always present in taxon S always_present_in_taxon T if every fully formed member of taxon T has part some S, or is an instance of S This is a very strong relation. Often we will not have enough evidence to know for sure that there are no species within a lineage that lack the structure - loss is common in evolution. However, there are some statements we can make with confidence - no vertebrate lineage could persist without a brain or a heart. All primates are limbed. never lost in brain always_present_in_taxon 'Vertebrata' forelimb always_present_in_taxon Euarchontoglires present in taxon