406 



A ZOOGEOGRAPHIC SCHEME FOR THE MID-PACIFIC, 



An examination of these data shows a deterioration of the fauna 

 from west to east, the five western archipelagoes mustering nearly 

 one-fifth more than the eastern. The even distribution 'of Cypnea 

 forms an exceptional case; it actually totals one species moi^e in 

 the Society Islands than it does in Fiji. With the exception of 

 CypTd'a there is a sharp fall in numbers on leaving the continental 

 area of Fiji. From the nearest to the farthest groups the loss 

 then continues in a parabolic curve. The most distant, the Mar- 

 quesas and Hawaii, being in every case the poorest. 



The Society Islands, however, possess a richer fauna than their 

 distance appears to entitle them to have. This comparative wealth 

 may be accounted for by their superior antiquity. The elevated 

 masses of Tahiti and associated islands have for long stood in the 

 drift track and intercepted migrant forms. It may even be that 

 Tahiti has been receiving such from a time which anteceded the 

 present form and population of the Melanesian area. A highly 

 curious Tahitian tree, Lepinia, was lately rediscovered in the 

 Solomons.* A subgenus of land shells. Libera, is peculiar to the 

 Cook and Society Islands. Thej^ also possess half the known 

 species of Partula.j Their poverty of insects as compared to 

 western continental islands greatly impressed D'Urville.J 



* Botting Hemsley — Science Progress, i. 1894, p. 30. 



t Hartman— Bull. Mus. Comp. Zool. ix. 1881, p. 173. 

 X D'Urville in Boisduval — Voy. Astrolabe, Entomologie, i. 1832, pp. 20-22. 



