WEEDS 205 



animals, especially the pond-snails, concerning which 

 we have unusually full information, exhibit a great 

 range in time, the genera being traced to Wealden 

 or even to Carboniferous times. They survive by 

 isolation, as certain ancient land-animals survive in 

 Australia or New Zealand. It is interesting to note 

 that our fresh- water areas can be successfully invaded. 

 The Anacharis of North America is a familiar instance. 

 The Azolla of North America is now plentiful in the 

 canals of Holland, and may easily spread to other 

 parts of Europe. 



The great land-mass of the northern hemisphere for a 

 long time past seems to have been the usual birth- 

 place of new forms of life. Here severity of competition 

 has created new races, which have spread into the 

 southern lands as opportunity offered, driving before 

 them the original inhabitants, and then themselves 

 becoming unprogressive by reason of their isolation. 



It is probable that since the time of the formation 

 of the Chalk the great oceans have always been pretty 

 much where they now are. There have been normally 

 one or two continents in the northern hemisphere. 

 When, as is now the case, North America has been 

 cut off from Asia, the barrier has not been of a very 

 permanent nature. An elevation of 180 feet would, 

 as Dana remarks, form a land-passage 30 miles 

 wide from Asia to America. The southern hemi- 

 sphere is mainly occupied by sea, but possesses three 

 continents, viz. South Africa, Australia, and South 

 America, which have sometimes existed as islands, and 

 have sometimes been joined to the northern continents. 

 There is no proof that any one of the three has ever 



