In addition, about 100 years ago, Carlo Emery observed that, "The slave-making temporarily and permanently parasitic ants originate from closely related forms which serve them as host." Jurgen Heinze did a series of experiments that prove this observation, known today as Emery's Rule. Rather than just relying solely on morphological cues, he used enzyme gel electrophoresis assays to create a detailed phylogenetic picture of host and parasite connections. His experimental results support a loose version of Emery’s rule. On one end of the spectrum, they saw near identical electromorphs between species in most host and parasite pairs studied. However, it also appears that at least one example does not support Emery’s rule in that ("Leptothorax paraxenus") differed from its host in several electromorphs. The selective forces involved in the evolution of social parasitism are not fully understood, and it is unlikely that one model fits all relationships.