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 alge, the ancestral forms of which are un- 
known. Nevertheless, the history of NV ematophycus shows that in the 
Silurian and extending through the Devonian, members of the brown 
alge 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 alge 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 Red 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- 
