In summary, the most-obvious trend in the evolution of social amoebas appears to be related to size. Evolutionary younger species both have larger sized spores and larger sized fruiting bodies. The latter is particularly evident in group 4 where large stalk and sori size is correlated with a tendency to form solitary and unbranched fruiting bodies. In addition to large size, group 4 displays other distinguishing features, such as formation of cellular support structures, loss of individual encystation, loss of spore granules, and the use of cAMP as attractant. The correlation of the latter two traits was also noted earlier by Traub and Hohl. (31) These workers also associated the presence of polar granules with a tendency to form clustered and/or branched sorocarps and a tendency for those sorocarps to be smaller than in species without polar granules, both of which are borne out by the recent analysis.