4 
Ridley.—The Distribution of Plants. 
bevmudianum , Linn., Eriocaulon septangulare, With, and two species of 
Spiranthes , while a later land connexion between Portugal and south-west 
England and Ireland have given us Pinguicula gran diflora, Linn., Ery- 
thraea Massoni, Sweet., Arbutus unedo , Linn., Saxifraga umbrosa , Linn., 
S'. Geum , Linn., and some other species. 
The same fluctuation of land and sea has been going on in the Malay 
Peninsula. This area has been connected, I gather from its flora, with 
Borneo at one time, and then separated by sea; with Sumatra ; with the 
Tenasserim region, and here, as I hope to show later, then broken off and 
joined again. Besides these land and sea changes, all of which altered to 
some extent the constituents of the flora, we have the constant and con¬ 
tinuous denudation of the mountains, causing the disappearance or scarcity 
of many high mountain plants and the formation of plains of sand or mud, 
according to the constituents of the mountains denuded, and producing 
a lowland area soon invaded and covered by a flora quite distinct from 
that of the mountain area. 
I give an instance of this factor in distribution from the Malay 
Peninsula. This region is now connected with Tenasserim by the Isthmus of 
Kra, and I have shown in a paper on the Flora of Lower Siam (‘ Journ. Roy. 
As. Soc. S. Br.’ 59 , p. 17) that the Isthmus of Kra contains a totally 
distinct flora from that of the Malay Peninsula, no less than forty genera 
occurring there which are absent from the Malay Peninsula, while sixty 
genera of the Malay Peninsula are missing altogether. The Isthmus of Kra 
is a sandy, flat area in which are scattered large islands of limestone, which, 
from the occurrence of sea-bird guano in their caves, shows that they were 
at no very distant period surrounded by sea. The climate is less wet than 
farther south, and there is a dry period in which most of the herbaceous 
plants wither. 
The mountains of the centre and west of the Malay Peninsula are of 
granite, and by denudation form mud alluvium, and on the west coast none 
of this flora occurs, but on the east coast the mountains are of sandstone 
and have been washed down to form large areas of sandy heaths running 
down to the east corner of Singapore. All along this coast you may find 
plants of the Isthmus of Kra flora, and the farther north you go the more 
you will find. 
Here we have a sample of the formation of a new sand-hill area being 
gradually invaded by a northern flora, since the land connexion was formed 
by the silting up of a shallow sea between the Burmese and Siamese 
regions and the Malay Peninsula, which was previously an island. 
Changes due to Human Agency. 
The alteration of a flora by human agency consists of the destruction 
of species, and the introduction of other species accidentally or intentionally. 
