564 Ridley.— On Endemism and the Mutation Theory . 
river is a rapid running, rocky mountain stream flowing through the forests. 
Along the banks of this stream I found a whole series of plants which 
possessed long narrow leaves of willow-leaf shape. They included species 
of Calophyllum (Guttiferae), Ixora (Rubiaceae), Hygrophila (Acanthaceae), 
Didymocarpns (Gesneraceae), Podochilus (Orchideae), Antidesma (Euphor- 
biaceae), Ficus (Urticaceae), M etas tom a (Melastomaceae), Rhyncopyle 
(Aroideae). These fringe the rocks and are often submerged by a very violent 
torrent. All or almost all are allied to species living out of the reach of the 
water, with broad lanceolate leaves. Should these broad-leaved plants be 
subjected to the rush of the torrent, their leaves would be torn to bits and the 
plants destroyed. But the form of the leaves varies occasionally, e.g. in Ixora , 
farther in the forest, we get forms with more narrow lanceolate leaves. 
According to the theory of natural selection by variation, the Ixoras with 
lanceolate leaves could establish themselves along the stream edge more 
readily than the broad-leaved ones which have their leaves destroyed by 
the torrent. A variation with narrow leaves could approach nearer the 
edge where the rush is more violent, and so it could go on till we found, as 
we do among the rocks where the torrent at certain times is excessively 
furious, the Ixora growing with foliage more like that of a stream-water 
plant, thriving and holding on with a mass of strong woody roots, and 
a tough short stem, with none of its leaves injured or its boughs broken. 
The theory of Natural Selection by infinitesimal variations will account for 
this, but the mutation theory will not. 
On the sandy shores of the Pahang river grows a species of Vitex , 
a prostrate creeping shrub, throwing up short branches four to six inches 
tall, with simple ovate blunt leaves 1 to i-io in. long ; the flower spikes are 
a little over an inch long, with rather showy blue flowers. As this looked 
likely to prove an ornamental plant suitable for bedding, I brought some to 
Singapore, where it immediately turned into Vitex trifolia ) an erect shrub 
or small tree about 10 feet tall with trifoliate leaves ; the leaflets obovate 
acute or elliptic, 2-5 in. long, 1 in. wide, with a petiole 0-4 in. long, and 
raceme or panicle of smaller flowers 6 inches long. The sea-shore form was 
abundant and occurs elsewhere, and would certainly be considered a distinct 
species, differing as it does importantly in all its parts — stem, leaves, inflo- 
rescence, and flowers. Yet Dr. Willis says that ‘ no evidence has ever been 
brought forward to prove that local species are adapted to local conditions ; 
it is simply an hypothesis’ (Ann. Bot., 1 . c., p. 15). 
Microcarpea muscosa , R. Br., is a small scrophularineous plant which 
grows on the edges of ponds. Where the water subsides and the plant is left 
on the bank it can be seen to be erect, 3-4 inches tall with little violet flowers, 
but beneath the water it forms large, short tufts, and the corolla scarcely pro- 
jects beyond the calyx, the limb of it, almost reduced to a rudiment, bearing 
mere traces of its violet colour. There are numerous instances of similar 
