x 288 “  Displacement and Replacement of Associations and Species. 
: - 
Elodea canadensis is extremely common in certain shallow, slow-flowing 
rivers of both the North and South Islands, 
Perhaps the most interesting case is that of Ranunculus aguatilis, which 
species was not noted until 1905, when it was discovered in South Canterbury. 
Since then, it has spread to Central Otago, North Canterbury, the North-eastern 
district and many dune lakes in the southern part of the North Island, its having 
crossed Cook Strait being a convincing proof that its rapid distribution is _ 
owing to water-birds. All plants of disconnected waters must be distributed 
in a similar manner. a 
Mimulus moschatus and M. luteus are frequent in many shallow lowland 
and even lower subalpine streams. Richar.dia africana grows luxuriantly in 
ditches in the Auckland and northern part of the Egmont-Wanganui districts. 
Mentha viridis make pure associations near rivers &c. in many places. 
5. Modified Associations. 
Steppe 
requent burning of the tussock and constant grazing by sheep, cattle, 
 scoparius an 
Bi 
