DEGENERA TION. 3 3 



or in imagination) that we speak of that animal as 

 an instance of Degeneration. 



Any new set of conditions occurring to an animal 

 which render its food and safety very easily attained, 

 seem to lead as a rule to Degeneration ; just as an 

 active healthy man sometimes degenerates when he 

 becomes suddenly possessed of a fortune ; or as Rome 

 degenerated when possessed of the riches of the 

 ancient world. The habit of parasitism clearly acts 

 upon animal organisation in this way. Let the 

 parasitic life once be secured, and away go legs, 

 jaws, eyes, and ears ; the active, highly-gifted crab, 

 insect, or annelid may become a mere sac, absorbing 

 nourishment and laying eggs. 



Reference was made above to the larval stage of 

 a certain shrimp (Figs. 4, 5, 6). Let us now compare 

 these with the young stages of a number of shrimp- 

 like animals, viz., Sacculina, Lernaeocera, Lepas, 

 Cyclops, Limnetis, (all drawn in Fig. 4), some of 

 which lead a parasitic life. All start equally with 

 the recapitulative phase known as the Nauplius ; 

 but whilst the Nauplius of the free-living shrimp 

 grows more and more elaborate, observe what 

 happens to the parasites — they degenerate into 



D 



