Main Lines of Evolution in Equisetum: I

In general, evolution may manifest itself in two ways in a group of plants. There may be progressive changes which lead up to a higher level and there may also be segregative changes giving rise to characters which are simply different in kind. By the development of segregative characters new groups or branches arise which may go through a series of progressive changes or steps paralleling more or less closely those which are appearing in the original group. In classification, therefore, it becomes important to recognize what is a low type from what is a high type of plant as well as what characters are persistent in the given group and, therefore, of phylogenetic value. The easiest way to decide on the relative positions of two plants is to look past the characters they may possess to the hereditary properties that produce these characters. For it is often true that an apparently simpler organ or organ complex represents the activity of a much more complex heredity than a more complicated organography which may merely represent repetition. Low types have fewer, more generalized, functional activities and high types have more specialized, complex, functional activities. Besides looking into the characters and the heredity of the plants under consideration themselves, a comparison must also be made with plants admittedly higher in the scale and with those lower. Applying these principles to the living species of Equiseta gives an arrangement nearly opposite to that which has been in vogue for a long time, and shows that