Vol. 6] 



Uliller: Avifauna of Fossil Lake. 



85 



A table of comparative measurements of the two species is 

 given below. 



The Pleistocene form of JEchmophorus could scarcely be 

 looked upon as a progenitor of the Recent species, even if the 

 latter were absent from the Fossil Lake Beds. The differences 

 displayed by M. lucasi are such as to indicate a greater degree of 

 specialization for the semi-aquatic life. The two seem rather to 

 have been of common origin and during the Pleistocene time 

 perhaps of about equal abundance. The less specialized and 

 more distinctly adaptive form has persisted until the present- 

 while the closely approximated phylogenetic twig became extinct. 



The extent to which the loss of this form was due to its weak- 

 ness in migration is an interesting subject for speculation. The 

 existing ^'E. occidental is is a regularly migratory form, indi- 

 viduals of which probably pass over a distance of twenty to thirty 

 degrees of latitude. Though far from conclusive, the indications 

 from Fossil Lake in Oregon and from the Pleistocene fauna 

 of Raneho La Brea in California suggest that the climatic 

 changes on the west coast since that time have been in the direc- 

 tion of lower temperatures and decreased humidity.'' The latter 

 condition would bring about greater extremes of climate, and 

 make more imperative the migratory movements of birds, and 

 especially of a form so intimately dependent upon aquatic fauna, 

 including invertebrates, for its food. The minute fresh-water 

 plankton forms are very delicately responsive to variations of 

 light and of temperature. The larger arthropods and the verte- 

 brates which might serve as food for grebes would respond less 

 delicately than the microscopic forms, yet their intimate depend- 

 ence upon the microplankton would cause their numbers to rise 

 and fall with the major fluctuations of the latter. 



The intermittent nature of smaller lakes incident upon a de- 

 creased humidity and the more marked seasonal variation in 

 food supply can readily be imagined to have rendered impera- 

 tive a degree of migratory activity to which Mchmophorus lucasi 

 was less able to respond than was its contemporary form, the 

 persistent M. Occident alis. 



••Miller, L. II., Univ. Calif. Publ. Bull. Dept. Geol., 5, nos. 19 and 30. 

 1910. 



6 Kofoitl, C. A., Bull. 111. State Lab. of Nat. Hist,, S, art. 1, p. 291. 1908. 



