Twenty first meeting. 



College of Physicians and Surgeons, Columbia University. March 

 20, igoy. President Flexner in the chair. 



44 (187) 



A study of the vital conditions determining the distribution 

 and evolution of snails in Tahiti, with illustrations. 



By H. E. CRAMPTON. 



[From the Department of Zoology, Columbia University^ 



In presenting the more important results of a recent study in 

 the field of terrestrial pulmonates of the island of Tahiti, belonging 

 to the genus Partula, it was shown that different valleys contain 

 forms that, on account of their more or less complete isolation, 

 have come to differ in correlation with their geographical proximity 

 or remoteness. The vital conditions that limit the snails of this 

 island to their particular stations are dryness peripherally, where 

 the valleys debouch upon the coastal alluvial plain, and lower 

 temperature centrally. Only rarely may stragglers pass from one 

 region to another. 



Evidence was adduced showing that " mutations" have arisen 

 at various recent times. The observations of Garrett and Mayer, 

 taken in connection with the results of the writer, make it certain 

 that at least three forms have thus originated, at dates that may be 

 determined with substantial accuracy. It was furthermore shown, 

 in corroboration of Mayer's contention, that the environmental 

 conditions cannot be regarded as the factors that have produced 

 the several specific and varietal differentia exhibited by the Tahitian 

 snails. 



(63) 



