Harvey E. Ballard, Jr., Kenneth J. Sytsma, Robert R. Kowal, Shrinking the Violets: Phylogenetic Relationships of Infrageneric Groups in Viola (Violaceae) Based on Internal Transcribed Spacer DNA Sequences, Systematic Botany, Vol. 23, No. 4 (Oct. - Dec., 1998), pp. 439-458
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Ballard et al. (1998) studied this question.
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