364 E. C. ANDREWS. 
Moreover, most of these great genera are either xero- 
phytic, or dwarfed, in nature. Supplementary evidence is 
also afforded by a study of Australian Myrtaceze where the 
great genera Hucalyptus, Melaleuca, and Leptospermum 
are xerophytic in nature, with the exception of the types 
which have been developed recently, in the moist eastern 
portions of the continent. 
In this connection it may be advisable to consider the 
environment of the plants. The Leguminose of the fertile 
tropics are subject to severe competition in the jungle, but 
there are limitations set to the struggle, and, moreover, 
such struggle proceeds along few lines. In the first place 
the climate is mild and equable, each plant protects the 
other in great measure from the storm and diurnal changes 
in temperature, and the competition resolves itself into a 
desperate struggle to reach the light and obtain food. With 
the plants of the open, sub-arid, or barren and sandy, plains 
or plateaus the struggle for existence is much more com- 
plicated. Great variation of conditions is ever to be 
expected. The food supply is scanty, the climate is torrid 
in summer and marked by cold desolating winds in the 
winter. The diurnal changes of temperature, moreover, 
are very great. Generally, also, the plants of these regions 
are more or less isolated, their foliage is not dense, and 
they are unable to protect each other from fierce and 
sudden climatic changes. The rain may fail to fall for 
months at a time, and the plants of such regions must 
develop special structures to minimise transpiration. Plants 
which can thrive under such hardships are possessed of 
wonderful powers of vitality. 
It may seem strange that the hardy xerophyte, being 
possessed of enormous vitality and rich in species, should 
not in turn, over-run and oust the jungle growths, never- 
theless a little reflection would supply the explanation. 
