V A GLIMPSE INTO THE PAST 137 



water liable to dry up, have special facilities for 

 distribution, such as resting eggs, which can be 

 transported by birds or wind in a dry state without 

 harm, but others, which habitually live in large 

 sheets of water, are not so easily dispersed. One 

 of the commonest elements in the plankton of the 

 Tasmanian lakes is the little Bosmina, which is 

 very abundant in the northern hemisphere and in 

 South America, but appears to be entirely absent 

 from the lakes of the tropical old world. Another 

 of the commonest plankton organisms in the Alpine 

 lakes and tarns of Tasmania is the Copepod 

 Boeckella, which displaces the northern genus 

 Diaptomus, and this genus is again represented 

 in temperate South America and New Zealand, 

 nowhere spreading into the tropics. 



These are the most striking instances among 

 the freshwater Crustacea of creatures confined to 

 temperate or Alpine stations which are widely 

 dispersed over the southern hemisphere, and 

 certainly cannot have reached southern Australia 

 through the tropics from the north. 



The freshwater fishes of the southern hemi- 

 sphere, about whose distribution something was 

 said in the last chapter, are another group confined 

 to the temperate regions of the southern hemi- 

 sphere which appear to have reached their present 

 stations in southern Australia and New Zealand 

 from an Antarctic connexion with South America. 

 The little pike-like fishes of the genus Galaxias 

 occur in southern Australia, Tasmania, New 



