is more definitely established in another species of the genus--_B. proliferum_. This plant, shooting up to a considerable height, and having a stem containing but little woody fibre, habitually breaks near the bottom while still in flower; and is thus generally prevented from ripening its seeds. The multiplication is, however, secured in another way. Before the stem is broken young plants have budded out from the pedicels of the flowers, and have grown to considerable lengths; and on the fall of the parent they forthwith commence their separate lives. Here natural selection has established a remarkable kind of co-ordination between a special habit of growth and decay, and a special habit of proliferation. § 285. The advance of physiological integration among plants as we ascend to the higher types, is implied by their greater constancy of structure, as well as by the stricter limitations of their habitats and modes of life. “Complexity of structure is generally accompanied with a greater tendency to permanence in form,” says Dr. [now Sir J.] Hooker; or, conversely, “the least complex are also the most variable.” This is the second aspect under which we have to contemplate the facts. The differences between the simpler _Algæ_ and _Fungi_ are so feebly marked that botanists have had great difficulty in framing definitions of these classes. This structural indefiniteness is accompanied by functional indefiniteness. _Algæ_, which are mostly aquatic, include many small forms that frequent the damp places preferred by _Fungi_. Among _Fungi_, there are kinds which lead submerged lives like the _Algæ_. Besides this indistinctness of the classes, there is great variability in the shapes and modes of life of their species--a variability so great that what were at first taken to be different species, or different genera, or even different orders, have proved to be merely varieties of one species. So inconstant in structure are the _Algæ_ that Schleiden quotes with approval the opinion of Kutzing, that “there are no species but merely forms of _Algæ_:” an opinion which though now rejected sufficiently implies extreme indefiniteness. In all which facts we see that these lowest types of plants, little differentiated, are also but little integrated.