82 AGE AND AREA [pt. i 



herbs, while the northern has 84 per cent, of trees and shrubs, 

 and, as we have pointed out above, the latter will be likely to 

 spread with vastly greater slowness. The average areas occupied 

 by the species of the two invasions are much the same. 



I am informed by the well-known palaeobotanist, Mrs Clement 

 Reid, that geology gives evidence that invasions follow directions 

 which offer stability of climatic conditions to their members; 

 polewards when climates are warming, equatorwards when 

 coohng. One feels inclined to infer, therefore, that at the time 

 of the northern invasion New Zealand was warm in the south, 

 whilst the Antarctic land was habitable to the northern types 

 of plants that largely compose the southern invasion, and which 

 perhaps reached Antarctica by way of the Andes, as most of 

 them occur in that chain. Then as the south cooled, the southern 

 invasion perhaps entered New Zealand, working northwards. It 

 is very noticeable in the ciuves for this invasion that they fall 

 off much more gradually to the north than to the south. 



Yet other probabilities may be deduced from the figures and 

 curves given. The curve in the southern invasion for endemics 

 that reach the outlying islands is flatter even than the curve 

 for wides, showing that they are probably older than the average 

 for wides, as we have shown ab^^^e (p. 69). But if we split the 

 curve for wides in the same way, into two, that for the wides 

 that reach these islands proves to be even flatter than that for 

 the endemics which do so, as we should expect by hypothesis. 



From the diagram given at the commencement of this chapter, 

 one may deduce that the average range of endemic species that 

 occur in the outer zones of New Zealand will be greater than 

 that of those that occur in the centre, for obviously those of 

 short range will be mainly concentrated towards the middle. 

 Examination of the actual figures for the southern invasion 

 shows that this is very strikingly the case, the average range of 

 all the endemics occurring in the northern half of the South 

 Island being about a third of that of those occurring to the 

 south. Not only so, but they belong in much greater proportion 

 to the smaller genera of New Zealand, i.e. by hypothesis (p. 71) 

 the younger. The long-ranging endemics of the outer zones be- 

 long mainly to large genera (of the New Zealand flora). 



It is clear that Age and Area can be used with considerable 

 directness in the study of the invasions by which a country has 

 received its present population of plants. 



