136 A NATURALIST IN TASMANIA ch. 



water shrimp ; in fact many of them belong 

 actually to the same genus (Gammarus), while the 

 majority approach rather more closely to the 

 blind Well-shrimp (Niphargus), which is frequently 

 found in wells, artesian springs, and the other 

 subterranean waters of the northern hemisphere. 

 Now although these and closely related forms 

 swarm in the rivers and lakes of the northern 

 hemisphere, and again in the temperate and Alpine 

 regions of southern Australia, there is no single 

 instance of a true freshwater Gammarus or of 

 any closely related genus occurring in the tropics. 

 Although we know of no records of Gammarus from 

 high elevations in tropical South America nor in 

 the temperate regions of that continent, there can 

 be small doubt that they have reached southern 

 Australia from the northern hemisphere by the 

 route across Antarctica and not through the tropics 

 of the old world. 



In Lake Titicaca, situated, it is true, in the tropics 

 but at an elevation of many thousand feet on the 

 Andes, freshwater Amphipods of the genus Hya- 

 lella occur ; and this again has its only closely allied 

 representatives in the extremely abundant Chil- 

 tonia of Victoria, Tasmania, and New Zealand. 



Another interesting piece of evidence is afforded 

 by some of the minute Entomostracous Crustacea 

 which form so important a part of the planJctoUy 

 or floating organisms, of freshwater lakes and 

 ponds. Of course many of these little creatures, 

 especially those which inhabit small pieces of 



