limb locomotion reaches its highest degree, as in birds, elongation disappears. These classes of familiar facts I have recalled to show that, in the course of evolution, achievement by plants of the all-essential elevation into the air and by animals of the all-essential power of movement have developed this trait of elongation in various types; and that in each kingdom acquisition of the common trait has had a tendency now to obscure morphological equivalence, and now to give the appearance of kinship where there is none. A further purpose has been to prepare the way for a question hereafter to be discussed--whether, in the various types of either kingdom, the elongation is effected in the same ways or in different ways. We shall have to ask whether the vertically-growing part is always, like that of _Lessonia_, a simple individual, or whether, as possibly in Phænogams, it is a united series of individuals; and similarly whether the elongated body is always single, like that of a mollusc, or whether, as possibly in annulose animals, it is a series of united individuals.] CHAPTER II. THE MORPHOLOGICAL COMPOSITION OF PLANTS. § 180. Evolution implies insensible modifications and gradual transitions, which render definition difficult--which make it impossible to separate absolutely the phases of organization from one another. And this indefiniteness of distinction, to be expected _à priori_, we are compelled to recognize _à posteriori_, the moment we begin to group morphological phenomena into general propositions. Thus, on inquiring what is the morphological unit, whether of plants or of animals, we find that the facts refuse to be included in any rigid formula. The doctrine that all organisms are built up of cells, or that cells are the elements out of which every tissue is developed, is but approximately true. There are living forms of which cellular structure cannot be asserted; and in living forms that are for the most part cellular, there are nevertheless certain portions which are not produced by the metamorphosis of cells. Supposing that clay were the only material available for building, the proposition that all houses are built of bricks, would bear about the same relation to the truth, as does the proposition that all organisms are composed of cells. This generalization respecting houses would be open to two criticisms:--first, that certain houses of a primitive kind are formed, not of bricks, but out of unmoulded clay; and second, that though other houses consist mainly of bricks, yet their chimney-pots, drain-pipes, and ridge-tiles, do not result from combination or metamorphosis of bricks, but are made directly out of the original clay. And of like natures are the criticisms which must be passed on the generalization, that cells are the morphological units of organisms. To continue the simile, the truth turns out to be, that the