3^4 A NATURALIST IN THE PACIFIC chap. 



remember that the small group of Tristan da Cunha, equally 

 isolated in the South Atlantic, possesses an endemic species of the 

 same genus. He will discover in the recognised dispersing 

 agencies of wild ducks and other waterfowl an explanation of the 

 occurrence in Hawaii of the aquatic genera Naias and Potamogeton ; 

 but he will be puzzled at their restriction to this group alone of the 

 three tropical Pacific archipelagoes here especially discussed. 



Amidst these various perplexities he will probably look with 

 relief on the appearance of Phytolacca brachystachys, an endemic 

 species of the American " pokeweeds " ; and he will feel grateful to 

 the American botanists like Professor Weed when they tell him 

 that in the United States crows, blackbirds, and other birds 

 successfully disperse these plants, the seeds of which are some- 

 times able to pass through the alimentary canal undigested. 



But by far the most significant lesson that the student of 

 distribution will carry away from his study of the Hawaiian 

 residual genera will be that which he learns from the genera 

 Embelia and Naias. He perceives here that not only with a 

 typical land-genus has specific differentiation occurred to much the 

 same extent in the continental and insular localities of its range, 

 but that even with a typical genus of submerged aquatic plants, 

 where the conditions of existence are as uniform as they are varied 

 in the case of land plants, the process of differentiation has pro- 

 ceeded on the same broad lines in the interior of a continent and 

 in an island in mid-ocean. 



The following notes on some of the residual genera refer more 

 particularly to matters connected with distribution and dispersal. 



Osmantkus (Oleaceae). — This genus, according to the Index 

 Kewensis, contains six species localised in their several habitats 

 of North America, Hawaii, Japan (two), Hongkong, and the 

 Himalayas. Its representative in this group is the Hawaiian Olive, 

 the Olea sandwicensis of Gray, a prevailing tree in the lower and 

 middle woods (i,ooo to 4,000 feet) of all the islands, which, like 

 other Hawaiian plants, such as those of the genera Eurya and 

 Antidesma, indicates that the group has been- sometimes indepen- 

 dently stocked from the regions of the northern hemisphere. The 

 drupe of this tree contains a stone two-thirds of an inch (17 mm.) 

 in length, and suitable for dispersal by frugivorous birds ; and 

 birds have evidently distributed the tree all over the group. 

 In fact Mr. Perkins in mentioning the favourite food of birds of 

 the Hawaiian genus, Phaeornis, refers to the fruits of this tree as 

 well as of the Opiko (Straussia) and of the Olapa (Cheirodendron). 



