AN EXAMPLE IN ACCORD WITH THE THEORY. 155 
that 1s variable but of very limited powers of migration has been for many 
generations surrounded by a mass of spectes possessing ordinary powers 
of variability and ordinary facilities for distributing themselves. 
If this prediction is found to be in accordance with facts, it will 
show that the explanation of divergent evolution to which we have 
been led by the investigations presented in the foregoing chapters is 
essentially correct. 
4. Prediction Confirmed by Partula of Tahitt. 
Since writing the preceding statement, I have read with the greatest 
interest Dr. A. G. Mayer’s memoir on ‘‘Some Species of Partula from 
Tahiti; A Study in Variation.”” The conditions of variation and 
migration which he brings to light in the case of some of the snails of 
Tahiti are a fine example of the conditions which I have found in the 
Hawaiian snails to be most favorable for the segregation of many 
closely related forms within a comparatively limited district, each 
section of which presents essentially the same environment. These 
conditions are, in the case of the closely related but divergent forms, a 
full degree of variability, but a very limited power of migration, and 
in the surrounding species the ordinary endowments in regard to 
variation and migration. Partula hyalina is found in all the valleys 
of the island of Tahiti; also on the Austral islands and on one of the 
Cook group. It may, therefore, have opportunities for migration that 
are not possessed by the other species of Partula found on Tahiti; and 
certainly it does not present the tendency to variation in form and 
color which we find in some of these species. 
Of these other types I will refer only to three species which are 
found in four valleys, in which the character of the vegetation is essen- 
tially the same. On the north side of the island are three approxi- 
mately parallel valleys, Pire, Fautaua, and Tipzrui. The first and 
last of these are about 3 miles apart, Pire lying on the east and 
Tiperui on the west, while Fautaua and several narrow gorges lie 
between them. ‘These three valleys are, however, ‘‘broad and well- 
watered, and contain a luxuriant growth of wild plantains and Cala- 
dium, upon which the snails are found in large numbers.’’ Besides 
Partula hyalina, mentioned above as found in all the valleys of the 
island, there are two species of Partula found in these valleys. Par- 
tula filosa is found only in Pir, and though constantly dextral pre- 
sents divers shades of color. Partula otaheitana is found in all three 
of the valleys, but presents hereditary tendencies differing in each of 
the valleys; for example, in Pire it is constantly sinistral, in Tipeerui it 
is constantly dextral, and in Fautaua dextral and sinistral forms are 
found in nearly equal numbers. 
