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At one time, the gymnosperms were considered to be a class (Class Gymnospermae), first within the seed plants (Division Spermatophyta; 1883~1950) and later within the vascular plants (Division Tracheophyta; 1950~1981). The class essentially encompassed the conifers and their allies (by which term is meant "related species of plants"), including several groups of extinct plants known only from fossils. In these earlier classification schemes, the "naked seed" plants were clearly set off from the other classes of higher plants (that is, the ferns and flowering plants), essentially as they are today. However, fossil evidence suggests that the angiosperms evolved from a gymnosperm ancestor, which would make the gymnosperm taxon paraphyletic. Modern cladistics attempts to define taxa that are monophyletic, traceable to a common ancestor and inclusive therefore of all descendants of that common ancestor. So, while the term gymnosperm is still widely used to distinguish the four taxa of non-flowering, seed-bearing plants from the angiosperms, plant species once treated as gymnosperms are distributed among four groups given equal rank as divisions within the Kingdom Plantae. These groups are: