XXV PSYCHOTRIA 315 



the tribe of the Rubiacese to which it has given its name. We have 

 here a genus that has overrun the tropical regions of the world, 

 probably originating in America ; and we may contrast it with the 

 relatively small Rubiaceous genus of Coprosma (with its three 

 score of species, and quite comparable with it from the stand- 

 point of capacity for dispersal), that, having its birthplace in New 

 Zealand, is only beginning to reach the mainlands of the New and 

 the Old World. 



One is a genus of the tropics and the other is a genus of 

 south temperate latitudes ; and both have occupied the Pacific 

 islands ; but Coprosma naturally finds its most appropriate station 

 on the cool uplands of Hawaii and Tahiti. We may ask, indeed, 

 whether the great contrast in the fecundity of the two genera, 

 dispersed as they are in the same fashion by the agency of fru- 

 givorous birds, is to be connected with questions of relative antiquity 

 or with geographical position. It would certainly have been a 

 more difficult task in the past, other things being similar, for a New 

 Zealand genus to stock the temperate regions with its species than 

 for a tropical American genus to overrun the warmer regions of 

 the globe. However that may be, the age of dispersal of both 

 genera is largely over now. 



A vast genus like Psychotria, that is not sharply defined from 

 other genera, presents difficulties to the systematic botanist which 

 are reflected in a complex synonymy ; but there are certain broad 

 facts which the student of dispersal can gather for himself without 

 much difficulty. When we look at its distribution in the islands 

 of the open Pacific, we find that the genus attains its greatest 

 development in the Western Pacific, there being from thirty to 

 forty species known from Fiji and quite a dozen from Samoa, and 

 that it shades away as we proceed eastward and northward, some 

 six species being recorded from Tahiti and the Marquesas, two 

 from Hawaii, and one from Juan Fernandez near the South 

 American mainland. The arrangement of the species shows fairly 

 conclusively that the genus Psychotria, as it is found in the Pacific, 

 has, like most of the other plants of this era of non-endemic genera, 

 been derived from the Asiatic side of the ocean. (The absence of 

 species of this genus from Mr. Cheeseman's Rarotongan collections 

 seems strange. It is represented by sonie species in Tonga, and it 

 is extremely probable that it will be subsequently found also in the 

 Rarotongan group.) 



That the age of dispersal of the genus Psychotria over the Pacific 

 islands has almost passed away is evident from the circumstance that 



