chap, xxiii.] ARCTIC PLANTS IN NEW ZEALAND. 
495 
The absence of Southern Types from the Northern Hemisphere. 
— We have now only to notice the singular want of reciprocity 
in the migrations of northern and southern types of vegetation. 
In return for the vast number of European plants which have 
reached Australia, not one single Australian plant has entered 
any part of the north temperate zone, and the same may he 
said of the typical southern vegetation in general, whether 
developed in the Antarctic lands, New Zealand, South America, 
or South Africa. The furthest northern outliers of the southern 
flora are a few genera of Antarctic type on the Bornean Alps ; 
the genus Accena which has a species in California; two re- 
presentatives of the Australian flora — Casuarina and Stylidium, 
in the peninsula of India ; while China and the Philippines 
have two strictly Australian genera of Orchidese — Microtis and 
Thelymitra , as well as a Restiaceous genus. Several distinct 
causes appear to have combined to produce this curious inability 
of the southern flora to make its way into the northern hemis- 
phere. The primary cause is, no doubt, the totally different 
distribution of land in the two hemispheres, so that in the south 
there is the minimum of land in the colder parts of the tempe- 
rate zone and in the north the maximum. This is well shown 
by the fact that on the parallel of Lat. 50° N. we pass over 240° 
of land or shallow sea, while on the same parallel of south 
latitude we have only 4°, where we cross the southern part of 
Patagonia. Again the three most important south temperate 
land-areas — South Temperate America, South Africa, and 
Australia — are widely separated from each other, and have in all 
probability always been so ; whereas the whole of the north 
temperate lands are practically continuous. It follows that, 
instead of the enormous northern area, in which highly organised 
and dominant groups of plants have been developed gifted with 
great colonising and aggressive powers, we have in the south 
three comparatively small and detached areas, in which rich 
floras have been developed with special adaptations to soil, 
climate, and organic environment, hut comparatively impotent 
and inferior beyond their own domain. 
Another circumstance which makes the contest between the 
northern and southern forms still more unequal, is the much 
