XX THE AGE OF FERNS 225 



Hawaii directly and not through South Polynesia. The mountain- 

 ferns of this group could hardly have been received by that route, 

 since, as is shown below, they do not as a rule occur in that 

 region. 



Some other interesting relations present themselves in con- 

 nection with the Hawaiian ferns and lycopods when we consider 

 the distribution of its non-endemic species in the other two groups 

 of Fiji and Tahiti. Out of these species, some eighty in all, not 

 more than half are common to all three groups, and about two 

 dozen have not been found either in Fiji or in Tahiti. Of these 

 last quite half are mountain species in Hawaii, having their station 

 at elevations exceeding those of the highest districts of Fiji and of 

 the several islands of the Tahitian area, excepting the limited 

 region comprised in the uplands of Tahiti itself 



A glance at the list, given in Note 65 of some of the mountain 

 ferns of Hawaii not recorded from Fiji and Tahiti will show that 

 these species are very widely distributed. Ferns .and lycopods 

 found in the Himalayas and in the Andes meet on the higher 

 slopes of the lofty mountains of Hawaii and in no other of the 

 less elevated island-groups of the open Pacific. This distribution 

 of the vascular cryptogams thus foreshadows a principle that will 

 come into prominence in the case of the flowering plants, namely, 

 that difference in elevation has been an important factor in 

 determining some of the contrasts between the Hawaiian, Fijian, 

 and Tahitian floras. The contrasts here implied are those 

 connected with the climatic conditions of station, since several 

 plants of temperate • regions, such as Aspidium filix mas, 

 Asplenium trichomanes, Asplenium adiantum nigrum, &c., 

 that are at home in the highlands of Hawaii, do not occur in 

 either Fiji or Tahiti. We can infer that widely ranging ferns 

 and lycopods have been dispersed over the oceanic groups of 

 the tropical Pacific with a fair degree of uniformity, and that 

 any marked contrasts in their distribution may be attributed to 

 considerable differences in the altitude of the islands. 



In appreciating such a conclusion, and in dealing with 

 apparent exceptions to the rule, the relation between the vertical 

 range of a species and its lateral distribution has to be considered. 

 We find, for instance, that whilst the Common Bracken (Pteris 

 aquilina) is a mountain plant in Hawaii, it occurs also in Fiji 

 and Tahiti. Since, however, it is found all over the temperate 

 and tropical regions, and has a vertical range in; Hawaii of from 

 800 to 8,000 feet, any difficulty in this respect is thus explained. 

 VOL. II Q 



