66 THE NEXT GENERATION 



The list might be made still longer. But no one explained 

 these rudiments of legs, teeth, gill-slits, etc. until Darwin 

 came with his five-linked chain and his proofs. He said that 

 through millions of years unused parts of the body become 

 more and more inefficient, until, in course of time, they have 

 no power left. 



To prove this, think of the ancestral horse and his useless 

 toes. Think of the fish in the dark recesses of Mammoth 

 Cave, Kentucky. They are blind, but they have rudiments 

 of eyes. 



Parasites in particular show the same degeneration from 

 lack of use. One of the most extraordinary among these is 

 »the sacculina, as described by Dr. David Starr Jordan. 



It begins life by looking very much like a young crab. 

 Both creatures have feelers, swimming apparatus, eyes, 

 heart, brain, etc. But soon a change sets in. The crab, on 

 the one hand, goes on developing. Feelers grow longer, 

 brain grows bigger, eyes continue active, the heart never 

 stops beating. In other words, the crab keeps active in every 

 part and grows as it should. Not so with the sacculina. 

 Soon after birth it fastens itself to the body of any conven- 

 ient crab and stays there the rest of its life. It first sends a 

 slender feeler down into the blood stream of the crab. This 

 feeler lengthens each day like a ramifying root. At the same 

 time branches of the root grow in this direction and that until 

 they have entered the entire system of crab blood vessels. 

 And as they ramify, they draw up from the blood of the crab 

 all the nourishment the sacculina needs. Feelers have there- 

 fore no work to do in hunting for food. Eyes are needed 

 no longer. Heart and brain cease to serve. As a result they 

 disappear, one after the other, until the sacculina finds itself 

 nothing but a sac fastened to the body of the crab — a sac 



