CHAP. XXIII ARCTIC PLANTS IN NEW ZEALAND 519 



as Pachychladon and Notothlaspi of New Zealand said to 

 have affinities with Arctic plants, while Stilbocarpa — 

 another peculiar New Zealand genus — has its nearest 

 allies in the Himalayan and Chinese Aralias. Following 

 these are a whole host of very distinct species of northern 

 genera which may date back to any part of the Tertiary 

 period, and which occur in every south temperate land. 

 Then we have closely allied representative species of 

 European or Arctic plants ; and, lastly, a number of 

 identical species, — and these two classes are probably due 

 entirely to the action of the last great glacial epoch, whose 

 long continuance, and the repeated fluctuations of climate 

 with which it commenced and terminated, rendered it an 

 agent of sufficient power to have brought about this result. 



Here, then, we have that constant or constantly 

 recurrent process of dispersal acting throughout long 

 periods with varying power — that ** continuous current of 

 vegetation '' as it has been termed, which the facts 

 demand ; and the extraordinary phenomenon of the 

 species and genera of European and even of Arctic plants 

 being represented abundantly in South America, Australia, 

 and New Zealand, thus adds another to the long series of 

 phenomena which are rendered intelligible by frequent 

 alternatrons of warmer and colder climates in either 

 hemisphere, culminating, at long intervals and in favour- 

 able situations, in actual glacial epochs. 



Geological Changes as Aiding Migration, — It will be well 

 also to notice here, that there is another aid to dispersion 

 dependent upon the changes effected by denudation 

 during the long periods included in the duration of the 

 species and genera of plants. A considerable number of 



different observers should all be illusory ; while the well established fact 

 of the former wide distribution of many tropical or now restricted types of 

 plants and animals, so frequently illustrated in the present volume, removes 

 the antecedent improbability which is supposed to attach to such identifi- 

 cations. I am myself the more inclined to accept them, because, according 

 to the views here advocated, such migrations must have taken place at 

 remote as well as at recent epochs ; and the preservation of some of these 

 types in Australia while they have become extinct in Europe, is exactly 

 paralleled by numerous facts in the distribution of animals which have 

 been already referred to in Chapter XIX., and elsewhere in this volume, 

 and also rex^eatedly in my larger work. 



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