60 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY - 



rents if they attach themselves to a moving object. Barnacles 

 {B alarms and Lepas) are found on driftwood; the bivalve mollusk, 

 Dreissena, attaches itself to wood by its byssus; and even reef corals 

 {Pocillopora) have been found attached to a floating piece of pumice. 11 



Transport by other marine animals may also occur. The ten species 

 of sucking fishes {Echene'is, Remora) , which attach themselves to 

 whales and sharks by means of their large sucking disks, are dis- 

 tributed to all tropical and warm seas by this means, though their 

 own swimming powers are slight. Many marine forms are transported 

 by shipping. Thus the American sea anemone Sagartia luciae was 

 carried along the coast from the south to New Haven and Boston, 

 and thence reached Europe, arriving at Plymouth in 1896, and at 

 Biisum in 1920. 12 



Barriers and means of dispersal for fresh-water animals. — The 

 conditions for direct dispersal are decidedly less favorable for fresh- 

 water animals than for marine forms. All the permanent fresh-water 

 basins are separated by land of greater or less extent. The rivers are 

 for the most part connected with the sea; but migration from one 

 river system to another is not facilitated by this means, since the sea 

 offers as effective a barrier to most fresh-water forms as does the land. 

 Fresh-water animals which can enter the sea may thereby have a very 

 wide range. Nearly all the genera of fresh-water fishes of Africa are 

 different from those of South America, and only those like Arius, 13 

 whose representatives enter the sea, are common to the two sides of 

 the Atlantic. Neighboring river systems are often decidedly different 

 in their fauna on this account, especially if they flow into different 

 oceans, and the watersheds then form the dividing lines between such 

 faunae. Thus the Danube basin is distinguished from that of the Rhine 

 by the difference in the migrating fishes, with different species of 

 sturgeon and salmon,* and the presence of eel and shad in the Rhine 

 which are absent from the Danube. The absence of the sticklebacks 

 (Gasterosteus) in the Danube and the presence there of numerous 

 eastern fishes such as Abramis sapa, Gobio uranoscopus, and Perca 

 volgenses, which are absent in the Rhine, differentiate the Danubian 

 fauna still more. The homing instinct of salmon which brings them 

 back to breed in the streams of their nativity after their sojourn in 

 the sea tends to prevent a mixing of the salmon populations of adja- 

 cent river systems. Conversely, although the Atlantic breeding grounds 

 of the European and American eels overlap, the species are distinct. 



* Acipenser ruthenus and Salmo hucho in the Danube, A. sturio and S. salar in 

 the Rhine. 



