THE PELAGIC DISTRICTS 989 



termingling of fresh water, or limno-pelagic, and salt water pelagic 

 types occurs in the estuaries and stream mouths, and it is notorious 

 that halo-pelagic fish will enter the limno-pelagic district in breed- 

 ing time. It is quite probable, as Sir William Flower suggests, that 

 the Cetacea, in their transition from a terrestrial to a marine life, 

 passed through a stage in which they lived in fresh water. A sim- 

 ilar transition for the sea-grasses is not improbable, though they can 

 no longer live in fresh water. Intercommunication between the 

 abysso-pelagic and pelagic districts also occurs, as well as between 

 the abysso-pelagic and abyssal. 



B. Fresh JVater. The pelagic life of ponds, lakes, and rivers 

 is much less varied than that of the sea, though, as will be seen 

 later, the number of individuals of a given species may often be very 

 great. In ponds and lakes no very distinctive characteristics not 

 found in shallow seas exist, aside, of course, from the distinctions 

 due to the difference in degree and kind of salinity. In rivers, on 

 the other hand, we must consider the importance of currents, which 

 vary greatly in strength, but are essentially constant in direction. 

 It is this constancy which, as Chamberlin has pointed out, favors 

 the development of a special body form, the resistance of which to 

 the current is at a minimum. Such a body form is found in fish 

 and in the ancient Eurypterida, and it is a possibility not to be 

 lightly set aside that both fish and eurypterids originated in the 

 rivers of the Palaeozoic and earlier lands. Indeed, this is all but 

 definitely proven for the Eurypterida (O'Connell-5) and seems to 

 be indicated for the fish as well. Significant in this connection is 

 the fact that the early remains of fish as of eurypterids are not 

 found in normal marine deposits, but in those which are at least 

 open to the suspicion that they are formed by rivers or at least at 

 the mouths of rivers, while the best preserved remains, and the 

 most abundantly represented in the Palaeozoic, are found in river 

 flood-plain deposits and in deltas. This subject will be more fully 

 discussed in the next chapter (Grabau-3). 



C. Terrestrial. The pelagic life of fresh water is, as we have 

 seen, much less abundant than that of the sea, and, again, the aerial 

 life, or that of the pelagic district of the land, is much less prom- 

 inent than that of the streams and lakes. This gradation is in direct 

 correspondence with that observed in the gradation in the density 

 of the medium. Moreover, in the terrestrial realm the pelagic dis- 

 trict is only temporarily inhabited, few, if any, terrestrial animals 

 or plants spending their entire existence suspended in the air. The 

 most familiar examples of animals spending a part of their lives 

 at least in the atmo-pelagic district are : insects among invertebrates, 



