CHAP. xxiiL] ARCTIC PLANTS IN NEW ZEALAND. 



497 



theory as to the cause of the peculiar biological relations be- 

 tween the northern and the southern hemispheres ; and no 

 better or more typical example could be found of the wide 

 range and great interest of the study of the geographical 

 distribution of animals and plants. 



The solution which has here been given of one of the most 

 difficult of this class of problems, has been rendered possible 

 solely by the knowledge very recently obtained of the form of 

 the sea- bottom in the southern ocean, and of the geological 

 structure of the great Australian continent. Without this 

 knowledge v/e should have nothing but a series of guesses or 

 probabilities on which to found our hypothetical explanation, 

 which we have now been able to build up on a solid foundation 

 of fact. The complete separation of East from West Australia 

 during the Cretaceous period, could never have been guessed 

 till it was established by the laborious explorations of the 

 Australian geologists ; while the hypothesis of a comparatively 

 shallow sea, uniting New Zealand by a long route with tropical 

 Australia, while a profoundly deep ocean ahvays separated it 

 from temperate Australia, v>'ould have been rejected as too 

 improbable a supposition for the foundation of even the most 

 enticing theory. Yet it is mainly by means of these two facts, 

 that we are enabled to give an adequate explanation of the 

 strange anomalies in the flora of Australia and its relation to 

 that of New Zealand. 



In the more general explanation of the relations of the 

 various northern and southern floras, I have shown what an 

 important aid to any such explanation is the theory of repeated 

 changes of climate, not necessarily of great amount, given in 

 our eighth Chapter ; while the whole discussion justifies the 

 importance attached to the theory of the general permanence 

 of continents and oceans, as demonstrated in Chapter YI., 

 since any rational explanation based upon facts (as opposed to 

 mere unsupported conjecture) must take such general perma- 

 nence as a starting-point. The whole inquiry into the pheno- 

 mena presented by islands, which forms the main subject of the 

 present volume has, I think, shown that this theory does afford 



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