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EFFECT OF SETTLEMENT UPON INDIGENOUS VEGETATION. 231 
found Verbena bonariensis so plentiful in the neighbourhood of 
Darling Downs, then only five or six years settled, that he named 
the place Vervain Plains. 
The injury done by introduced weeds will be almost entirely by 
competition, but it is possible that in time, the Australian plants 
may begin to hold their own and even to some extent drive out 
the others. ‘This will be more especially the case with the group 
of plants which are found on the barren and sandy tracts 
wherever the Hawkesbury Sandstone formation occurs. In such 
land few aliens get a footing. On the sandstone about Sydney 
as a rule, and in the Blue Mountains where the same soil occurs, 
the foreign weeds have no chance. But wherever the soil is fairly 
good, or where it has been broken up, there they triumph and 
exclude the indigenes. 
To some extent however, the weeds will work their own destruc- 
tion. They increase so rapidly that competition is most severe, 
not between them and the natives, but between individuals of the 
same alien species, or between distinct alien species. Szwsymbrioum 
officinale was oncea pest near Mudgee, the fallow and unoccupied 
land being covered with a thick mass of it ; but after the lapse of 
a few years it became quite rare, and Hrigeron canadense took 
its place. I think that in some cases the fact of a heavy crop of 
weeds occurring ina locality one or more years is a reason for 
expecting its scarcity in the following years. The soil becomes 
exhausted of the particular constituents demanded by the plants, 
and they failin consequence. I had often read doleful prophecies 
of the damage that might be expected when the Cape weed 
(Cryptostemma calendulaceum) became common. When I first 
saw it appear in Illawarra, I was therefore prepared to see much 
land infested by it in a short time. It spread to a great extent 
in certain spots for a couple of years and then almost disappeared. 
In my garden half-a-dozen vigorous plants came up, and as I left 
them for the purpose of observation, they flowered and seeded 
plentifully. I fully expected a large crop the following year, but 
