7 
southern China, including Hongkong. Their migration northward into 
that region was almost certainly independent of that into the Philippines. 
Of the few remaining species, a few stop at Formosa and a few others 
reach as far as Japan. Of the entire 139 species, only a single one, 
Pteridium aquilinum, the most cosmopolitan of ferns, has such a dis- 
tribution that it could as well be regarded as an immigrant from the north 
as from the south. Since it is impossible that many other ferns, and 
improbable that any others have come to Mindanao from north of Luzon, 
it is likewise unlikety that Pteridium is an immigrant from that side, 
especially since we have here the lanuginose form usual in the Tropics. 
If, as is commonly done, we picture the migrations of plants as waves 
rolling on to a land from all sides on which it has points of biological 
contact, we should expect, on geographical grounds, to find such waves 
to have rolled on to the Philippines from Malaya on the south and 
Formosa on the north. The strength of these waves must depend on 
the ease of migration and on the “pressure” of the flora back of them. 
The character of the waves must be determined by the partial presssure 
of the different constituents of the parent flora. 3 The ease of migration 
depends upon the distance and the natural obstacles. Whatever their 
means of travel, ferns readily cross bodies of water. 4 
From its greater proximity and its incomparable wealth of ferns, 
it was easy to expect that Malaya would furnish by far the larger part 
of the San Eamon fern flora. Still, I did not at all anticipate that the 
entire flora would prove Malayan, and I know of no other case of any 
very rich flora which, aside from endemic species, is derived entirely 
from immigration from one side. Nor does the endemic element really 
constitute an exception here. Polysticlium nudum belongs to the cos- 
mopolitan “aculeatum” group. Loxogramme conferta is a-, very isolated 
species but in a Malayan genus. Two species are placed in Schizostege , 
a genus hitherto known only from Hawaii. Schizostege seems to be a 
generalized type, intermediate between Clieilanthes and Pteris, and its 
discontinuous distribution may be another evidence of antiquity, the 
Hawaiian fern flora being strongly Austro-malayan. The othet local 
ferns all belong to groups characteristic of the islands to the south. 
This is no less true of the 14 species peculiar to the Philippines, of 
which only Psomiocarpa apiifoiia is without very close Malayan cousins. 
The non-polypodiaceous fern flora yields altogether harmonious evi- 
dence. A notable example is Dicksonia chrysotricha Hassk., the genus 
not being known hitherto from the Philippines. 
The conclusion derived from the study of the San Eamon collections 
is applicable to the whole of Mindanao. I have at present fully 100 
3 Of course the effectiveness of migration depends also on the power of the 
migrants to live, with or without modification, in the new environment. 
4 Treub: Notice sur la nouvelle flore de Krakatoa, Ann. Jard. Buitenz. (1888) 
7. Schimper: Plant Geography, Engl. Translation (1903), 80. 
