1) Clausen, J., Keck, D. D., & Heisey, W. M. (1940). Experimental studies on the nature of species. I. Effect of
varied environments on western North American plants. Carnegie Institution of Washington Publications, 520, 1–452.


2) Clausen, J., Keck, D. D., & Heisey, W. M. (1945). Experimental studies on the nature of species. II. Plant
evolution through amphiploidy and autoploidy, with examples from the Madiinae. Carnegie Institution
of Washington Publications, 564, 1–174.


3) Clausen, J., Keck, D. D., & Heisey, W. M. (1958a). Experimental studies on the nature of species. III. Environmental
responses of climatic races of Achillea. Carnegie Institution of Washington Publications, 581, 1–129.


4) Clausen, J., Keck, D. D., & Heisey, W. M. (1958b). Experimental studies on the nature of species. IV.
Genetic studies of ecological races. Carnegie Institution of Washington Publications, 615, 1–312.