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 littoral 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 facilities—that is, facilities 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 peculiarities of littoral 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- 
