132 A COMPARISON OF SOME PECTENS FROM THE EAST 



During the Pliocene depression many of the valleys which had been developed 

 in the post-Miocene interval of high altitudes, were filled to the brim with delta 

 deposits. . . . Numerous islands, large and small, fringed the coast of California. 

 There were numerous submerged valleys, so that the coast was well supplied with 

 harbors. In a word, the coast of California at the close of the Pliocene had the 

 aspect of an archipelago. The archipelagic condition endured into the early Pleisto- 

 cene, and from this condition it [the coast] has been gradually recovering up to the 

 present day." From the beginning of the Pleistocene, then, the available areas for 

 the littoral Pectens have been rapidly diminishing. This has been a time of stress 

 for the shallow- water fauna, a period of violent, relentless selection. 



Considering the causes of variation in connection with what has been stated 

 as to differences of the geographic history of the two regions, this conclusion seems 

 to be justified: The greater variability of the individuals from San Diego is due to 

 the more varied present environment which tends to make some shells deviate one 

 way and others in another, and to the past rapid changes in the physiographic con- 

 ditions which have favored the more responsive, adjustable individuals, and so have 

 given a race of which the different individuals are easily modified by the diverse 

 environments offered. The geographic history has given San Diego a plastic race; 

 the diversity of the present environments of San Diego have determined the excessive 

 variability of that race. 



5. If the greater variability of the San Diego Pectens is the result of 

 the geographical changes of the Californian coast, then we should expect to find 

 other species of this coast exceptionally variable. Unfortunately few quantitative 

 studies have been made on the variability of American animals. Eigenmann ('95, 

 '95 a ) has stated that "in all families but the Cyprinodontidse with more than one 

 species on the Pacific slope the extent of variation is greater than in the same families 

 on the Atlantic slope." Eigenmann's method of measuring variability by the total 

 range is not altogether satisfactory. So far as they go, however, his studies on river 

 fishes lead to the same conclusion as these on Pecten the aquatic species of the 

 Pacific coast are extremely variable owing to the great physiographic changes which 

 that coast has undergone. 



IX. BIBLIOGRAPHY. 



Baldwin, J. M. 



'97. Organic Selection. Nature, vol. 55, p. 558. 

 Bumpus, H. C. 



'98. The Variations and Mutations of the Introduced Littorina. Zool. Bull., vol. 1, no. 5, pp. 247-259, 



charts 1-14. 

 Dall, W. H. 



'98. Contributions to the Tertiary Fauna of Florida. Part 4. Trans. Wagner Free Inst. Sci. Philadelphia, 

 vol. 3, pt. 4, pp. i-viii, 571-947, pis. 23-35. 



