206 Willis . — The Relative Age of Endemic 
Natural Selection (6, pp. 6-16), which may be exactly repeated with illustra- 
tions drawn from the flora of New Zealand, where the Ceylon estimates are 
replaced by actual measurements. If these questions cannot be parried, the 
case for Natural Selection is a very forlorn hope. 
A very valuable result of Mr. Ridley’s paper is the stress which it lays 
upon the various causes which may modify the action of my age and area 
law. Some of these causes probably come into action in almost every 
single case of any one individual species, though upon large numbers, and in 
the long run, they cancel out. All might have been covered had I added to 
my tentative statement of the law the phrase ‘ so long as conditions remain 
constant or words to that effect. We may enumerate some of these factors 
here, but the list will no doubt be largely added to. The law itself, how- 
ever, in my opinion, will stand as valid, when it is applied, like Mendel’s, 
only to groups of forms. 
Chance (the operation of causes as yet not understood) ; 
Action of man in opening up a country, cutting of forest, exploring, 
making fires, &c., &c. ; 
Interposition of barriers, such as mountains, broad rivers, deserts, arms 
of the sea, sudden changes of climate from one district to the next, 
and the like ; 
Geological changes, especially if involving change of climate ; 
Serious changes of climate ; 
Natural selection ; 
Local adaptation (a species may have a peculiarity which is useful 
in one country and valueless in another) ; 
Dying out of occasional old species ; 
Arrival of a species at its climatic limit ; 
Density of vegetation upon the ground at the time of arrival of 
a species ; 
Presence or absence of mountain chains in the land over which the 
species has to travel in arriving ; 
Relative width of the union between the country of departure and that 
of arrival (the wider it is the more rapid may be the spread of 
the species in the new country), 
and so on. There are numerous factors which may exert a disturbing 
influence, but that no more affects the validity of my law than does the 
resistance of the air, which prevents a thing from falling in exact accor- 
dance with the law of gravity, affect the validity of that law. I am far 
from denying that in individual cases plants may be relic endemics, or 
may have had their area of distribution greatly extended by the action 
of Natural Selection, or in other ways altered ; but in large numbers and 
the long run such things do not show in my figures, which indicate that 
the overwhelming factor in distribution is simply age . 
