On Endemism and the Mutation Theory. 
BY 
H. N. RIDLEY, F.R.S. 
T HE history of the rise and fall of species of plants or animals is 
a subject of great interest, and it is one which has very seldom been 
worked out. Dr. Willis has, however, lately, in a paper to the Royal Society, 
Phil. Trans., B, ccvi, p. 307, and one published in the ‘ Annals of Botany ’ 
(vol. xxx, 1916, p. 1), attempted to formulate a law dealing with the rarity 
or commonness of species and its bearing on the endemic plants of Ceylon. 
He bases his argument on a study of Trimen’s 5 Flora of Ceylon’, in which 
work Dr. Trimen states under the description of each plant whether it is 
very common (VC), common (C), rare (R), and very rare (VR). Now it is 
known that Trimen in putting these notes merely referred to the number of 
specimens in the Ceylon herbarium, and Dr. Willis (Ann. Bot, 1 . c., p. 4) 
admits that the figures ‘ are based on herbarium specimens ’. Dr. Trimen 
himself in vol. i, p. ix, writes as to general distribution and comparative 
frequency : ‘ Very much has yet to be done in tracing out the distribution of 
plants through the island, and the information here given is very imperfect 
and will be much modified and increased by further investigation.’ 
No further investigation on these lines has apparently been done in 
Ceylon. In Dr. Trimen’s day Ceylon was not so easy of exploration as it 
is now, and even now large areas are difficult to get at, and would require 
a large staff of botanical collectors to take a thorough census of the various 
species throughout the island. I do not think that this has been done 
indeed for any region of the world except the British Isles. 
The number of specimens of a species in a herbarium does not show at 
all the abundance or rarity of any given plant. Frequently a tree in the 
tropics does not flower for very many years, and collectors do not collect 
specimens which are not in flower or fruit. One of the commonest trees in 
Sarawak is Koompassia parvifolia , theTualang. The only flowering specimen 
known is one in the Florence herbarium picked up by Beccari, and the only 
fruiting ones were collected by me last year. Thus in herbaria this would 
figure as VR (very rare), whereas it should be VC (very common). 
Conversely, a plant only known from a single tree often in flower and 
easy of access might be extensively represented in herbaria, as every 
[Annals of Botany, Vol. XXX. No. CXX. October, 1916.] 
P p 2 
