XI, C, 4 
Copeland: Natural Selection 
165 
in itself to make each of these species a rather rare plant. 
The effect of this development of numerous species has probably 
been in each genus to increase the area available, and so the 
aggregate commonness. But the improvement in this respect 
cannot be expected to suffice to offset the rareness resulting to 
the single species from the fact of their considerable number, 
unless the differences between these single species are such 
as completely to remove them from competition with one another. 
This probably never happens in any genus. Therefore, in every 
genus, the more numerous the species, the greater their indi- 
vidual rarity. “When the genus contains one or two species 
only, rarity is 4, when it contains more than two it is 4.3” (p. 
331). If this rather elementary application of mathematics 
does not torture the phenomena in question into any kind of 
support for a theory of advantageous evolution, it certainly 
leaves nothing that can be construed as in opposition to such 
theory. 
As to the relative rarity of Doona and Stemonoporus, Doctor 
Foxworthy points out that Stemonoporus is a decidedly isolated 
group, while Doona is closely related to the widespread genera 
Hopea and Shorea. If Stemonoporus is the rarer, as well as 
the more isolated morphologically, this is another illustration 
in support of Doctor Willis’s general thesis. 
In several places, Doctor Willis notes the general tendency 
of endemic species to occur in the wet and mountainous districts 
rather than in the dry. Thus, page 319 : 
It is also very noteworthy that in the dry zone there are only 28 endemic 
species against 743 in the wet, though the species of wide distribution are 
only in the proportions of 304 to 648, and the dry zone has twice the area 
of the wet. 
There appear to be two valid explanations of this condition. 
The dry region is closer to the dry region of continental India, 
and there is, accordingly, a better chance for species to spread 
across the channel in either direction ; that is, the dry region 
of Ceylon is much less isolated from the continent than is the 
wet region. A second explanation is that conditions are much 
more varied in the wet region and that any given set of condi- 
tions is much more restricted. If, therefore, a plant varies so 
as to produce a new species in any given spot in the wet region, 
its favorable field for dispersal is almost sure to be decidedly 
circumscribed, as compared with that of a new form adapted 
to dry-country conditions. High mountains are particularly 
“local” in their conditions, and, accordingly, in Ceylon, as is 
true everywhere in the tropics, each mountain of any age has 
