Ridley . — On Endemism and the Mutation Theory . 571 
Just in the same way in Ceylon we have isolated species like the 
Dipterocarpeae and Acrotrema , a native of South India and of the Malay 
Peninsula, both countries formerly connected with Ceylon, and from both of 
which mutations or species may have come, and by crossing, formed the 
seven recorded species of Acrotrema (the largest number of species of the 
genus known anywhere) now inhabiting Ceylon. Indeed both Thwaites 
and Trimen considered it probable that some of these species were natural 
hybrids. I have only been able to outline this suggested cause of the state 
of affairs which the distribution of plants and the great preponderance 
of species in one group and the rarity of others seem to evidence. To give 
fuller instances and to give such evidence as is known would require 
a large-sized volume. Nor do I at all intend to intimate that this alone 
accounts for the immense number of species and genera we see in the world, 
and the facts of their comparative abundance or rarity. The whole subject 
is more complex than that. 
It is perhaps as well to point out that it is not necessarily the change 
to another country that is the cause of fixation of mutations ; every country, 
every district, has a variety of localities into which a plant may push by 
a suitable mutation. Given just the similar conditions for the plant, there 
is no probability of any mutation for the form which finds its way there 
being in any way more suitable, but where conditions are different, a muta- 
tion even slightly more suitable would be necessary for the plant to establish 
itself. Thus Ixora Lobbii , Loud., is a tall, broad, oblong, lanceolate-leaved 
shrub, the leaves about two inches across. It grows in thick forest, where, 
owing to the comparatively scanty sunlight, it requires broad leaves to 
receive a sufficient supply of light. Through the forest runs the rapid 
stream Tahan. The seeds of the Ixora (dispersed by birds) must frequently 
be dropped nearer the river edge. The vegetation is thinner here, and 
there is more light. At certain times the river rushes in spate through the 
wood-edge, and a narrow-leaved form would be less injured by the water- 
rush than a broad-leaved one. On the actual rocky edge of the stream the 
rush is much more violent and constant, and only very narrow leaves can 
stand against it. Gradually, by eliminating the wider-leaved mutations, 
there appear forms with very narrow willow-like leaves, van salicifolia. 
This edge of the riyer is in bright sunlight, so that the advantage of 
broad leaves to the plant is lost. Now suppose a fall of the lofty hill- 
side so as to block up and divert the Tahan river, leaving the Ixora in 
an area quickly covered by dense forest. Owing to the reduction of 
sunlight, the narrow leaves are now an injurious mutation, the plants 
with slightly wider leaves would supplant those with willow leaves, and 
eventually by the elimination of the narrow-leaved forms the plant would 
revert to the jungle form. 
The ‘ struggle for existence ’ is not of course merely the struggle 
