 Darwin would have been surprised to hear that his theory of natural selection was circumscribed so as to apply only to evolving populations. He himself constructed an explanation of a persistent polymorphism, heterostyly, using his own theory. Plants exhibiting heterostyly develop two, or sometimes three, different forms of flower whose reproductive organisms vary in a number of ways, principally length. Some plants exhibit different forms of flower on the same plant, while some are dimorphic and trimorphic, with only one sort of flower per plant. Darwin interpreted the flower variations as conducive to intercrossing, which he thought was beneficial, at least for many organisms. Populations should not evolve directionally such that a single form of flower spreads throughout the population; instead, multiple variants should be retained, a polymorphism. Darwin thinks it clear that heterostyly is an adaptation: 