199 
Willis.—Aw and Area. 
time is slow and uniform, or fairly so, and which thus allows acclimatization 
to go on at the same time. Age thus becomes a measure of dispersal, or 
vice versa, but, as already explained, the measure probably differs for every 
species, though all will spread, if unimpeded, about twice as far in 100,000 
years as in 50,000. The spread is everywhere impeded by ecological and 
physical barriers, and will ultimately be stopped in one or more, or all 
directions, by one of these barriers, perhaps most often by a physical 
barrier, or by a sudden change of climate. There seems no reason to 
suppose, however, when one looks at such a map as that of the species of 
Ranunculus in New Zealand, that most species have reached their limits of 
dispersal. 
Age and Area, then, implies that, on the whole, area occupied increases 
with age, and that as a general rule the very localized species are not the 
failures, as used to be imagined, but the young beginners. Whether the 
increase with age is in direct proportion or not, we do not yet know. There 
seems reason to believe that at first, when a species is probably represented 
by few individuals on a small area, the rate of dispersal will be much slower 
than when it becomes more common, but when once it has reached reason¬ 
able commonness on a considerable area it would seem probable that 
further dispersal will be at a more or less uniform rate. 
The study of the localized species (cf. maps in 8, p. 343, or 11, 
p. 156) shows that they appear within, or very close to, the boundaries then 
occupied by the genus, but in what to us at present appears a casual way. 
A consideration of the map of the New Zealand species of Ranunculus 
upon p. 156 of my book, and of the diagram upon p. 76, will make this clear. 
Even in the case of the most localized species of all, confined to a mere 
spot of ground (cf. p. 151), where the conditions now existing must be 
practically the same as those under which it arose, one can see no special 
reason for its appearance, nor anything of special local adaptation in the 
characters that distinguish it (cf. pp. 325-7). 
Now, as has been shown in the book, chaps, xvi, xx, genera appear 
to obey the same rules as species, also appearing in this casual manner ; 
and they spread in the same way with age, increasing their number of 
species on the whole as they do so. As has already been frequently 
pointed out, no two genera or species will spread at the same rate, though 
all will follow the law of Age and Area. The result will therefore be what 
we actually see in nature—a vast multitude of more or less rounded 
areas, overlapping one another in every conceivable way, and of every 
possible size. 
One of the most essential differences between this and the Natural 
Selection position is that under Age and Area one need no longer engage 
in the hopeless task of finding differences between species in characters that 
are of importance in the struggle for existence. If they exist, well and 
