a Study in Taxonomic Distribution . 45 
The endemics of Stewart are in the largest (in general, oldest) 
families of New Zealand. 
The endemics of Stewart are in the larger (older) genera of New 
Zealand. 
The Kermadec, Chatham, and Auckland Islands have proportion- 
ately more in common with Stewart than they have with New Zealand 
itself, although they are nearer to New Zealand than to Stewart. 
All species that occur both in the Chathams and the Aucklands 
are also found in Stewart. 
Species common to Stewart and the Chathams (84) range in New 
Zealand, with 4 (really 3) exceptions, up to Lake Taupo or beyond (in 
the North Island), i.e. to the circumference of the circle passing through 
Stewart and the Chathams. 
The species common to Stewart and the Kermadecs (22) are most 
widespread in New Zealand (rarity i-o, the minimum), those common 
to Stewart and the Chathams next, and those to Stewart and the 
Aucklands least so. 
The plants common to Stewart and the Kermadecs, being the 
oldest, show the greatest proportion of wides, while endemic forms 
show in increasing proportion in the plants common to Stewart and the 
Chathams, or Stewart and the Aucklands. 
The relationship of Stewart to the two great invasions of plants into 
New Zealand is then considered, and it is shown that the northern invasion 
was perhaps the earlier, and that Stewart did not lie in the track of the 
southern invasion, but received the plants thereof from the north side of 
Foveaux Strait. In each invasion the plants which occur in Stewart are 
selected from the largest families and genera. 
Finally, it is pointed out that as so many predictions may be success- 
fully based upon age and area alone, the operation of this factor must be the 
principal positive determining cause in geographical distribution, while the 
operation of barriers is the principal negative cause. Some difficulties 
which various workers have experienced in thinking of the hypothesis are 
also pointed out, notably that the area occupied, in the absence of barriers 
or other causes of limitation, increases with the square of the diameter, the 
latter probably increasing uniformly with the passage of time. 
