Eichler's rule
Eichler's rule is one of several coevolutionary rules which states that parasites tend to be highly specific to their hosts, and thus it seems reasonable to expect a positive co-variation between the taxonomic richness of hosts and that of their parasites. == History == A rule to describe the taxonomic relationship between parasites and their hosts was developed in 1942 by Wolfdietrich Eichler (1912–1994), a German authority in zoology and parasitology who served as a professor of parasitology at Leipzig University.