2 
Willis.— The Evolution of Species in Ceylon , 
assortment of species occupying spots characterized by special local 
conditions, but are distributed in the island according to certain fairly 
definite rules, which obtain equally well in other parts of India and the 
East, and which I found with much interest to hold also in the case of 
the numerous endemic species of the state of Rio de Janeiro. These rules, 
as I have already indicated, and shall endeavour further to prove below, 
appear to me entirely out of harmony with the current idea that the great 
differences in the geographical distribution of species are largely due to 
the operations of Natural Selection. When one works with any number 
exceeding, say, 15 or 20 allied species of similar distributional origin, one 
finds that any one group behaves like any other group. 
In one of the papers referred to 1 I published the statement that the 
Ceylon endemic species were rarer than those of wider distribution that 
occurred amongst them, and made up my mind to enumerate the whole 
flora in this respect at the first opportunity. This arose with the publication 
of my £ Revised Catalogue of the Ceylon Flora and at the same time 
it struck me that the evidence I was collecting would be rendered much 
more conclusive were the non-endemic species divided into two groups, 
those found also in Peninsular India, and those with yet wider dispersal 
than this. 
Marking the species in the Catalogue thus, and entering for each the 
degree of rarity given by Trimen, who in his great Flora of Ceylon divides 
all species into six classes— Very Common, Common, Rather Common, 
Rather Rare, Rare, and Very Rare— -I had not done many pages before 
I realized that I had come upon a general law, which shows as clearly in 
the figures as does Mendel’s Law in any table of results of crossing. Pages 
II to 15 of the Catalogue, for instance, show the following: 
Table I. 
Ceylon spp. 
Total. 
Ceylon- Peninsular- 
Indian. 
Total. 
Wider. 
Total. 
1 . VC 
— 
— — — 
1 
1 
— 1 — — 1 
2 
4 
3 
2 3 
4 
16 
2. C 
1 
— 1 1 
2 
5 
— — — — 1 
1 
5 
8 
7 3 
3 
26 
3 . RC 
— 
1 — 2 
5 
8 
- 1 - - 3 
4 
2 
3 
6 3 
H 
4 . RR 
— 
— — — 
0 
O 
3 
— — — — — 
— 
3 
2 
1 — 
— 
6 
5 . R 
2 
— 1 2 
3 
8 
— 1 1 1 1 
4 
2 
1 2 
— 
5 
6. VR 
8 
212 
4 
17 
- - 3 ~ ~ 
3 
3 
2 
1 3 
***“ 
9 
If VC (Very Common) be marked 1 and the 
others 
up 
to VR 6 , 
we 
may 
easily calculate the average rarity by multiplying the total under each 
head of VC, &c., by the mark for that head, and dividing the grand total 
by the total number of species. This shows that the mean rarity (for these 
1 Some Evidence against . . , Natural Selection, Ann. Perad., vol. iv, 1907, p. 12, 
