XX THE AGE OF FERNS 227 



" The evolution theory (writes Dr. Hillebrand, p. xxix) could 

 hardly find a more favourable field for observation than an isolated 

 island-group in mid-ocean, large enough to have produced a number 

 of original forms, and at the same time so diversified in conditions 

 of temperature, humidity, and atmospheric currents as to admit an 

 extraordinary development in nearly every direction of vegetable 

 morphology, uninfluenced by intercrossing with foreign elements." 

 Isolation thus admittedly offers the preliminary determining or 

 favouring conditions. This is directly indicated by the fact that 

 Hawaii possesses fewer genera of ferns and lycopods than either 

 Fiji or Tahiti, notwithstanding that it has the same area as Fiji, 

 and is in extent three or four times the size of the whole Tahitian 

 area. One effect of isolation in Hawaii has, therefore, been greater 

 room for the development of new forms. It has, however, already 

 been remarked that the islands of the Fijian area are much less 

 isolated than those of the Hawaiian group, and that in consequence 

 the free immigration possible in the one group has been checked 

 in the other. Fiji possesses in respect to vascular cryptogams at 

 least half as many species again as Hawaii, but Hawaii has three 

 or four times the number of peculiar species. Yet before this great 

 contrast can be ascribed to different degrees of isolation, it is 

 necessary to exclude another possible cause presented by the 

 greater range of life-conditions in Hawaii. It is possible that all 

 the Hawaiian peculiar species may belong to the higher levels, 

 elevations, as before shown, not represented in the Fijian islands, 

 which correspond only to the lowlands of Hawaii, that is, to levels 

 below 4,000 feet. If this is the case, the contrast between Fiji and 

 Hawaii would be connected mainly with a difference in life- 

 conditions, and, however potent the isolating influences might have 

 been in Hawaii, they could hardly have been concerned with this 

 striking difference. 



In order to determine this point, I went carefully through the 

 account given by Hillebrand of the Hawaiian ferns and lycopods, 

 noting the altitudes there given, and making use of the maps and 

 of my own local knowledge of the islands of Oahu and Hawaii, 

 where the elevation is neither directly nor indirectly implied. As 

 a result, I found that out of sixty-six endemic species available for 

 my purpose, forty-seven had their stations at levels below 4,000 feet, 

 that is in the region corresponding to Fiji, and nineteen at eleva- 

 tions exceeding this height. This, however, did not finally decide 

 the question, since the proportion of endemic species may be much 

 smaller in the region below 4,000 feet than in that above it. I, 



Q 2 



