1(56 The Philippine Journal of Science im 



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 1 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. 



'Ann. Roy. Bot. Gardens Peradeniya 4 (1908) 135. 

 "Variation in California plants, p. 413. 



