XI, C, 4 
Copeland: Natural Selection 
153 
lolepis,” Niphobolus, Drynaria, Dryostachyum, Thayeria and Lecanopteris, 
and the American Lepicystis, Campyloneuron and Phlebodium must all be 
younger groups than the cosmopolitan Polypodium, ancestor and cousin at 
once of them all. A group with wide and discontinuous distribution must 
be ancient enough to have become widely distributed, and to have died 
out in the intermediate territory; it may not be older than a group with 
equally wide continuous distribution, but its minimum probable age is 
greater. 
The relation between commonness in one country and distribu- 
tion outside that country has not been in the past so well appre- 
ciated as the relation between age and distribution, but is made 
equally clear by Doctor Willis’s tables. It is unquestionably true 
that in almost any country such a relation exists, and a little re- 
flection suffices to show that it must in general exist. Without 
ever putting it into words, I have for years acted on the assump- 
tion that there is such a relation. It has repeatedly happened 
that after describing a species from a single collection, under the 
impression that it was quite local, and after a reasonably care- 
ful search for previous description elsewhere, I have found the 
plant to be rather common in the Philippines, and have then 
made a renewed search for previous description from some neigh- 
boring land; the idea being exactly that which Doctor Willis 
has demonstrated to be sound — that a species common and of 
rather wide distribution in a given island, or group, or region, 
is therefore to be expected to occur in other islands or regions. 
Locally extensive distribution and commonness are evidence both 
of considerable age and of the ability of the species to maintain 
itself and to spread, and age and the ability to spread are in 
themselves reasons to consider it likely that the plant has spread 
extensively. 
For the sake of emphasis, I repeat that both time and the 
ability to survive and spread are necessary in order that any 
plant can have become widely disseminated. In the four-year 
course of our College of Agriculture, some students graduate in 
three years, some graduate after five years, while the largest 
number of those who enter the course graduate in four years 
or disappear without finishing the course. In determining when 
any student will graduate, time is one paramount factor. There 
is probably no human being who could finish the course in one 
or two years; but, because time is a paramount consideration 
in determining who will graduate during any given year, shall 
we conclude that ability has nothing to do with it? Surely not. 
In the case of any given student, the date of graduation is fixed 
by the date of entrance, by his ability, and by other considera- 
tions (sickness or death, for instance) ; and the fact that time is 
