16 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
is roughly triangular in outline and consists of a single volcanic mass; the crater of 
this has broken outward in such a way as to form two bays into which the sea has 
entered. Huahine (plate 3) is a double island, surrounded by a coral reef, whose two 
parts are separated by a narrow strait only a few score yards wide at its narrowest 
point and less than o.5 mile broad at its greatest width. Raiatea and Tahaa 
(plate 4) are sister islands still encircled by a continuous line of reef; each of them 
is a single complex with one main crater only. Finally, Borabora is a sharply 
sculptured volcanic peak bordering a lagoon whose other boundaries are reefs for 
the most part. 
BELLINGSHAUSEN 
: LEEWARD ISLANDS 
MOTUITI 
a 
SCILLY. 
8 MAUPITI ® Gees 
TAHAA ‘GQ 
MOPELIA RAIATEA 2 
17° (MAPIHAA OR MAPETIA} 
@ MAIAO-ITI 
(TUBUAI MANU) 
155° 154° 153° 152° 151° 150° 149° 
Text-Fic. 1—Chart of the Society Islands. The islands with underlined names are inhabited by species of the 
genus Partula. 
In the aforesaid geographical features we find most favorable conditions for 
the inter-insular and intra-insular analysis of the snails, in so far as different degrees 
of isolation are displayed by comparable land-masses. The two parts of Tahiti 
are still connected, while those of Huahine are narrowly separated by water, giving 
a more advanced stage in the disconnection of peaks formerly united. Raiatea and 
Tahaa are 2 miles and more apart, but a single barrier-reef surrounds them, so that 
they show a stage beyond that of Huahine, albeit one that falls short of actual and 
complete disconnection. The final condition is displayed by Moorea in its relation 
to Tahiti as a whole. The inference is obvious that at a prior time the larger 
islands of this group were connected peaks, which have reached their present degrees 
of virtual, partial, or complete isolation by a process of subsidence. Certainly we 
can not adopt the alternative hypothesis of uplift for these islands, so as to interpret 
