it shows the older sheaths in process of being split open. That is to say, there must result the mode of growth which helped to give the name Endogens to this class. [Illustration: Figs. 92–94.] [Illustration: Figs. 95–99.] The other way in which an integrated series of fronds may acquire the rigidity needful for maintaining an erect position, has next to be considered. If the successive fronds do not acquire such habit of curling as may be taken advantage of by natural selection, so as to produce the requisite stiffness; then, the only way in which the requisite stiffness appears producible, is by the thickening and hardening of the fused series of mid-ribs. The incipient axis will not, in this case, be inclosed by the rolled-up fronds; but will continue exposed. Survival of the fittest will favour the genesis of a type, in which those portions of the successive mid-ribs that enter into the continuous bond, become more bulky than the disengaged portions of the mid-ribs: the individuals which thrive and have the best chances of leaving offspring, being, by the hypothesis, individuals having axes stiff enough to raise their foliage above that of their fellows. At the same time, under the same influences, there will tend to result an elongation of those portions of the mid-ribs, which become parts of the incipient axis; seeing that it will profit the plant to have its leaves so far removed from one another, as to prevent mutual interferences. Hence, from the recumbent type there will evolve, by indirect equilibration (§ 167), such modifications as are shown in Figs. 92, 93, 94; the first of which is a slight advance on the ideal type represented in Fig. 76, arising in the way described; and the others of which are actual plants--_Haplomitrium Hookeri_, and _Plagiochila decipiens_. Thus the higher Archegoniates show us how, along with an assumption of the upright attitude, there does go on, as we see there must go on, a separation of the leaf-producing parts from the root-producing parts; a greater development of that connecting portion of the successive fronds, by which they are kept in communication with the roots, and raised above the ground; and a consequent increased differentiation of such connecting portion from the parts attached to it. And this lateral bulging of the axis, directly or indirectly consequent on its functions as a support and a channel, being here unrestrained by the early-formed fronds folded round it, goes on without the bursting of these. Hence arises a leading character of what is called exogenous growth--a growth which is, however, still habitually accompanied by exfoliation, in flasks, of the outermost layers, continually being cracked and split by the accumulation of layers within them. And now if we examine plants of the exogenous type, we find among them many displaying the stages of this metamorphosis. In Fig. 95, is shown a form in which the continuity of the axis with the