1913.] NATURAL SCIENCES OF PHILADELPHIA. 381 



of West Coast shells were collected, but unfortunatelj^ the localities 

 were badly mixed up, as in the Sulphur shells. 



Good accounts of the explorations of Veatch, Gabb, Scammon, and 

 others of that period together with much historical and other infor- 

 mation concerning the peninsula may be found in a book issued by 

 the Lower California Company, entitled, "Lower California: Its 

 Geography and Characteristics," New York, 1868. The most exten- 

 sive recent reports by scientific observers are those published by the 

 California Academy of Sciences. These deal chiefly with the Cape 

 region, which differs widely from the peninsula further north by reason 

 of its far greater rainfall. 



Helicoid and Bulimoid landsnails living in exposed places usually 

 become more variegated than the related forms from sheltered or 

 shaded places. When exposed to the sun, the ground color or the 

 whole shell becomes opaque white, an adaptation to exclude light, 

 which would shine through a brown, corneous, or yellowish shell, 

 probably to the detriment of the snail. Compare Cerion, Hemi- 

 trochus, Bulimulus schiedeanus, Leucochroa, etc. Among snails living 

 in drj^, sunny places, there is also very often a tendency to multiply 

 color-bands, in snails derived from stocks with one or few bands, 

 as in Euparypha, Hemitrochus, Plecto stylus, etc. This may possibly 

 be protective, as a variegated shell is less conspicuous on a bush or 

 weed than a one-colored shell. 



The enhanced color development and variabilitj^ of snails living 

 on trees or other plants lead to the formation of color varieties from 

 diverse variations in different colonies. Where the topographic 

 and other environmental conditions favor isolation of colonies, 

 permanent races or species result, as among the Partulce and Achati- 

 nellce. When the colonies occasionally mingle by changing condi- 

 tions of vegetation, etc., we have polymorphic hybrid races — 

 colonies where great individual diversity of pattern or form are seen. 



Lower California, exclusive of the Cape region, is probably an area 

 of increasing aridity, like southern Arizona and Sonora, so that there 

 is now a strong tendency towards restriction and consequent isolation 

 of the snail colonies. The rather wide distribution of some of the 

 species and the polymorphic coloration of many colonies leads us to 

 believe that conditions formerly were much more favorable to migra- 

 tion and mingling of land snails. Subsidence has also played a part 

 in isolating the snails of the small islands along the coast. That the 

 changes have often been unfavorable is shown by the dwarfed size of 

 many races, as on San Geronimo, Natividad, and Asuncion Islands. 



