290 Willis . — The Floras of the Outlying Islands of 
consideration of the various papers that have been written on the subject, 
and after devoting much time to further work upon it, I am inclined to word 
the hypothesis thus : 
The area occupied at any given time, in any given country, by any 
group of allied species at least ten in number, depends chiefly, so long as 
conditions remain reasonably constant, upon the ages of the species of that 
group in that country, but may be enormously modified by the presence of 
barriers such as seas, rivers, mountains, changes of climate from one region 
to the next or other ecological boundaries, and the like, also by the action 
of man, and by other causes. 
In other words, age and area is the chief positive, the action of barriers 
the chief negative, factor in plant distribution, while in recent times the 
action of man has become of greater importance than either. 
It is clear that in general this law also covers the case of a genus of 
more than a very few species, for a genus is in general a group of allied 
species, though it is becoming every year more clear (cf. 1 and 12, p. 446) 
that many genera are based upon too few characters, and are in reality 
polyphyletic. 
The acceptance of this hypothesis will involve various changes in our 
methods of handling problems of geographical distribution, and in further 
papers I shall go on to indicate some of these. 
Summary. 
In this paper the chief attention is devoted to the islands outlying 
around New Zealand, especially the Kermadecs, Chathams, and Aucklands, 
and the following points are indicated, chiefly by the method of prediction 
and subsequent verification, which is here used successfully no fewer than 
32 times.' It is first shown that the floras of these islands must be very 
old, from their far outlying position, and is then indicated that — 
i. 1 The islands have proportionately more families in common with 
Stewart, whose flora is also old, than with New Zealand proper (Table I). 
2. The great proportion of the families in the different islands are the 
same, and most are selected from the Stewart list (Table II). 
3. The families reaching three groups of islands are on the whole the 
largest (oldest), those reaching two next, and then those reaching one or 
none (Table III). The same is true for the Stewart families (Table IV), 
and the total result is summed up in Table V. 
4. The Stewart families that are missing in the islands are on the whole 
the smallest. 
5. The Aucklands and the Chathams contain most of the families of 
the southern invasion, and those that are missing in the Chathams are the 
smallest, numerically, of that invasion. 
1 Predictions numbered as in text above. 
