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 



