Two-species studies should not be used to infer adaptive relationships between ecological variables, but may be useful to generate hypotheses for comparative testing, to obtain data for comparative analysis, or to obtain information specific to a particular paired comparison. I present a case in which a two-species study was essential to obtaining data for comparative analysis. Measurements of multiple individuals per taxon are needed to provide estimates of variables and their differences, but comparative methods using dichotomous variables cannot themselves demonstrate the existence of true differences among taxa. Previous studies using the concentration of correlated changes test and the method of independent contrasts showed that correlated evolution has occurred in lizards between foraging mode and detection of prey chemicals by tongue-flicking. Active foragers exhibit prey chemical discrimination, but ambush foragers do not. Effects of changes in foraging mode among very close relatives are unknown and more data on changes in foraging mode are needed to confirm and extend the relationship. In a two-species experiment using congeneric lacertid lizards differing in foraging mode, both species discriminated prey chemicals from control substances, but the discrimination was significantly stronger in the active forager. Change in foraging mode was accompanied by rapid change in prey chemical discrimination that could not be detected by correlative methods. Concentration of correlated changes tests including data from the experiment strengthened evidence for correlated evolution between foraging mode and prey chemical discrimination in lizards as a whole and the major subtaxa Scleroglossa and Autarchoglossa, and permitted its first detection in Scincomorpha.