Ridley On Endemism and the Mutation Theory . 573 
plant with the same plume-borne seeds. It was probably introduced into 
Singapore about the same time as the Lalang, but it has no deeply protected 
rhizome. It is unable to defend itself against great heat, drought, and 
fire. It occurs only sporadically here and there, and is in fact comparatively 
rare, and does not grow near volcanoes nor survive a forest fire. 
Some years ago, in writing about cultivation of lawns, I had to point 
out that only Grasses with creeping rhizomes should be used, because 
in dry weather those which had not creeping rhizomes died out from the 
heat, while the rhizomatous Grasses creeping over each other prevented the 
ground from giving off the moisture (chiefly dew) all day during the heat. 
P asp alum platyc aide, Sw., is a creeping Grass, native of South America. 
It is now very common in Java. One day I found a patch of it in Singa- 
pore by the road-side, the only patch I had ever seen ; in fact, it was VR. 
I removed some to the gardens, where it grew very fast ; it now spread all 
over the Singapore road-sides. It has since become a nuisance, having 
become in a few years VC. 
But the whole story of weeds (i. e. plants introduced by man) is the 
same. Look at Ageratum conyzoides , an American plant now extraordi- 
narily common all over the Eastern tropics ; Clitoria cajanifolia , formerly a 
rather rare local plant in Pernambuco, whence for many years only one or two 
specimens had been obtained, now common in Java and in Singapore, 
spreading to Borneo, thanks to the fact that, unlike other species of the 
genus, the seeds are viscid and adhere to the passing cattle ; or again 
Tithonia diver sifolia, Gray, a Mexican plant— though only introduced 
(into Ceylon) as a garden plant so recently as 1851 it is now one of 
the commonest and most conspicuous plants in the island’ (Trimen’s 
Flora). 
The obvious reason why wide range (as in the case of Imperata 
cylindrica) involves greater commonness is that for some reason the plant 
has advantages which enable it to spread and propagate its seed successfully 
and continuously, and has nothing to do with the period at which it 
entered the country. 
What the advantages are in each plant is only to be known by careful 
study in the field, and an extremely interesting study it is, especially as they 
are different in almost every species, and due to modifications in all parts of 
the structure from root to seed, and even in some cases the chemical contents 
of the cell. Let the botanist find out why Ageratum conyzoides has spread 
over the whole tropics in spite of m^n, and A. peruvianum has failed 
though helped by man ; why Capsicum minimum has established itself on 
tropical Eastern limestone rocks in immense abundance and C. annuum 
failed. 
These stories can be elucidated in the field, but not in the library. 
