Hutchinson's (1959) seminal paper, "Homage to Santa Rosalia or Why are there so many kinds of animals?" raised many issues that have subsequently dominated evolutionary ecology. One answer Hutchinson proposed to his question was that two species in the same trophic level can coexist if the ratio of sizes of their trophic apparati is sufficiently large, since this would allow them to avoid competitive exclusion. For birds and mammals Hutchinson suggested a critical size ratio of 1.3. Two species whose sizes (or whose skulls' or culmens' sizes) differ by a factor less than 1.3 would not be able to occur sympatrically and syntopically: "This latter figure may tentatively be used as an indication of the kind of difference necessary to permit two species to co-occur ...." He arrived at this ratio inductively, by examining a few sets of birds and mammals, and he interpreted his results as examples of competitively induced character displacement (Brown and Wilson, 1956). In the 20 years since Hutchinson's suggestion, interest in the importance of size differences for coexistence has led to two related ideas, codified in textbooks (e.g., Ricklefs, 1973; Pianka, 1978) and cited very frequently: