CHAP, xxiii.] ARCTIC PLANTS IN NEW ZEALAND. 



495 



The ahsence 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 be 

 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 Stylidinm, 

 in the peninsula of India ; while China and the Philippines 

 have two strictly Australian genera of Orchide£e — 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 p?.rallel 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°, Avhere 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, but 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 



