Harrison's rule

Launcelot Harrison 1880-1928
Schematic diagram of Harrison's rule: small host species harbor small, large host species harbor large parasite species
Schematic diagram of Harrison's rule with Poulin's supplement: small host species harbor small, large host species harbor both small and large (on average: larger) parasite species

Harrison's rule is an observation in evolutionary biology by Launcelot Harrison which states that in comparisons across closely related species, host and parasite body sizes tend to covary positively.