MOUNTAINS AND THEIR EFFECT ON NATIVE VEGETATION. 269 
used), largely a tropical element, which has entered the 
continent, probably through New Guinea, before the last 
land connection was severed, and this is practically con- 
fined to the coastal strip and eastern face of the Main 
Range in Queensland and New South Wales. Now it is 
interesting to consider why this brush vegetation has such 
a limited range in Australia. We can readily understand 
that, being of tropical origin, its progress southwards will 
be arrested by the cold of southern latitudes, but seeing 
that it enters Australia in the north, it is not clear why it 
does not extend right across the continent from east to 
west, and come an equal distance south throughout. The 
reason which suggests itself is that this distribution is 
regulated by climate and rainfall. It would seem however, 
that these factors are directly the result of certain topo- 
graphic conditions, and had the topography of northern and 
eastern Australia been similar, there would not have been 
such a wide difference in the two floras. 
There is perhaps nothing which shows more evident 
response to certain physiographic features than the resul- 
tant native flora. This response is due to certain natural 
laws, an important one being that it is the cooling, and, 
therefore, often the ascending cloud which precipitates 
most of the rain. The result of this law is a good rainfall 
throughout practically the whole of the eastern slopes of 
Australia, for the rainy weather comes from the ocean, and 
ascends these mountains to their summits, there being none 
so high as to reach above the rain zone, and the clouds are 
chilled in their ascent. It seems unquestionable that had 
this great plateau been only half its present height, and 
one or two hundred miles further inland, as well as reached 
by a gentle slope instead of a fairly abrupt face, the rain- 
fall over the present coastal belt, though considerable, 
would have been less, while that of the area now occupied 
by the lower western slopes would have been increased. 
