itself with that idea of periods of creative activity which we have already had occasion to notice. [Illustration: FIG. 155.—_Sassafras cretaceum_ (Newberry).] [Illustration: FIG. 156.—_Liriodendron primævum_ (Newberry). A Cretaceous Tulip-tree.] [Illustration: FIG. 157.—_Onoclea sensibilis._ Eocene.—After Newberry.] [Illustration: FIG. 158.—_Davallia tenuifolia._ Eocene.—After Dawson. Natural size and enlarged.] I have dwelt principally on the phænogamous plants of the Cretaceous, as presenting the most noteworthy and new features of the time; but we must not forget that though cryptogams were deposed from the high position they held in the Palæozoic, they still existed; and there are more especially many interesting species of ferns and equisetums in the Cretaceous and Eocene rocks. These are, however, of modern types; and it is remarkable that some of them appear to have continued without even specific change from the later Cretaceous up to the present time. A striking illustration of this is afforded by two ferns discovered side by side in the oldest Eocene beds[72] of the plains west of Red River, and described in Dr. G. M. Dawson’s report on the 49th parallel. One of these is the well-known and very common _Onoclea sensibilis_ (Fig. 157), or sensitive fern of Eastern America.[73] This species came into existence at latest at the close of the Cretaceous, and has apparently been continued in America up to the present time. In Europe, where it does not now live, it occurs as a fossil in Eocene beds in the Isle of Mull. The other is _Davallia tenuifolia_ (Fig. 158), a delicate little plant belonging to a genus not now represented in America, and to a species at present found only in Asia. Yet this species also lived in America in early Eocene times, but has since been banished, though its former companion, the _Onoclea_, still holds its ground. Such cases of specific persistence along with great changes of habitat are very instructive as to the permanence of species. Count Saporta, whose just remarks on the marvellously sudden incoming of the Cretaceous flora we have already referred to, also notices the fact that the families and genera represented in this flora are a most miscellaneous and unconnected assemblage, showing either the simultaneous appearance of many dissimilar types, or requiring us to believe in the existence of these and of intermediate forms for a very long period before that in which they are first found. This may, however, be placed in connection with the appearance of an exogenous tree (_Syringoxylon_) in the Devonian, referred to in a previous chapter. It would be a strange and now little suspected case of imperfection of the record, if it should be found that trees of this type were lurking in exceptional corners through all the vast periods between the Devonian and the Cretaceous, to burst forth in unwonted variety and luxuriance in the latter period.