28 The Philippine Journal of Science 1914 
that are equally definitely of oriental origin, and about 20 re- 
garding whose origin there is at least a reasonable doubt, but 
which are probably for the most part natives of the Old World. 
I consider that the pantropic species, for the most part, at least, 
have been transmitted from one hemisphere to the other through 
the agency of man. Admitting that at least 50 of Seemann’s list 
of Vitian weeds are now found in the tropics of both the Old 
and New Worlds by no means proves that these weeds have 
originated in America. I believe that it is safer to conclude 
that the most of the weeds enumerated by Seemann have origi- 
nated in the Old World, that many have been introduced into the 
New World, and that their presence in Polynesia is due to their 
introduction into that region through the Polynesians themselves 
in their migrations from island to island before the advent 
of the Europeans, and, at a more recent date, by the increase 
in intercommunication after the advent of the Europeans in Poly- 
nesia. 
That Polynesia has acted as a bridge by means of which the 
weeds of the Old World have passed over to the New and vice 
versa impresses me as being improbable, at least for any great 
number of species. That most of the pantropic weeds can be 
distributed over fairly long distances by the means of winds, 
migratory birds, and some, perhaps, by water, must be admitted, 
but it is hardly conceivable that the majority of them can pass 
unaided such a great expanse of water as the eastern Pacific. I 
am of the opinion that only those species are capable of being 
transmitted very great distances, by water, that have very special 
adaptations for dissemination by ocean currents, that is, floating 
seeds or fruits, that not only retain their buoyancy for many 
weeks or months, but whose seeds also retain their germinating 
power for equally long periods when immersed in salt water. 
Guppy ® has given considerable attention to this matter, and an 
examination of the lists of species whose seeds and fruits he 
investigated,’® as to buoyancy or non-buoyancy, shows that among 
all the species of pantropic weeds investigated a single one, 
Eclipta alba Hasak., has seeds that float for many months. All 
the other weeds and weed-like plants which he investigated have 
seeds or fruits that sink at once or within a day or two, those 
tested being in such genera as Abrus, Ageratum, Canna, Phyl- 
lanthus, Portulaca, Urena, Argemone, Triumfetta, Tephrosia, 
Bidens, Cassia, Commelina, Ipomoea, Jussiaea, Sida, and Wal- 
° Observations of a Naturalist in the Pacific 2 (1906) Seed Dispersal. 
*L. c. 529-533. 
