ON SHALLOW WATER FAUNAS. 41 



certain groups have pelagic members. These facts, taken in 

 conjunction with the further fact that pelagic animals are, as 

 a rule, highly modified in accordance with their habits, point 

 strongly to the pelagic fauna being a derived, and not a 

 primitive, one ; and if this be true, they must be derived 

 from the shallow water fauna, which would thus become the 

 most primitive of the faunas, and the parent of all the others. 

 On the other hand, we have the fact, already alluded to, 

 that the larvae and young of almost all marine animals are 

 free swimming, and if these represent ancestral forms, as by 

 the Recapitulation Theory we are compelled to suppose they 

 do, then we must conclude that the ancestors of all marine 

 animals were free swimming, presumably pelagic forms. It 

 seems scarcely possible to regard these larval forms as 

 secondarily acquired, and hence the conclusion to which we 

 are led is that, while existing pelagic animals have probably 

 with few exceptions acquired pelagic habits secondarily, yet 

 that the most primitive animals were primarily pelagic, and 

 that from them have sprung the shallow water fauna, from 

 which in turn the others have been derived later on. 



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