166 The Philippine Journal of Science 1916 
a considerable endemic flora. What we have here is merely a 
manifestation of the principle, which various evolutionists have 
strongly emphasized, that isolation is essential or at least favor- 
able to the establishment of new species. “Isolation, as isolation, 
favours the production of new forms.” 8 
Even this does not exhaust the explanations. On the whole, 
the plants of dry districts are probably more likely to scatter 
their seed to great distances than the plants of wet districts. 
Facility for wide dispersal of seed is of course conducive to 
commonness and inimical to prolonged endemism. And, still 
again, in a district where there are marked local differences of 
conditions natural selection tends to permit a wider freedom 
of variation than it does where conditions are uniform or 
comparatively uniform over considerable areas. 9 For this reason, 
variation being more frequent and wider in the more diversified 
wet region, more new forms, susceptible of recognition as 
species, are continually appearing there; and the more such 
forms appear, the more are likely to be perpetuated and to 
attain recognition. 
Doctor Willis is also puzzled by the fact that (p. 319) — 
Adding up all the species of the dry zone, we find 472 confined to it 
with 1809 marks, or a rarity of 3.8; those of the wet zone only are 1692 
with 6497 marks, or also a rarity of 3.8. But the species that occur in 
both zones, 645 with 1505 marks, are much commoner in both, and show 
a rarity of 2.3, i. e. are fairly near to the level of “Common.” How this 
result is to be interpreted it is difficult to say. 
To this, as to the rarity of numerous species in a single genus, 
a merely mechanical explanation suggests itself. The common- 
ness of a species being graded according to the number of 
collections and their remoteness of locality from one another, 
the fact that collections are possible in both districts must 
operate to make the plant seem common, even though a plant 
is rated very common, if sufficiently abundant in the district 
climatically suitable. The fact, moreover, that a plant can 
produce seed under a variety of conditions, and have these seed 
likely to grow likewise under a variety of conditions, gives it, 
in the struggle for existence, a material handicap over any plant 
that can thrive only under comparatively restricted conditions; 
and this handicap, given sufficient time, will inevitably make 
the more adaptable plant the commoner. 
s Ann. Roy. Bot. Gardens Peradeniya 4 (1908) 135. 
8 Variation in California plants, p. 413. 
