CHAP. XXII TFIE FLORA OF NEW ZEALAND 507 



the specially Australian genera in New Zealand are 

 largely tropical or sub-tropical, the specially Australian 

 species are wholly temperate or alpine ; for these are 

 comparatively recent arrivals, they must have migrated 

 across the sea in the temperate zone, and these temperate 

 and alpine forms are exactly such as would be best able to 

 establish themselves in a country already stocked mainly 

 by tropical forms and their modified descendants. This 

 hypothesis further fulfils the conditions implied in Sir 

 Joseph Hooker's anticipation that — " these great differ- 

 ences (of the floras) will present the least difficulties to 

 whatever theory may explain the whole case," — for it 

 shows that these differences are directly due to the history 

 and development of the Australian flora itself, while the 

 resemblances depend upon the most certain cause of all 

 such broad resemblances — close proximity or actual land 

 connection. 



One objection will undoubtedly be made to the above 

 theory, — that it does not explain why some species of the 

 prominent Australian genera Acacia, Eucalyptus, Melaleuca, 

 Grevillea, &c., have not reached New Zealand in recent 

 times along with the other temperate forms that have 

 established themselves. But it is doubtful whether any 

 detailed exj^lanation of such a negative fact is possible, 

 while general explanations sufficient to cover it are not 

 wanting. Nothing is more certain than that numerous 

 plants never run wild and establish themselves in countries 

 where they nevertheless grow freely if cultivated ; and the 

 explanation of this fact given by Mr. Darwin — that they 

 are prevented doing so by the competition of better 

 adapted forms — is held to be sufficient. In this particular 

 case, however, we have some very remarkable evidence of 

 the fact of their non-adaptation. The intercourse between 

 New Zealand and Europe has been the means of intro- 

 ducing a host of common European plants, — more than 

 150 in number, as enumerated at the end of the second 

 volume of the Handbook; yet, although the intercourse 

 with Australia has pi'obably been greater, only two or 

 three Australian plants have similarly established them- 

 selves. More remarkable still, Sir Joseph Hooker states : 



