THE ANIMALS OF LAKES AND RIVERS 189 



marine animals. Tides and currents sweep the members 

 of the plankton hither and thither, but the regions 

 into which they are swept may be more suitable than 

 those they have left — ^in the case of the Mttoral animals 

 the currents are the great distributors. But for a lake 

 or river animal to be swept out to sea must in the 

 general case mean death, for it means the passage from 

 a specialized environment to which the animal has 

 become adapted through progressive variation, to 

 another to which it has no adaptations. 



In the nature of things littoral animals must have 

 always had special faciMties — that is, faciHties in regard 

 to space — ^for conquering fresh water. So far as their 

 relations in space go, it is relatively easy for them to 

 progress, actively or passively, from the shore up 

 rivers and estuaries. But we have already seen that 

 it is one of the great pecuKarities of Httoral animals 

 that they tend to produce free-swimming young, or 

 free-swimming stages, whose purpose it is to ensure 

 distribution along the shore, and the existence of these 

 free-swimming stages or young offers a serious obstacle 

 to the colonization of fresh water. For example, in 

 some artificial ponds at the mouth of the Ganges 

 a very curious fauna has been described {Records Indian 

 Museum, i. (1907) p. 35). These ponds are sometimes 

 connected with the sea, and sometimes shut off from 

 it, and are sometimes filled with water rendered very 

 salt by evaporation, while at the rainy season the water 

 is nearly fresh. In spite of these variations in salinity, 

 a number of marine forms occur, including a sea- 

 anemone, a kind of sea-fir (Hydromedusan), and a 

 polychaete worm, showing that it is not its freshness 

 alone which prevents such forms from colonizing fresh 

 water. Now sea-anemones produce usually free- 



