128 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



that of Pleurocystis. Originally descended from a free form, a stat- 

 ozoic existence was acquired, which was maintained long enough to 

 impress a degree of radial symmetry upon the theca, and for the 

 acquisition of a jointed column. In the decadence of the stock a 

 prostrate existence was gradually assumed, and the animals crawled 

 about, dragging an all but useless stem behind them. Such a failure 

 as regards the majority of the Cystidea well illustrates the fate of any 

 major tendency becoming effective in a large body of organisms. 

 When a group of animals tends to assume a decided change in struc- 

 ture and habit of life, the tendency becomes elTective in varying 

 degrees among different genetic lines. One or two lines, perhaps, out 

 of the whole group will carry the innovation through successfully. 

 The other members equally affected by the tendency, but incapable 

 of pushing it to a successful issue, form short-lived lines that as a rule 

 show obvious indications of degeneracy. 



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