336 THE POPULAR SCIENCE MONTHLY 



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 un- 

 known. 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 char- 

 acter 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, bear- 

 ing in turn conceptacles containing both micro- and mega-sporangia. 

 Some of these latter further contain prothalli in various stages of de- 

 velopment. 



The earliest form of gymnosperm is that which we recognize in the 

 genus Cordaites from the Devonian. The highly developed and dicoty- 

 ledonous 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 develop- 

 ment 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 succes- 

 sion, 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 evo- 



