything else 

 is gone, and, as the illustration shows, the parasite finally 

 looks more like a growth of the crab itself than like a sepa- 

 rate creature. The sacculina lives as long as the crab lives, 

 and when the crab dies, it must die too. 



Now this record is simply an illustration of the result of 

 disuse. Somewhat the same thing happens in other animals. 



Sacculina on the Abdomen of a Crab 



Notice how the root processes of this parasite ramify through the legs and 

 the body of the crab. (Adapted from Parker and Haswell) 



In many cases, however, unused parts remain as mere rudi- 

 ments through later generations. Darwin makes three state- 

 ments concerning this whole matter, which may be condensed 

 as follows : 



1 . All organs degenerate by disuse. 



2. As any race of animals grows more and more success- 

 ful in the struggle for existence, it develops new organs and 



