Page:Popular Science Monthly Volume 77.djvu/342

336 history of development is enforced upon our consideration in a great many subordinate ways.

Recognizable plant remains first occur in the Silurian in the form of certain highly organized algas, the ancestral forms of which are unknown. Nevertheless, the history of Nematophycus shows that in the Silurian and extending through the Devonian, members of the brown algae directly comparable with the modern kelps, both in general character and in detailed structure, had attained to a development unknown to any of the marine algas of to-day. Arborescent forms with stems two feet in diameter and a corresponding height lead to the inference that they not only represent the culmination of the phylum at that time, but that they must have been preceded by a long line of ancestral forms, extending far back into the earlier horizons, possibly into the Eozoic itself.

Parka decipiens from the old Bed Sandstone of Scotland affords striking illustration of the very early period at which heterospory was developed among vascular plants, which, according to the evidence now available, are comparable with the genus Marsilea among existing types. In these remains we meet with prostrate stems often one to two inches in diameter, from which slender, upright branches are produced, bearing in turn conceptacles containing both micro- and mega-sporangia. Some of these latter further contain prothalli in various stages of development.

The earliest form of gymnosperm is that which we recognize in the genus Cordaites from the Devonian. The highly developed and dicotyledonous character of the stem affords abundant evidence that the ancestral type must be looked for in some remote and earlier horizon, but, taken as an isolated case, it affords no clue whatever to the origin of that particular phylum, although the subsequent course of development may be traced with considerable certainty to comparatively recent times.

The obvious conclusion to be drawn from the geological relations presented by such illustrations as those recited, is, that the evolution of even very simple forms from the most primitive plants must have called for enormously lengthy periods of time. Even the most liberal application of the law of mutation would fail to adequately account for the extensive gaps which are recognized as occurring between the simpler types and those which lie in the same general line of succession, but with greatly advanced organization.

We are now led to ask, how far have paleontological studies carried us in our knowledge of plant life from the earliest times, that is, do they enable us to trace an unbroken series of steps from the first to the last? To this the answer must be that, while paleobotany has been of the greatest service in supplying missing data, in filling great gaps in a supposed sequence and in giving the fullest support to the law of