 One central issue in the philosophy of biology concerns the nature of biological species, which have traditionally been held to be paradigmatic natural kinds. The traditional Linnean binomial system of classification groups organisms into species and genera in virtue of their overall physical similarities (their morphology). However, only the taxa species and genus were held to reflect ontological divisions in nature. The higher taxa (e.g., family, order, class, phylum, kingdom) are merely conventional divisions, which are of heuristic use in biology.