If the genus is considered to be more than a category of convenience, that is to say, a group of related species having a common origin, the primary problem is to determine whether or not the five species form a phylogenetic unit.
"A Review of the Middle American Tree Frogs of the Genus Ptychohyla"
William E. Duellman
The medial migration of the thoracic artery appears to have some phylogenetic significance as yet not understood.
"Thoracic and Coracoid Arteries In Two Families of Birds, Columbidae and Hirundinidae"
Marion Anne Jenkinson
From the fact that a phylogenetic race is thrown repeatedly among different external conditions, it may at last unite in its idioplasm a large number of developing, mature, and vanishing adaptation determinants.
"A Mechanico-Physiological Theory of Organic Evolution"
Carl Von Nägeli