216 FRESHWATER FAUNA 



freshwater organisms, besides swift-moving forms, like 

 fish, may have entered a river system directly from the 

 sea. In our islands we might be apt to underestimate the 

 chances in favour of this method ; our rivers where they 

 approach the sea are burdened with sediment, our tides 

 run high, and in estuaries where tide and river meet, a 

 vast quantity of mud is constantly suspended in the 

 water. As a consequence, the tides, though they bring 

 with them flotsam and jetsam from the sea, do not succeed 

 in introducing any additions to our freshwater fauna. It 

 is quite possible to conceive of far more favourable con- 

 ditions than these, and such appear to exist in a number 

 of cases. Dr. von Kennel mentions marine mussels 

 (Mytilus), a small species of Pholas, and a marine worm, 

 Lumbriconereis, as living in the fresh water of the river 

 Ortoire, Trinidad, eight miles above its mouth. Dr. 

 Perceval Wright describes a freshwater Teredo, Nausitora 

 Dunlopi, as occurring in the river Hurreegovga, a 

 tributary of the Ganges ; and we have already called 

 attention to the great jelly-fish, which ascends the Tagus 

 till it meets with fresh water. A sea snail, Natica helioides, 

 which lives off the coast of America, has found its way 

 into the rivers of Mexico. The list might be added to, 

 but these examples will suffice to show that direct 

 introduction of slow-moving marine forms does some- 

 times occur, and even if it happen very rarely, yet rare 

 events scattered through the many millions of years of 

 past time may sum up to a very respectable total. It thus 

 becomes an extremely difficult matter to assign their true 

 relative values to the tw r o principal methods by which the 

 freshwater world has been peopled from the sea. 



Let us now cast a brief retrospective glance over our 

 somewhat discursive argument. The first step taken by 

 the marine organism in qualifying for a freshwater 

 existence is to surmount the obstacle presented by the 



