ON THE RIVERS AND CREEKS. 183 



progress there is no difficulty in appreciating the 

 fact that the fittest may survive even by chance. 

 While quite prepared to doubt that fortuitous 

 circumstances alone decide as to what shall or 

 shall not remain, we are willing to allow that a 

 great deal depends on accident. In the Essequebo 

 river are hundreds of islands in all stages of 

 growth, each forming a little world of its own, 

 and covered with dense forest or jungle. Those 

 near to the mouth have been mainly the work of 

 the courida and mangrove, of which we shall have 

 something to say in another chapter, but away 

 from the mingling of sea-water all the others have 

 been gained from the flood by the fortunate 

 deposit of seeds and plants floated down by itself. 

 Most of the forest trees provide their seeds with 

 spongy or cellular pods, by which they are carried 

 away to long distances and deposited by accident 

 along the banks of every river and creek. Were it 

 not for the openings produced by the flood hardly 

 a seed would ever get an opportunity of gaining 

 a position for itself, but as it is the water under- 

 mines one forest giant and carries seed to replace 

 it by another. It almost seems at first as if the 

 mocca-mocca provides a breakwater for the other 

 plants, but when we see that they have been push- 

 ing it farther and farther into the stream for ages, 



