EFFECT OF SETTLEMENT UPON INDIGENOUS VEGETATION. 203. 
ledge of such subjects in Australia, no more can be done than 
indicate how modification, other than direct, might occur. 
The alteration in the mechanical and chemical structure of the 
soil is a point that must not be lost sight of. The burning of 
quantities of vegetable matter must add potash to the soil, while 
the great heat chars some of the organic constituents of the soil, 
and alters the clays and stiff soils mechanically. Such changes 
must benefit some plants more than others, and here again another 
cause of change occurs. 
Again after a bushfire, the surface of the ground having lost its 
protecting investment of grass, etc., would be more liable to erosion 
in a wet season and consequently alterations of surface drainage 
would take place, with consequences to plants that have already 
been pointed out, and will be alluded to again in Division II. 
Since then bushfires have so great an influence, direct and in- 
direct, on the flora, it must be regarded as proved that the checking 
of the fires results in as great an effect in the opposite direction. 
But besides all the effects indicated above, there is still another 
though obscure one, which must be touched upon: the destruction 
or modification of a flora reacts on the animal kingdom, and that: 
acts again on the flora. The complex network of relations between 
organisms is such that any disturbance of any part, alters the 
balance in all parts, just as a stone thrown into a quiet pool pro- 
duces waves which not only proceed to all parts of the shore, but 
are reflected back again in all directions and thus produce most 
complex wave systems by interference with each other. It is 
difficult, if not impossible to say what effects may have been pro- 
duced in this way, but that there are such effects is a point beyond 
all dispute. The sweeping away of whole groups of plants must 
necessarily destroy the means of subsistence of groups of insects. 
which live on these plants. But as many insects fertilise plants 
(and usually different species to those on which their larve live) 
some flowers would remain partly or entirely unfertilized, which 
would be a disadvantage to them in the struggle for existence, 
