VOL. XIIII, NO. 511 



JULY, 1909 



THE 



AMERICAN 

 NATURALIST 



A MONTHLY JOURNAL 



Devoted to the Advancement of the Biological Sciences with 

 Special Reference to the Factors of Evolution 



Selection Index Numbers and their Use in Breeding. Dr. RAYMOND PEARL 



and FRANK M. SURFACE 385 



A Contribution to the Theory of Orthogenesis. Dr. ALEXANDER G. RUTHVEN 401 

 The "Presence and Absence" Hypothesis. Dr. GEORGE HARRISON SHTJLL . 410 

 Present Problems in Plant Ecology: Vegetation and Altitude. Professor 



CHARLES H. SHAW 420 



Shorter Articles and Correspondence : Pleistocene Swamp Deposits in Virginia. 



Dr. EDWARD W. BERRY 432 



Notes and Literature : Heredity— & Case of Non-Mendelian Heredity. Db. W. 



THE SCIENCE PRESS 

 LANCASTER, PA. GARRISON, N. Y. 



NEW YORK : SUB-STATION 84 



