CRUSTACEA OF FRESH WATERS 159 



towards the sea, the smaller feebly swimming forms 

 tend to be swept down with the current, and ulti- 

 mately carried to perish in the sea. It follows that 

 only those forms which possess special adaptations 

 for dispersal are able to flourish in fresh water. In 

 many cases, as will be described below, the eggs of 

 the smaller Crustacea can survive being dried up, 

 and in this state they may be blown about by wind 

 or carried to great distances in mud, adhering to the 

 feet of migratory wading birds. Darwin says : " The 

 wide-ranging power of fresh-water productions can, 

 I think, in most cases be explained by their having 

 become fitted, in a manner highly useful to them, 

 for short and frequent migrations from pond to 

 pond, or from stream to stream, within their own 

 countries ; and liability to wide dispersal would 

 follow from this capacity as an almost necessary 

 consequence " (" Origin of Species," sixth edition, 

 chapter xiii.). In accordance with this, we find that 

 it is just those groups of Crustacea which show these 

 adaptations for dispersal that are most universally 

 distributed in fresh water. On the other hand, the 

 larger Crustacea, like the Crayfishes and River 

 Crabs, which cannot so easily be transported from 

 one locality to another, have as a rule a more 

 restricted range. These larger forms, from their 

 size and powers of swimming or creeping, can make 

 their way upstream and spread throughout a river 

 system, and in some cases they can leave the water 



