18 THE DISPERSAL OF PLANTS [pt. i 



distance from the mainland, but as regards frequency of arrival 

 of species from elsewhere. There are few local island species 

 among the beach plants, which are continually arriving with the 

 ocean currents, more among the mountain-top plants, where 

 probably birds most commonly alight on arrival, and most 

 among those of intermediate elevation. 



He regards as the oldest, on the whole, those groups with 

 actual genera confined to the island or group of islands, then 

 those with genera all of whose species are endemic, followed by 

 those having genera with some species endemic and some widely 

 distributed, and as the youngest, on the whole, those having 

 only genera with no species endemic. He regards the develop- 

 ment of endemic species as due to what he calls the principle of 

 differentiation. They are most often allied to some common 

 widely ranging and polymorphous species which he regards as 

 the parent. To this very important conclusion he returns in 

 other papers (45-6), and in his later book upon the Atlantic 

 Ocean (47), where he comes to much the same general conclu- 

 sions upon distribution as in the case of the Pacific. 



In fact, as wc shall see in more detail in the course of this 

 book, Guppy arrived at, and published a year sooner, the same 

 general conclusions to which I also have been driven by a life- 

 time spent, like his, in travel and botanical investigation, chiefly 

 in the tropics. 



Interesting facts in regard to the distribution of the Compositae 

 have been worked out by Small (103). The fruits of these plants 

 are usually carried by aid of a parachute-like tuft of hairs, as 

 may be well seen in the dandelion. The general evidence that he 

 marshals goes to show that the fruits may frequently be dis- 

 persed to a distance of from four to twenty miles, and even at 

 times over one hundred (cf. Ritigala and Krakatau above). His 

 experimental observations show that so long as the relative 

 humidity of the air remains at a figure that keeps the pappus 

 open, a wind of two miles an hour (barely perceptible) is enough 

 to keep the fruit floating in the air for an indefinite period, but 

 if the moistness increases, the pappus closes, and the fruit soon 

 falls to the ground. Thus the dispersal of these j^lants on land, 

 where the air in general is drier, may at times be to great dis- 

 tances, but over the sea such conditions of dryness will com- 

 paratively rarely occur. 



Important papers have also been published by Ridley on the 

 actual facts of spreading observed by him (91-2). For example. 



