VOLUME 6 
DECEMBER, 1920 
NUMBER 12 
PROCEEDINGS 
OF THE 
National Academy 
OF Sciences 
OF THE 
UNITED STATES 0# AMERICA 
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EDITORIAL BOARD 
Raymond Pearl, Chairman Edwin B. Wii^son, Managing Editor 
C. G. Abbot, Home Secretary George E. Hale, Foreign Secretary 
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J. M. Clarke 
A. L. Day 
Gano Dunn 
E. B. Frost 
LuDviG Hektoen 
L. J. Henderson 
H. S. Jennings 
Jacques Loeb 
J. C. Merriam 
R. A. MiLLIKAN 
PUBLISHED MONTHLY 
E. H. Moore 
W. A. NOYES 
W. J. V. Osterhout 
E. L. Thorndike 
w. M. Wheeler 
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1 Publication Office: Eschenbach Printing Company, Easton, Pa. 3 
1 Editorial Office: Massachusetts Institute of Technology, Cambridge I 
= Home Office of the Academy: Washington, D. C. 1 
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lintcred as second-class matter June 21, 1920, at the postofiBce at Easton, Pennsylvania, under the act of August 24, 1912 
Acceptance for mailing at special rate of postage provided for in Section 1103, Act of October 3, 1917. Authorized June 18, 1920. 
^ Price per annum, $5.00 
