558 Ridley. —On Endemism and the Mutation Theory. 
Endemic Species. 
An endemic species or genus is one confined, so far as is known, to one 
definite area, and is not known to occur outside that area. Such a species 
may be one which has evolved within that area, and for some reason never 
spread farther, or it may have at one time or other occupied a wider 
area from which, except in one region or locality, it has disappeared. 
Before we can say definitely a plant is endemic we must have a complete 
knowledge of the flora of the nearest countries, but this in many cases we 
do not possess. Thus, though there are a very large number of what have 
to be recorded as endemic plants in the Malay Peninsula, we know com- 
paratively little of the floras of the adjacent countries, Sumatra, Borneo, and 
the islands south of Singapore, Siam, and Cochin China. The nearest land 
to Ceylon is southern India, Madura, and the Carnatic. Has this region 
been so thoroughly explored that we can say for certain that many of these 
so-called Ceylon endemics do not still occur there ? I doubt it very much. 
This part of India has been very long heavily cultivated and thickly 
populated. How many of the now endemic plants of Ceylon were not 
formerly abundant over this area ? 
Dr. Willis himself, on p. 12 of his paper in these Annals, gives an impor- 
tant clue to the history of endemics in Ceylon when he says, ‘ The second 
point that shows at once in these diagrams is that the enormous majority of 
the endemic species are in the wet zone \ Exactly what would be expected 
if the climate of southern India and the remainder of Ceylon had been 
formerly a wet district like the Malay Peninsula, and by land changes, 
denudation of mountains, felling of forests, changing rainfall or other 
climatic changes had become xerophytic except in the still wet zone of 
the south-western quarter of the island. In that case we should find 
exactly what we do find — the remains of an old rain-forest flora isolated in 
the wet zone. 
We find evidence of a reverse action curiously in the case of the lime- 
stone rocks of Selangor in the Malay Peninsula. These rocks, attaining 
a considerable altitude (about 1,000 feet), lie at present thirty-two miles 
from the sea, though there is still a tradition of the sea having washed their 
bases in the time of man. Between them and the sea is a flat area of wet 
rain-forest which has crept up their sides for some distance. At the top of 
these precipitous rocks there is much mica in the soil, which must have come 
from granite mountains in close proximity to the limestone and higher than 
it. These mountains are now gone. On the top of the limestone rocks we 
get a distinct flora largely identical with that of the Tenasserim limestone, 
including two species of Boea closely allied to plants of Tenasserim and 
Borneo, and Calanthe vestita, only known from the limestone rocks of 
Tenasserim and Borneo. Here we have the remains of a xerophytic flora 
