GLASS-SPONGE COLONIES OF THE DEVONIAN 37 



8. Their departure from the Mesozoic epicontinents came with 

 the opening of the Tertiary. They have never returned to epi- 

 continental waters. 



9. Today their descendants present a wide vertical range in the 

 ocean waters, indeed an extraordinary adaptation through 2,800 

 fathoms. The deepest water forms are hopelessly isolated — ^they 

 cannot cHmb the hill back to the zones of evolution and they will be 

 as they are for future generations of observers. It looks as though 

 all of them wete travehng down to the depths; in which case the 

 race will become stabilized and "immortal." 



10. The history then is one of cycles of migration and develop- 

 ment, of compelHng impulses governing the former and probably 

 inducing the latter. 



