The Nature of Species: Experimental Studies on the Nature of Species. II. Plant Evolution through Amphiploidy and Autoploidy with Examples from the Madiinae . By Jens Clausen, David D. Keck and William M. Hiesey. Carnegie Institution of Washington Publ. 564. vii+174 pp. 86 figs. 1945. Paper, $1.25, cloth, $2.00.