BARRIERS AND MEANS OF DISPERSAL 61 



Remarkably long distances by water connect portions of different 

 drainage basins which are separated by only a few miles of land. 

 Thus in New York, Lake Chautauqua of the Mississippi River drain- 

 age is about 125 miles from Lake Seneca of the Atlantic drainage, yet 

 to go from one to the other by water would require traversing thou- 

 sands of miles, almost half of which would be through salt water. 

 Before the opening of the Chicago Drainage Canal in 1900, the water 

 journey from Lake Chautauqua to Lake Erie, which lie eight miles 

 from each other, would have been even longer. 



Dispersal within a body of quiet water meets with little or no 

 hindrance. In running water, the current has an important influence. 

 In strong currents upstream dispersal is made difficult; bottom forms 

 like snails and eels or powerful swimmers like the Salmonidae are 

 able to cope with a current most easily. Suspended animals are af- 

 fected by even a slight current; thus in the Havel lakes near Berlin 

 the lower ones have a larger number of the cladoceran Bosmina 

 coregoni than the upper, since they can spread with the current but 

 not against it. 14 Waterfalls and rapids accordingly form well-marked 

 barriers to dispersal. The salmon is unable to pass the falls of the 

 Rhine, and is therefore absent from Lake Constance. Above the 

 Trollhattanfall in Sweden there were formerly no eels, either in Lake 

 Venern or in the streams draining into it, because the young eels were 

 unable to pass the fall; with the building of locks at the beginning of 

 the nineteenth century this condition was changed. 15 



Passive dispersal plays a very important role in inland waters. 

 Fishes, which are the only active swimmers in fresh water, frequently 

 serve other animals as a means of transport. Thus the larvae of river 

 and pond mussels {Unio and Anodonta) clamp themselves to the fins 

 or gills of various species of fishes, living on them for a short time as 

 parasites, during which period they are transported by their hosts. 

 The marine fishes which enter the rivers of Malaysia bring their para- 

 sitic isopods with them. Thus Rosinella typus, a cymothoid, is known 

 from the Gulf of Bengal, the Capuas River in Borneo, and the Sea 

 of Sinkarah. This isopod is known to leave the fish, maintain itself 

 for a time on the bottom, and then attach itself to another, which 

 may be a fresh-water species. The isopods of the family Bopyridae 

 parasitize marine crustaceans (Palaemon and others) , and are carried 

 into rivers by their hosts, where they establish themselves. 16 



The passive distribution of fresh-water animals by flying forms is 

 more frequent and effective. Birds are the principal transporting 

 agents. Eggs and forms in a dormant state become attached to the 

 feet, bills, feathers, and tongues of swimming and wading birds. Such 



