Archegoniates present a like relation between the small specialization of functions which constitutes physiological differentiation, and the small combination of functions which constitutes physiological integration. “Mosses,” says Mr. Berkeley, “are no less variable than other cryptogams, and are therefore frequently very difficult to distinguish. Not only will the same species exhibit great diversity in the size, mode of branching, form and nervation of the leaves, but the characters of even the peristome itself are not constant.” And concerning the classification of the remaining group, _Filicales_, he says:--“Not only is there great difficulty in arranging ferns satisfactorily, but it is even more difficult to determine the limits of species.” After this vagueness of separation as well as inconstancy of structure and habit among the lower plants, the stability of structure and habit and divisibility of groups among the higher plants, appear relatively marked. Though Phænogams are much more variable than most botanists have until lately allowed, yet the definitions of species and genera may be made with far greater precision, and the forms are far less capable of change, than among Cryptogams. And this comparative fixity of type, implying, as it does, a closer combination of the component functions, we see to be the accompaniment of the greater differentiation of those functions and of the structures performing them. That these characters are correlatives is further shown by the fact that the higher plants are more restricted in their habitats than the lower plants, both in space and time. “The much narrower delimitation in area of animals than plants,” says Sir J. Hooker, “and greater restriction of Faunas than Floras, should lead us to anticipate that plant-types are, geologically speaking, more ancient and permanent than the higher animal types are, and so I believe them to be, and I would extend the doctrine even to plants of highly complex structure.” “Those classes and orders which are the least complex in organization are the most widely distributed.” § 286. Thus that which the general doctrine of evolution leads us to anticipate, we find implied by the facts. The physiological division of labour among parts, can go on only in proportion to the mutual dependence of parts; and the mutual dependence of parts can progress only as fast as there arise structures by which the parts are efficiently combined, and the mutual utilization of their actions made easy.