140 W. K. BROOKS. 



If this were a solitary case it would not deserve notice ; but ex- 

 amination will show that no highly organized animal has ever been 

 evolved at the surface, although all depend on the pelagic food- 

 supply. 



The animals which now find their home in the o])eu waters of 

 the ocean are, almost without exception, the descendants of forms 

 which live upon or near the bottom or along the sea-shore or upon 

 the land, and the exceptions are all simple animals of minute size. 

 The metazoa which are primitively pelagic, that is, those which 

 have been pelagic throughout their whole history and do not owe 

 their structure to competition with improved forms from the bottom 

 or the shore, are astonishingly few, and these few are among the 

 smallest and simplest of the metazoa. 



It is only necessary to review the chief groups of metazoa in 

 order to perceive that most of their pelagic representatives exhibit 

 the clearest evidence of descent from forms which lived upon or 

 near the bottom or the shore. Many indeed have no pelagic mem- 

 bers, but are restricted to the bottom. 



The sponges are obviously a bottom group ; most of them are 

 fixed, all are sedentary, and their whole organization is an adapta- 

 tion for life in the bottom. 



The coral polyps, actinias and alcyonarias, are among the most 

 characteristic bottom forms, and the abundance of the fossil remains 

 of polyp skeletons proves that these animals became established on 

 the bottom very early, and that the whole history of their evolution 

 has taken place at the bottom. The acraspedote medusae are uni- 

 versally and justly regarded as the descendants of fixed polyp-like 

 ancestors, and we may state with confidence that they are not primi- 

 tively pelagic, but that a fixed period in their history has come 

 between the modern swimming jelly-fish and its remote and un- 

 known primitive pelagic ancestor. 



The veiled medusae are usually held to have had a similar history, 

 but I shall soon give my reasons for holding that some of these at 

 least are primitively pelagic. There can, however, be no doubt 

 that the evolution of hydroid cormi has taken place at the bottom. 

 The siphonophores are descended from ancestors like the antho- 

 medusae, and the various families and genera and species of siph- 

 onophores have most certainly been produced by divergent special- 



