96 A NATURALIST IN THE PACIFIC chap. 



I do not, therefore, think that the buoyancy of seeds and fruits 

 has had any relation either in the present or in the past to the 

 density of the sea. Nor is it to be supposed that any slight 

 variations in density in the course of ages would have materially 

 affected the dispersal of plants by currents. It is to be inferred 

 that the physicist and the geologist would be prepared to grant 

 only small variations, such as a change from i'020 to ro25. It 

 will be gathered from what has been said before that changes of 

 this nature would have a very slight influence on the buoyancy of 

 seeds and fruits, since the plants they would affect would be very 

 few. The change that the student of plant-dispersal would require 

 to produce any marked alteration in distribution would be in 

 amount alarming to the physicist. 



Whether or not the oceans have been getting fresher or salter 

 in the course of ages (see Note 42), we will be moderate in our 

 demands, and will listen to the physicist when he argues that a 

 diminishing density, for instance, from i'035 to 1*025, in the course of 

 ages might explain some of the peculiar features in the present 

 isolation of insular floras. Many seeds, he would contend, that 

 could float across an ocean having a density of r035 would be 

 unable to accomplish it when the density fell to ro25. It has, 

 however, been remarked that the critical point of density for the 

 flotation of seeds or fruits that sink under present conditions is 

 probably about I'loo. Cases of such a fine adjustment to the 

 density of sea-water are too few to endow this argument with any 

 weight. Or it might be suggested that with a gradual increase in 

 density in the lapse of ages seeds might float now that sank before, 

 or they might float for a longer period. Such a change, however, 

 would not have much effect, since nearly all the seeds and seed- 

 vessels that sink in our rivers sink also in our seas, and a much 

 greater increase of density is required to make any difference. 



Yet, although we might term the sinking of a seed or fruit an 

 accidental attribute of certain plants, just as we might regard the 

 floating of a log as an accidental attribute of a pine, since in either 

 case the specific weight might have been acquired without any 

 direct relation to the density of water, still the sinking of the seed 

 or fruit signifies a profound distinction not only, as is stated below, 

 in plant distribution, but, as we shall see later on, in plant-develop- 

 ment. Especially striking, says Prof. Schimper (p. 153), is the 

 dependence between an over-sea area of distribution and a station 

 at the coast in the case of species of the same genus of which some 

 belong to the littoral and some to the inland flora. In the first 



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