NATURAL SELECTION AND ENVIRONMENT 155 



only one example where a thousand could be given. 

 Place is not long reserved for mere cumberers of the 

 ground. There are so few exceptions to this state- 

 ment that we might almost call it a law of biology. 

 Let us see how it fares with an animal which retreats 

 to a lower plane of life. A worm, rather than seek its 

 own food, becomes a parasite. It degenerates, but still is 

 easily recognized as a worm. A crustacean tries the 

 same experiment, though living outside of its host in- 

 stead of in it. It sinks to a place even lower, if possi- 

 ble, than that of the parasitic worm. A locomotive form 

 becomes sessile. It loses most of its muscles and the 

 larger part of its nervous system ; and even the di- 

 gestive system, which it has made the goal of its exist- 

 ence, is inferior to that of its locomotive ancestors and 

 relatives. But to the vertebrate these lowest depths of 

 stagnation and degeneration are, as a rule, impossible. 

 From true fish upward parasitism and sessile life are 

 practically impossible. Here stagnation and degenera- 

 tion mean, as a rule, extinction. Of all the relatives of 

 vertebrates back to worms only the very aberrant lines 

 of amphioxus and of the tunicata remain. Of the 

 rest not a single survivor has yet been discovered. 

 And yet what hosts of species must have peopled the 

 sea. The primitive round-mouthed fishes have prac- 

 tically disappeared. The ganoids survive in a few 

 species out of thousands. The amphibia of the car- 

 boniferous and the next period and the reptiles of the 

 mesozoic have disappeared ; only a few feeble degen- 

 erate remnants persist. And this was necessarily so. 

 Each advancing form crowded hardest on those which 

 occupied the same place and sought the same food, that 

 is, the members of the same species. And the first to 



