1911.] NATURAL SCIENCES OF PHILADELPHIA. 119 



provinces, or even neighboring islands, would be given distinctive 

 specific names by any systematist, and yet may have come from a 

 single hill slope where the animals were free to migrate from one part 

 to another. But, on the other hand, in another species which we 

 might examine from the same locality we would find great uniformity 

 in a large series of specimens. 



Adams also calls attention to the fact that an insular fauna resem- 

 bles a continental fauna in little, "the island is a miniature continent." 

 "Among the terrestrial shells typical forms exist in great profusion; 

 these forms are of every conceivable grade of value, from varieties 

 up to genera. They also have a determinable geographical distribu- 

 tion." This makes the island of Jamaica an excellent field for the 

 study of variation and its controlling factors. A complete zoological 

 survey of even such a contracted area as is covered in this paper, 

 (better still if extended to a wider area or to the entire island), would 

 be of the greatest value in the study of the causes of variation, so far 

 as they may be worked out by morphology. The possibilities in this 

 direction appealed very strongly to me during a visit to the island in 

 February-March, 1910, and on a subsequent visit in April-May I 

 collected large series of specimens from definite colonies with this end 

 in view. From a study of the large series of species of Pleurodonte 

 taken at this time some facts in regard to the causes of the variation 

 observed seem to be indicated, and they will be found embodied in 

 this paper. In my first visit in February-March I was assisted in the 

 collecting by Mr. Stewardson Brown, who was studying the flora of 

 the region. At this time I had not recognized the importance of 

 collecting by isolated colonies, and the series of specimens we obtained, 

 while useful for biometric measurements on the species of the regi6n 

 as a whole, do not give the data necessary for tracing out the pro- 

 gressive variation from point to point. In my second visit I collected 

 by colonies, keeping each one separate, and thus obtained material 

 for a comparative study. This material has been gone over and 

 individual measurements of each shell made; these measurements 

 have been plotted in various ways and the results compared. The 

 variation in measurement gives a quantitative method of comparison 

 which eliminates very largely the personal equation, and therefore 

 it has been resorted to first in all of these comparisons. The two 

 dimensions of altitude and diameter, plotted as a point, give the best 

 comparison of the dimensions, their ratio to each other, or the index, 

 may likewise be used; and the indices arranged in order and plotted 

 for each colony on one diagram show the variations in the slope of 



