52 The Medium 



The American species of eel breeds in an area that overlaps the 

 spawning zone of the European eel. The Gulf Stream circulation 

 similarly plays an essential role in carrying its larvae northward. 

 Nevertheless, no record exists of European eels entering an American 

 river, or of American eels being found in Europe. The metamorphosis 

 of the American eel takes place at the end of one year when it is 

 opposite the American shore. At this time it is ready to respond to 

 some influence which orients it toward the mouths of the rivers. 

 However, its European cousin, mingled with it in the plankton, is not 

 yet responsive to shore influences and continues drifting with the Gulf 

 Stream water until it reaches the European coast. The eventual ar- 

 rival of the two species of eel on opposite sides of the Atlantic is a 

 spectacular illustration of the integration of the action of the environ- 

 ment with genetic differences in the timing of responses. 



Harmful Transport 



In our discussion of the favorable results of transport by the medium 

 in some natural situations we must not lose sight of the fact that this 

 influence, like most others, may also produce harmful effects under 

 different circumstances. Some terrestrial organisms benefit by the 

 distribution provided by the wind, but a great many others are carried 

 into regions where they cannot possibly survive. Many land animals 

 and plants are blown out to sea or over lakes, where they fall into 

 the water. Insects and birds permanently established in windy 

 regions have been forced to develop reactions that protect them from 

 being blown away. The insect fauna of islands and mountain 

 regions frequently includes a disproportionate number of wingless 

 species. Although in some circumscribed situations flightless forms 

 may have survived chiefly from lack of need of flight (Darlington, 

 1943), it is probable that certain flying species have been positively 

 eliminated from many exposed islands by wind action. 



Inhabitants of rapid streams are frequently swept down into slug- 

 gish rivers or into lakes in which conditions are unfavorable for them. 

 Fresh-water forms are carried into the ocean, where usually they are 

 quickly killed by the change in salinity. Similar harmful transport 

 occurs in the marine environment. All the Arctic organisms that are 

 carried southward by the Labrador Current are killed when the water 

 in which they are living is mixed with warmer water in the vicinity 

 of the Grand Banks. All tropical phytoplankton and zooplankton 

 which are swept by eddies out of the northern edge of the Gulf Stream 

 succumb as a result of excessive cold. 



