Stebbins, G. L. (University of California, Davis, California 95616) 1974. Adaptive Radiation and the Origin of Form in the Earliest Multinuclear Organisms. Syst. Zool. 22:478–485.—The unicellular eukaryotes are used as an example of extensive adaptive radiation, which in a few evolutionary lines led to the origin of more complex multicellular organisms. The particular kinds of multicellular organisms which were the descendants of a given group of unicellular eukaryotes, whether algae, fungi, metazoa, or slime molds, depended upon particular characteristics which their unicellular ancestors acquired in association with their adaptive radiation. The most significant determining characteristics were autotrophy vs. heterotrophy, presence vs. loss of cellular polarization, rigid vs. flexible cell wall or membrane, and the size and shape of the cell. The most continuously successful groups, the multicellular plants, metazoa and fungi, retained cellular polarity, but their ancestors differed from each other with respect to the remaining characteristics. Based upon this review, the author recognizes three kinds of evolutionary success: narrow success, broad but terminal success, and continuing or progressive success. Hypotheses are suggested about the kinds of characteristics which contribute to each of these kinds of evolutionary success.