J 



68 A NATURALIST IN THE PACIFIC chap. 



island of Hawaii. The mature fruits of this genus have never been 

 experimented on by me ; but there is nothing in the structure of 

 the fruits to indicate that they have any buoyancy, or to show that 

 they differ in this respect from the fruits of other completely 

 submerged aquatic plants like Ceratophyllum, Ruppia, and some 

 of the Potamogetons. It is to ducks and other waterfowl that we 

 must attribute the dispersal of this and the other genera just 

 mentioned over wide tracts of ocean, a subject dealt with in dis- 

 cussing those plants. 



The Hawaiian Group probably represents the most isolated 

 locality occupied by this genus, since none of the other islands from 

 which species have been recorded, such as New Caledonia, Mauritius, 

 and Bourbon, are so far removed from continental regions. The 

 source of the Hawaiian form of Naias marina lies evidently on the 

 Asiatic side of the Pacific, since it is referred by Mr. Rendle to the 

 variety " angustifolia," an Asiatic plant found also in the island of 

 Bourbon and in West Australia, but not recorded from the New 

 World. The important little monograph of the genus by Mr. Rendle 

 (" Naiadaces," in Engler's Das Pfianzenreich, 1901) is full of sugges- 

 tiveness for the student of plant-distribution. His interest is excited 

 when he discovers that one of the most typical genera of aquatic 

 plants displays the same principle of differentiation at work that is 

 so well illustrated by many of the land genera of the Pacific islands. 

 I refer to the principle implied in the existence of a widely-ranging 

 genus comprising " a polymorphic species occurring over almost 

 the whole area of the genus," as well as a number of less widely 

 distributed species, most of which have " restricted areas and fall 

 for the most part into small geographical groups." I have just 

 been quoting Mr. Rendle's description of the distribution of Naias, 

 the " polymorphic " species concerned being N. marina ; but it need 

 scarcely be remarked that it would apply just as well to several of 

 the land genera dealt with in the previous chapter (XXVI.), such as 

 Alphitonia, Metrosideros, Pisonia, &c. 



Although there is such a contrast in the degree of uniformity of 

 their life-conditions between land and water plants, a strictly 

 aquatic plant being but slightly affected by changes in the physical 

 conditions that are accompanied by a complete transformation in 

 the character of the terrestrial vegetation, yet — and this is the 

 important point — we find the same principle of differentiation at 

 work with both land and water plants. If one wished to produce 

 proof of the contention that the production of new species is largely 

 independent of external conditions, one could not do better than 



