62 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



transport has been confirmed by direct observation for rhizopods, 

 statoblasts of bryozoans, winter eggs of cladocerans, threadworms, 

 rotifers, 17 and eggs of snails. Frog spawn may be kept in the air for 

 considerable periods (up to four days), if the weather be cool and 

 damp, without losing its vitality. 18 The conditions for its transport are 

 therefore favorable. Bivalves occasionally clamp themselves to the 

 webs of ducks' feet and may be transported, if they are not too large. 19 



Transport of tiny animals from one body of water may also take 

 place through the agency of insects. The larval stages of hydrachnids 

 parasitize aquatic beetles and bugs, and are carried away by them 

 on their aerial journeys. Snails of the genus Ancylus have several 

 times been found attached to the wing covers of beetles. 20 Dytiscus 

 marginalis, a water beetle with strong flight, has repeatedly been 

 found with the small bivalve Sphaerium attached to its legs, 21 and 

 Pisidiwn has been found attached to a water bug. The small ostracod, 

 Cyclocypris laevis, is transported in this way by flying insects. 22 Such 

 means of passive transportation are of little consequence to marine 

 animals, on account of their continuous habitat; but they are impor- 

 tant to fresh-water animals, on account of the discontinuity of theirs. 



Fresh-water animals have been known to be carried up by torna- 

 does and deposited at a distance. Gudger 23 has given an excellent 

 resume of authentic "rains of fishes." Other animals such as tadpoles, 

 frogs, salamanders, and mollusks have been transported in this way. 



Barriers and means of distribution for terrestrial animals. — 

 Barriers to distribution are of especial importance for terrestrial ani- 

 mals. Whereas the oceans are connected, the principal land masses 

 are separated by sea water into three huge blocks, Eurasia and Africa, 

 the Americas, and Australia. In addition, there are a great number of 

 islands of all sizes, entirely separated from other land masses. The 

 oceans are thus the principal barriers to the free dispersal of land 

 animals. These separations are bridged at only a few places in the 

 polar seas, where ice may connect various islands; Saghalien, and 

 the polar islands of North America, have thus been connected with the 

 mainland. In general, however, a separation by the sea conditions the 

 distribution of animals, and narrow straits may separate very dis- 

 tinct faunae, as does the Mozambique Channel between Madagascar 

 and Africa which is only 400 km. across. 



Rivers are of much less importance as barriers. Great streams like 

 the Amazon and its tributaries may limit the range of many forms, 

 such as forest birds or butterflies. The La Plata estuary forms the 

 southern limit of the range of the capybara and the northern limit for 

 the viscacha. 25 The south Russian Dnieper separates the ranges of 



