Harrison's rule
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. == Parasite species' body size increases with host species' body size == Launcelot Harrison, an Australian authority in zoology and parasitology, published a study in 1915 concluding that host and parasite body sizes tend to covary positively, a covariation later dubbed as 'Harrison's rule'.