CHAP. XXIII ARCTIC PLANTS IN NEW ZEALAND 



527 



the influence of changed conditions, which are manifested 

 by the extreme peculiarity of many of these interesting 

 endemic forms. 



The Absence of Southern Types from the Northern Hemi- 

 sphere. — 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 Acsena which has a species in California ; 

 two representatives 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 Resti- 

 aceous genus. Several distinct causes appear to have 

 combined to produce this curious inability of the southern 

 flora to make its way into the northern hemisphere. 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 

 temperate 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 Tem- 

 perate 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 soutli three comparatively small and detached areas, in 

 which rich floras have been developed with special adapta- 



