New Zealand Flora , with a Reply to Criticism . 341 
of Ceylon only and those of wider distribution than Ceylon and southern 
India. 
Professor Sinnott seems to think that I am propounding age and area 
as a kind of master-key that is to unlock everything. Naturally I have 
laid most stress upon it in my papers, for I am trying to get it established 
as a law. But it would be as reasonable to try to explain everything by 
it as to try to explain the upward movement of an aeroplane or a balloon 
by appeal to the law of gravity. What I am endeavouring to make clear 
is that, though plants (and I am inclined to think that it applies to animals 
also) are determined in their existing ultimate distribution by the operation 
of very numerous causes, they all obey as much as possible the law of age 
and area, which shows quite clearly in the figures of distribution of any 
group of plants, even though it may not always show in individual cases. 
Contentions based upon probabilities cannot weigh so heavily against 
age and area as the very clear and decisive figures which have appeared 
weigh in its favour. These figures, it must not be forgotten, do not depend 
in any way whatsoever upon the acceptance or rejection of the hypothesis, 
but simply represent the plain unvarnished facts of distribution. They 
were discovered by aid of age and area, it is true (except the first ones 
relating to Ceylon, which originally suggested age and area), but that was 
simply because the hypothesis acted as a guide .to directions in which to 
seek. This alone is a very powerful argument in its favour, that by its 
use one is able to discover new facts and new methods of looking at them, 
which may lead to advances in our knowledge of geographical distribution. 
It is quite impossible to predict by aid of Natural Selection what will be 
the actual facts of the geographical distribution of any plants >in any 
country. For instance, one cannot predict the distribution in New Zealand 
of the plants of the Chathams, or that, to quote de Vries, ‘ the endemics 
with a small distribution are heaped up in the centre of the country', or 
again, that an endemic in New Zealand will occupy a greater area than 
in Ceylon. 
Dr. Sinnott is inclined to say that ‘ so-and-so must be so 5 , but these 
statements are sometimes rested upon assumptions which are rather 
difficult to prove. On p. 210, for example, he says, ‘a species with means 
for rapid dispersal will evidently overrun a wider area in a given length of 
time than will a more slowly moving type’, and on p. 314, speaking of new 
arrivals, he says, ‘ after its first rapid spread ’, therein assuming that under 
untouched natural conditions that spread is rapid. The facts at our 
disposal do not warrant such assumptions. We have all but no information 
as to spread under untouched natural conditions ; the only material 
available refers to spread under conditions altered by man, whose inter¬ 
ference in a country may rapidly become of supreme importance to dispersal 
of plants or animals therein. 
