Greenberg began writing during a time when phylogenetic systematics lacked the tools available to it later: the computer (computational systematics) and DNA sequencing (molecular systematics). To discover a cladistic relationship researchers relied on as large a number of morphological similarities among species as could be defined and tabulated. Statistically the greater the number of similarities the more likely species were to be in the same clade. This approach appealed to Greenberg, who was interested in discovering linguistic universals. Altering the tree model to make the family tree a phylogenetic tree he said: