4 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
In accordance with the general principle that the degree of resemblance be- 
tween two species, or between distinguishable varieties of a single species, is 
correlated with the degree of their geographical proximity or remoteness, the Par- 
, 
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J AGIGUAN I, 
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145° 

Fic. 1.—Southernfislands of the Mariana or Ladrone Group. (From Natural History, vol. 
xxi, No. 2, 1921. 
tule of one group of islands are different from those which exist in another archi- 
pelago, while the individual islands of one and the same geographical association 
have their own unique species; furthermore, the forested areas of the valleys in a 
