396 A NATURALIST IN THE PACIFIC chap. 



The most reasonable explanation of the extraordinary dis- 

 tribution of Lindenia is that it was in a past age found over the 

 tropical regions of both America and the Old World, and that it 

 has died out over the greater part of its original area. To study 

 the means of dispersal of plants with such a distribution seems 

 almost futile. I am inclined to think that the limited range of 

 Dolicholobium, so frequently its station-companion in Fiji, may be 

 similarly explained. 



LimnantJiemiun (Gentianaceae) 



This interesting genus of aquatic plants is dispersed over the 

 tropical and temperate regions of the globe, but with the exception 

 of Fiji and the New Hebrides it is not found in oceanic groups, 

 though it occurs in large continental islands like New Caledonia 

 and Cuba. About twenty species are enumerated in the Index 

 Kewensis, but it is stated in the Genera Plantaram that they can 

 probably be reduced to ten, the reduction being chiefly applicable 

 to the tropical species, nearly all of which are reducible to varieties 

 of L. indicum, the temperate species being often very distinct. It 

 would thus appear that although dispersal is still active in the 

 tropics, it is in part suspended in the temperate zone, and we seem 

 to possess in L. indicum a typical polymorphous species that has 

 played the role of Naias marina in the warm, fresh waters of the 

 globe (see page 368). 



Although some of the temperate species, like Limnanthemum 

 nymphaeoides in Europe and Northern Asia, have a wide range, 

 it is probable that this is connected not so much with means of 

 dispersal, as with its relation to present and past drainage-areas. 

 Rivers in the lapse of ages change their courses and carry their 

 aquatic floras with them, leaving, however, a few of their plants 

 around the springs and in the lakes which serve still as centres of 

 dispersal. Rivers may even exchange their plants in flood-time in 

 extensive level districts. Nor is the occurrence of the genus in the 

 Old and New Worlds in the northern hemisphere to be connected 

 with questions of dispersal across an ocean. Except in the case of 

 small-seeded plants, like Nasturtium and Lythrum, where the 

 dispersal could be carried on by water-fowl, the plant-species being 

 often identical on both sides of the Atlantic, it is probable that 

 most of the large-seeded river-side genera common to Europe and 

 North America, such as Iris and Acorus, had in past ages their 

 home in the extreme north, whence the plants spread as from a 





