vi PREFACE 



His constructive criticism of all phases of the work has been 

 of inestimable service. Thanks are also due to Mr. George 

 T. Kline, biological artist of the University of Missouri, for 

 the drawing of Figs. 28, 29, and 30, and for assistance with 

 some of the other figures. 



Formal acknowledgments are due to the Scientific Ameri- 

 can for Fig. 7; to the Clarendon Press for Fig. 8; to the 

 Pierpont Morgan Library, as the source of Fig. 9, reproduced 

 from the paper by W. A. Locy; to the Macmillan Company 

 for the reproduction of Figs. 23 and 24, and for the quotation 

 from Metcalf's "Organic Evolution," as it appears on page 

 61; to the American Museum of Natural History and the 

 F. A. Stokes Company for Fig. 25; and to Henry Holt and 

 Company for the tabulation of the linked characters in 

 Drosophila, which appears on page 133. Lesser quotations 

 from copyrighted works are duly acknowledged in the foot- 

 notes. 



W. C. C. 

 University of Missouri, 



Nov. 10, 1921. 



