XX THE AGE OF FERNS ' 223 



the fern and lycopod floras of Fiji and Hawaii are similar in this 

 respect. Yet in each the average number of species to a genus has 

 a separate significance. A genus may acquire its species through 

 immigration, or they may arise from its formative energy within 

 the particular area. The first principle has been largely dominant 

 in Fiji, the last in Hawaii, and the resemblance between the 

 average number of species in a genus in these two groups is to a 

 large extent accidental. Between the vascular cryptogams of Fiji 

 and Tahiti, however, such a comparison is legitimate ; and since 

 the average formative energy is in these groups about the same, 

 the difference is to be attributed to a lessened number of immi- 

 grants into the Tahitian area. 



The results, so far mentioned, are in the main consistent with 

 the geographical position and the degree of isolation of these three 

 areas. From their proximity to the large continental islands of 

 the Western Pacific, the Fijian islands would have readily received 

 a great number of immigrants from the west, since the intervening 

 sea is not over 500 miles in breadth. They lie in the track of the 

 main line of migration into and across the South Pacific, a track 

 which has been followed by flowering plants and animals as well as 

 by aboriginal man. Assuming that the migration of the vascular 

 cryptogams extended from Fiji eastward to Tahiti, fewer of the 

 immigrants would reach the last-named group. Fewer still would 

 reach the Hawaiian islands, which excluding the groups of low 

 coral islands to the southward are cut off on all sides, whether from 

 the Fiji-Samoan and Tahitian areas, from the coasts of North 

 America, or from the regions north and west, by a breadth of 

 ocean that is never less than 1,500 miles. 



That the main track of the ferns and lycopods across the South 

 Pacific to Tahiti has been eastward there can be little doubt. This 

 is indicated in the tables given by Drake del Castillo for Eastern 

 Polynesia, and also by an analysis I have prepared of the distribu- 

 tions that he gives for the species of the Tahitian region (see Note 

 64). Out of the 154 species there are only two that belong exclu- 

 sively to the American side of the Pacific ; whilst 58 are derived 

 exclusively from the Asiatic side, and mainly from Indo-Malaya. 

 The drift of the ferns and lycopods eastward from Fiji is also 

 brought out in the number of Tahitian species common to Hawaii 

 and Fiji. Of these about "j^ per cent, are common to Fiji or to 

 the groups around, and only 30 per cent, occur in Hawaii. The 

 Tahitian species found in Hawaii occur also in Fiji with the excep- 

 tion of two or three mountain species which have doubtless failed 



