62 ISOLATION [ch. vii 



mechanism of their formation might be the action of cosmic rays, 

 which would be more marked at high elevations. 



It was clear that these mountain tops showed a distribution of 

 plants like that which was shown by a group of islands forming 

 an archipelago. Now the only thing obviously in common be- 

 tween the two was isolation, and I therefore drew the conclusion 

 that isolation as isolation favoured the production of new forms. 

 At that time we knew little or nothing about the genes and 

 chromosomes, and since then Harland has put forward the likely 

 suggestion that long continued gene separation may lead to gene 

 change, which of course in its turn might lead to definite mutation. 

 Since about three-quarters of this mountain-top flora has no 

 special adaptation for distribution by wind or by animals, it is 

 highly probable that individuals of the more widely distributed 

 species lower down would very rarely reach the higher summits, 

 whose plants would be, and remain, very isolated. In this con- 

 nection it is worth special notice that the islands which show con- 

 siderable local endemism, like the Hawaiian islands, are very 

 commonlv mountainous. 



Whether the mutation w^hich the author considers to have been 

 the origin of any one of the species was due to one or the other 

 cause, or to both, there would not be, in either case, any serious 

 opening for gradual adaptation under the influence of natural 

 selection. It must also be remembered that the number of indi- 

 viduals is very small (cf. p. 25). In this connection, I may quote 

 from Age and Area the footnote on p. 206: "A few days before 

 I left Rio, Dr Lofgren found, on a little island about three miles 

 off the coast, a new and very distinct Rhipsalis, of enormous size. 

 He told me that there were only four examples on the island." 

 I may also refer to the examples given in the same book on 

 p. 151. 



Sixty-eight of the 108 endemics (including the eleven varieties) 

 are found upon one mountain only, the other forty upon more 

 than one. It is a very striking fact that these mountain endemics 

 belong, not to small and local genera, but chiefly to large and 

 widespread ones, as was shown on p. 26. 



The general conclusion from this piece of work is that isolation 

 favours the development of new forms, and that local conditions 

 have but little eff'ect in developing, though they may have much 

 in determining the survival of, these new forms, and that conse- 

 quently natural selection, upon adaptational grounds, is unlikely. 

 It is more than doubtful whether any given species has been 



