AN EXAMPLE IN ACCORD WITH THE THEORY. 155 



that is variable but of very limited powers of migration has been for many 

 generations surrounded by a mass of species 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 o] Tahiti. 



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, Pirae, Fautaua, and Tipaerui. The first and 

 last of these are about 3 miles apart, Pirae lying on the east and 

 Tipaerui 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 iiyalina, 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 Pirae, and though constantly dextral pre- 

 sents divers shades of color. Partula otalicitana is found in all three 

 of the valleys, but presents hereditary tendencies differing in each of 

 the valleys ; for example, in Pirae it is constantly sinistral, in Tipaerui it 

 is constantly dextral, and in Fautaua dextral and sinistral forms are 

 found in nearly equal numbers. 



