Ridley .— The Distribution of Plants. 21 
There are a few species of plants which occur in tropical Asia and 
South America which do not appear to be sea-borne, and as far as can be 
judged are not weeds. These are chiefly small-seeded Cyperaceae such as 
Cyperns Hasp an, Linn., Heleocharis capitata , Br., and H. chaetaria , R. and 
S., Fuirena umbellata , Rothb., and Rhynchospora aurea , Vahl., all inhabit¬ 
ing damp, swampy, open spots or marshes, and Scteria lithosperma , Sw., 
which is an open-forest plant. All these plants occur all over Asia and 
Africa as well as South America (except the Scteria not recorded from 
Africa), but they are absent from oceanic islands. The swamp species are 
perhaps carried about by wading birds, as they have the habit of appearing 
very soon on the edges of artificial ponds where sandpipers constantly 
come. More research is required into this form of dispersal, but I cannot 
otherwise account for the appearance of these plants and some other swamp 
and aquatic plants in artificial lakes such as those in the Singapore Botanic 
Gardens. Wide as their distribution is, none are nearly as widely dis¬ 
tributed as the Reed and Cynodon dactylon , as they are exclusively tropical, 
and cannot stand even a temperate climate. Polygonum hydropiper , Linn., 
and P. minus , Huds., are similarly widely distributed, though absent from 
South America, and probably dispersed by wading birds. They range 
from Europe, through India, the Malay Peninsula, and Java, to Australia, 
and P. hydropiper to North America. P. minus is absent from America 
and New Zealand. P. hydropiper first appears fossil in the Neolithic period, 
P. minus is found in the Tegham beds. 
In a paper on the Cyperaceae of the Welwitsch Herbarium (‘ Trans. 
Linn. Soc. 5 , ii, p. 122), I called attention to the extraordinarily large propor¬ 
tion of Cyperaceae common to Africa, chiefly the West Coast, and South 
America and the West Indies, and showed that these plants were largely 
forest plants. The proportion of species of other flowering plants common 
to the two continents is small, but I would add Pothomorplie peltata , Miq. 
(Piperaceae), and Lophiocarpits guyanensis , Mich. (Alismaceae), both of 
which range from South America, through Africa, into the Malay Islands ; 
neither of these species is likely to have been dispersed by man, nor could 
they be dispersed by birds. The Pothomorplie is not cultivated nor, so far 
as I am aware, used by man, and it does not produce drupaceous fruit like 
the Pipers. Lophiocarpits occurs in the Malay Peninsula in rice-fields only, 
and I had thought it might have been carried about in rice-seed, but the large 
size of its seeds makes this improbable, and prevents its being carried on the 
feet of birds. 
Arldt, in ‘ Die Pmtwicklung der Kontinenten und ihre Lebenswelt ’, 
gives a series of maps showing the distribution of land and water from early 
periods. Assuming that these are approximately correct geologically, a land 
connexion between Africa and Brazil appears in Silurian times and con¬ 
tinues through the Chalk period to the Neocomian. This land area includes 
