CHAP. XXII.] 
THE FLOE A OF NEW ZEALAND. 
475 
for all the main features of the New Zealand flora. It shows 
why the basis of the flora is fundamentally Australian both as 
regards orders and genera, for it was due to a direct land con- 
nection between the two countries. It shows also why the great 
mass of typical Australian forms are unrepresented, for the 
Australian flora is typically western and temperate, and New 
Zealand received its immigrants from the eastern island which 
had itself received only a fragment of this flora, and from the 
tropical end of this island, and thus could only receive such 
forms as were not exclusively temperate in character. It shows, 
further, why New Zealand contains such a very large proportion 
of tropical forms, for we see that it derived the main portion of 
its flora directly from the tropics. Again, this hypothesis shows 
us why, though the specially Australian genera in New Zealand 
are largely tropical or sub-tropical, the specially Australian 
species are wholly temperate or alpine; for these are com- 
paratively recent arrivals, they must have migrated across the 
sea in the temperate zone, and these temperate and alpine 
forms are exactly such as would be best able to establish them- 
selves in a country already stocked mainly by tropical forms 
and their modified descendants. This hypothesis further fulfils 
the conditions implied in Sir Joseph Hooker’s anticipation that 
— “ these great differences (of the floras) will present the least 
difficulties to whatever theory may explain the whole case,” — 
for it shows that these differences are directly due to the history 
and development of the Australian flora itself, while the resem- 
blances depend upon the most certain cause of all such broad 
resemblances — actual land connection. 
One objection will undoubtedly be made to the above theory, 
— that it does not explain why some species of the prominent 
Australian genera Acacia , Eucalyptus, Melaleuca, G-revillea, &c., 
have not reached New Zealand in recent times along with 
the other temperate forms that have established themselves. 
But it is doubtful whether any detailed explanation of such 
a negative fact is possible, while general explanations sufficient 
to cover it are not wanting. Nothing is more certain than that 
numerous plants never run wild and establish themselves in 
countries where they nevertheless grow freely if cultivated; 
