Experimental evolution will solve a taxonomic problem as yet untouched, namely, the effect of recent environment upon the production of species. It is well understood that some species grow in nature in various habitats without suffering material change, while others are modified to constitute a new form in each habitat. It is at once clear that these forms (or ecads) are of more recent descent than the species, i. e., of lower rank. It must also be recognized that a constant group and a highly plastic one are essentially different. If constancy is made a necessary quality of a species, one is a species, the other is not. If both are species, then two different kinds must be distinguished. Among the species of our manuals are found many ecads, alongside of constant and inconstant species. These can be distinguished only by field experiment, and their proper coordination is possible only after this has been done. Indeed, the whole question of the ability or the inability of environmental variation to produce constant species is one that must be referred to repeated and long-continued experiment in the field. A minor service of considerable value can be rendered taxonomy by working over the diagnosis from the ecological standpoint. Many ecological facts are of real diagnostic value, while others are at least of much interest, and serve to direct attention to the plant as a living thing. The loose use of terms denoting abundance, which prevails in lists and manuals, should be replaced by the exact usage which the quadrat method has made possible for vegetation. The designation of habitats could be made much more exact, and the formation, as well as the habitat form or ecad, and the vegetation form or phyad, should be indicated in addition. The general terms drawn from pollination, seed-production, and dissemination might also be included to advantage.