EFFECT OF SETTLEMENT UPON INDIGENOUS VEGETATION. Dd 
(b) Action on local flora. 
As already pointed out, Australian plants from their long 
isolation, and their having little competition of a severe kind, 
settled down into a state of balance or rather of slight oscillation, | 
governed by a few causes, which themselves varied but little. In 
the older continents, however, from the intercommunication of 
the various nations, and from the fact that men continually add 
to their stock of cultivated plants, there is severe competition ; 
the struggle for existence goes on continually and aided by natural 
selection and domestication some plants gain an advantage. 
Among other useful habits acquired by plants under competition 
is a certain plasticity of constitution which enables them to bear 
changes to different climates with equanimity. On this account 
the old world weeds when brought to Australia are able to beat 
the native plants. They are mostly plain dwellers, and as such 
accustomed to the heat of the sun in the open, and the bitter 
blasts of the winter, better than forest plants. When forests are 
cleared and brought under cultivation, the weeds soon beat the 
former occupants out of the field. Again many old world weeds 
are plants of wide range, and on this account have an advantage 
over those of more restricted habitat. ‘Widely varying species 
abounding in individuals which have already triumphed over many 
competitors in their own widely extended homes, will have the 
best chance of seizing on new places when they spread into new 
countries. In their new homes they will be exposed to new con- 
ditions, and will frequently undergo further modification and 
improvement ; and thus they will become still further victorious 
and produce groups of modified descendants.”* As before remarked 
their success in competition implies a plasticity of organism which 
is an advantage to them also; on this subject Darwin says, SSilE 
a number of species, after having long competed with each other 
in their old home, were to migrate in a body into a new and after- 
wards isolated country, they would be little liable to modification 
or variation; for neither migration nor isolation in themselves 
* Origin of Species, 6th Ed. p. 319. 
