496 
Willis.— Endemic Genera of 
‘ The area occupied at any given time, in any given country, by any 
group of allied species at least ten in number, depends chiefly, so long as 
conditions remain reasonably constant, upon the ages of the species of that 
group in that country, but may be enormously modified by the presence of 
barriers such as seas, rivers, mountains, changes of climate from one region 
to the next or other ecological boundaries, and the like, also by the action 
of man, and by other causes.’ 
The Ceylon figures were strong evidence in favour of Age and Area, 
but as soon as possible I obtained confirmatory evidence by work upon the 
flora of New Zealand (16, also 17-24), where I was able to give actual 
longitudinal measurements of the areas where the species occurred, and 
where the results came out with much greater clearness. Still further work 
(19) on the Orchids of Jamaica, on Callitris (a Conifer) in Australia, and on 
the flora of the Sandwich Islands (both flowering plants and ferns, separately 
treated), also confirmed my conclusions. It will be well to quote here the 
distribution figures for New Zealand, as illustrating what I have said about 
the graduation of endemics one way and wides the other : 
Range in N . Z . 
Wides . 
Endemics . 
881-1080 miles 
2or 
112 
641-880 
11 
77 
120 
401-640 
11 
53 
184 
161-400 
11 
38 
190 
1-160 
H 
30 1 
296 
Many people who have tried to apply Age and Area to their own work 
have become dissatisfied with it because they have ignored the reservations 
which are given in the statement of the rule above, and we must now 
devote a few moments to their consideration. To begin with, it must never 
be applied to single species, but only to groups of at least ten allied forms, 
in order to cancel out the effects of differences among them in degree of 
local adaptation, of luck in transportation in the earlier rarer stages, and 
other more or less chance effects. In the second place, allied species must 
be taken, as in general they will belong to the same ecological type, and 
will react to their surroundings in more or less the same manner. It 
cannot be too often repeated that Age and Area does not as yet offer any 
means of distinguishing the co7nparative ages of differing ecological types, 
though I hope to show in a later paper that this is beginning to be possible 
to a slight extent. A group of Compositae may occupy the same area in 
the same country as a group of Dipterocarpaceae, but no one would suggest 
that they were of the same age within the country, though both groups obey 
the Age and Area rule. An area which might take the latter one thousand 
years to cover might be covered by the Compositae in one or two. Again, 
the conditions must remain reasonably constant. Any serious change 
might kill out some species. In essence, a change of conditions means 
1 Largely undoubted introductions of recent years. 
