It is at this point that the fragmentary geological evidence sheds its strongest light on the problem under consideration. In the Cretaceous and Tertiary floras which preceded the modern, the present degree of differentiation had not as yet been attained and but few modern species made their appearance before the close of the Tertiary.* The species, however, which immediately preceded those which now exist were very closely related to them, being their immediate ancestors, and differed from them only in showing a somewhat lower degree of differentiation, and their leaf-forms are accordingly more primitive than those of the ex isting species which have descended from them.
Now it is a well-established fact that the lower leaves of young branches and shoots, and especially of those which spring from the stumps of felled trees, are frequently unlike the adult forms which occur higher up and bear a close resemblance to the fossil leaves of extinct species, so close indeed, as oftentimes to be in distinguishable from them. This is strong evidence in favor of the doctrine that the lower foliar organs represent not reduced leaves, as botanists have commonly supposed,t but the primitive foliar organs,and that in an ascending series from the lowest scale to the mature adult leaves of the upper part of the stein, giving a more or less perfect summary of the phylogenetic development of the foliar organ from the most primitive type upward to the most highly differentiated./ In other words, a single stein may represent the whole phylogeny of the foliar organs of its type. It is true that there are simple leaf-forms which have become so by reduction but, as an organ cannot be reduced until it has been developed, these are to be looked for above and not below the perfect leaves, and are found in bracts, involucral scales and the parts of floral envelopes, reduction taking place in inverse order to the course of development, and only the most primitive structure, the simple sheath, persists in the petals of most flowers. Re duced leaves are also common in parasites, and in the flora of desert regions as is well illustrated in some of the Leguminosre of Australia the leaves of which are little more than spines, or are developed into bladeless phyllodia, while in the seedlings the ancestral pinnate or bipinnate forms occur.* We thus have shown in each season's growth of a plant, though not clearly in annuals because disguised in the seedling, a more or less complete series of foliar organs which may for illustration lie compared with the vertebrate series among animals, the lowest leaf-scales being comparable in degree of development to the sim ple structures of the fishes and the most highly developed leaves to the complicated ones of mammals. Each leaf in the series is
equally perfect for the function it is intended to perform, but the lowest of a lower type of organization, as are the fishes, and rep resenting an earlier stage in the phylogenetic series.
Now in animals we look to the developing egg of the more gen eralized fishes for the least abbreviated embryological recapitula tion of the early development of the vertebrate branch, for in the mammals the early stages are passed through so rapidly and with so many disguises as to be of comparatively small importance in giving the history of the branch, unless viewed in the light of the embryological development of the lower types. So the lower foliar organs of a branch or shoot are embryologically of far greater importance than the upper, for in the beginning of the de velopment of one of the upper leaves we have but the early stages of a highly organized appendage. These early stages are conse quently abbreviated and more or less disguised. The formation of the stipules in the growth of the upper leaves is therefore not a salient point in the consideration of our problem though it has had much stress laid upon it, yet it is of interest to note that in general the stipules appear earlier than the leaf-blade, thus giving evidence that they are of more ancient origin. It may be added, *See Sir Jobn Lubbock. On Seedlings. 1: 474. 1592. See also p. 440 as to the similar case of Lethyrus Aphrfca.
and it is a matter of common observation, that the petiole is the last portion of the leaf to develop ontogenetically and is therefore to be regarded as the most recent part to be added phylogeneti cally. This helps to explain the common occurrence of sessile and petiolate leaves even in different species of the same genus, as variation more readily occurs in recent than in ancient struc tures, while on the contrary it has been a matter of remark among even the earlier botanists that stipules when they occur usually characterize all the species of a family, an additional evidence of the antiquity of their origin.