

JU.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 



