Endemic Genera of Plants in their Relation to Others. 
BY 
J. C. WILLIS, M.A., Sc.D., F.R.S., 
European Correspondent of the Botanic Garden , Rio de Janeiro. 
With six Diagrams in the Text. 
Contents. 
PAGE 
Review of the Hypothesis of Age and Area 493 
Endemic Genera 501 
Review of the Hypothesis of Age and Area. 
I NQUIRY is frequently made as to where a brief summary can be 
found of the ideas meant to be conveyed by what I have termed the 
hypothesis of Age and Area, and it will be well perhaps to commence this 
paper, which is the first of a new series in which somewhat larger issues will 
be treated, with such a summary. The new facts to which attention will be 
called below were discovered by aid of the hypothesis. 
Examining on many occasions, from 1896 onwards, the volumes of the 
Flora of Ceylon ( 7 , 8), so carefully worked up by Thwaites and Trimen, 
I gradually found, somewhat to my surprise, that the strictly local species 
confined to that island, or endemic species, as we usually call them, which 
are very numerous in Ceylon, showed on the average the smallest areas of 
distribution there, whether in the grand total or in individual families (cf. 10, 
p. 12). On the older view of the meaning of endemic species, which I then 
held, this seemed a very remarkable thing — that species which were 
generally looked upon as having been specially evolved to suit the local 
conditions should be so rare in those very conditions. If these species were 
specially adapted to Ceylon, therefore, it could not be to the general condi- 
tions of the island, but must be to strictly local conditions within its area. 
There was clearly no difference between island endemics and those of the 
mainland. Accordingly, still more remarkable did it seem when I came to 
study in detail the local distribution of these endemic species in Ceylon, and 
found that, as a rule, they were not confined each to one spot or small 
region characterized by some special local peculiarity in conditions, to suit 
[Annals of Botany, Vol. XXXV. No. CX. October, 1921.] 
K k 
