When "The Origin of Species" was written, it was necessary to show that
the Geological Record was favourable to, or at least consistent with,
the Theory of Descent. The point is argued, closely and fully, in
Chapter X. "On the Imperfection of the Geological Record," and Chapter
XI. "On the Geological Succession of Organic Beings"; there is, however,
little about plants in these chapters. At the present time the truth
of Evolution is no longer seriously disputed, though there are writers,
like Reinke, who insist, and rightly so, that the doctrine is still
only a belief, rather than an established fact of science. (J. Reinke,
"Kritische Abstammungslehre", "Wiesner-Festschrift", page 11, Vienna,
1908.) Evidently, then, however little the Theory of Descent may be
questioned in our own day, it is desirable to assure ourselves how the
case stands, and in particular how far the evidence from fossil plants
has grown stronger with time.
As regards direct evidence for the derivation of one species from
another, there has probably been little advance since Darwin wrote, at
least so we must infer from the emphasis laid on the discontinuity
of successive fossil species by great systematic authorities like
Grand'Eury and Zeiller in their most recent writings. We must either
adopt the mutationist views of those authors (referred to in the last
section of this essay) or must still rely on Darwin's explanation of the
absence of numerous intermediate varieties. The attempts which have been
made to trace, in the Tertiary rocks, the evolution of recent species,
cannot, owing to the imperfect character of the evidence, be regarded as
wholly satisfactory.
When we come to groups of a somewhat higher order we have an interesting
history of the evolution of a recent family in the work, not yet
completed, of Kidston and Gwynne-Vaughan on the fossil Osmundaceae.
("Trans. Royal Soc. Edinburgh", Vol. 45, Part III. 1907, Vol. 46, Part
II. 1908, Vol. 46, Part III. 1909.) The authors are able, mainly on
anatomical evidence, to trace back this now limited group of Ferns,
through the Tertiary and Mesozoic to the Permian, and to show, with
great probability, how their structure has been derived from that of
early Palaeozoic types.
The history of the Ginkgoaceae, now represented only by the isolated
maidenhair tree, scarcely known in a wild state, offers another striking
example of a family which can be traced with certainty to the older
Mesozoic and perhaps further back still. (See Seward and Gowan, "The
Maidenhair Tree (Gingko biloba)", "Annals of Botany", Vol. XIV. 1900,
page 109; also A. Sprecher "Le Ginkgo biloba", L., Geneva, 1907.)
On the wider question of the derivation of the great groups of plants,
a very considerable advance has been made, and, so far as the higher
plants are concerned, we are now able to form a far better conception
than before of the probable course of evolution. This is a matter
of phylogeny, and the facts will be considered under that head; our
immediate point is that the new knowledge of the relations between the
classes of plants in question materially strengthens the case for the
theory of descent. The discoveries of the last few years throw light
especially on the relation of the Angiosperms to the Gymnosperms,
on that of the Seed-plants generally to the Ferns, and on the
interrelations between the various classes of the higher Cryptogams.
That the fossil record has not done still more for Evolution is due to
the fact that it begins too late--a point on which Darwin laid stress
("Origin of Species" (6th edition), page 286.) and which has more
recently been elaborated by Poulton. ("Essays on Evolution", pages 46
et seq., Oxford, 1908.) An immense proportion of the whole evolutionary
history lies behind the lowest fossiliferous rocks, and the case is
worse for plants than for animals, as the record for the former begins,
for all practical purposes, much higher up in the rocks.
It may be well here to call attention to a question, often overlooked,
which has lately been revived by Reinke. (Reinke, loc. cit. page 13.) As
all admit, we know nothing of the origin of life; consequently, for all
we can tell, it is as probable that life began, on this planet, with
many living things, as with one. If the first organic beings were many,
they may have been heterogeneous, or at least exposed to different
conditions, from their origin; in either case there would have been a
number of distinct series from the beginning, and if so we should not
be justified in assuming that all organisms are related to one another.
There may conceivably be several of the original lines of descent still
surviving, or represented among extinct forms--to reverse the remark
of a distinguished botanist, there may be several Vegetable Kingdoms!
However improbable this may sound, the possibility is one to be borne in
mind.
That all VASCULAR plants really belong to one stock seems certain, and
here the palaeontological record has materially strengthened the
case for a monophyletic history. The Bryophyta are not likely to be
absolutely distinct, for their sexual organs, and the stomata of the
Mosses strongly suggest community of descent with the higher plants; if
this be so it no doubt establishes a certain presumption in favour of
a common origin for plants generally, for the gap between "Mosses and
Ferns" has been regarded as the widest in the Vegetable Kingdom. The
direct evidence of consanguinity is however much weaker when we come to
the Algae, and it is conceivable (even if improbable) that the higher
plants may have had a distinct ancestry (now wholly lost) from the
beginning. The question had been raised in Darwin's time, and he
referred to it in these words: "No doubt it is possible, as Mr G.H.
Lewes has urged, that at the first commencement of life many different
forms were evolved; but if so, we may conclude that only a very few
have left modified descendants." ("Origin of Species", page 425.) This
question, though it deserves attention, does not immediately affect the
subject of the palaeontological record of plants, for there can be no
reasonable doubt as to the interrelationship of those groups on which
the record at present throws light.
The past history of plants by no means shows a regular progression from
the simple to the complex, but often the contrary. This apparent anomaly
is due to two causes.
1. The palaeobotanical record is essentially the story of the successive
ascendancy of a series of dominant families, each of which attained
its maximum, in organisation as well as in extent, and then sank into
comparative obscurity, giving place to other families, which under new
conditions were better able to take a leading place. As each family ran
its downward course, either its members underwent an actual reduction in
structure as they became relegated to herbaceous or perhaps aquatic life
(this may have happened with the Horsetails and with Isoetes if derived
from Lepidodendreae), or the higher branches of the family were crowded
out altogether and only the "poor relations" were able to maintain their
position by evading the competition of the ascendant races; this is also
illustrated by the history of the Lycopod phylum. In either case there
would result a lowering of the type of organisation within the group.
2. The course of real progress is often from the complex to the simple.
If, as we shall find some grounds for believing, the Angiosperms came
from a type with a flower resembling in its complexity that of Mesozoic
"Cycads," almost the whole evolution of the flower in the highest
plants has been a process of reduction. The stamen, in particular, has
undoubtedly become extremely simplified during evolution; in the most
primitive known seed-plants it was a highly compound leaf or pinna; its
reduction has gone on in the Conifers and modern Cycads, as well as in
the Angiosperms, though in different ways and to a varying extent.
The seed offers another striking example; the Palaeozoic seeds (if we
leave the seed-like organs of certain Lycopods out of consideration)
were always, so far as we know, highly complex structures, with
an elaborate vascular system, a pollen-chamber, and often a
much-differentiated testa. In the present day such seeds exist only in a
few Gymnosperms which retain their ancient characters--in all the higher
Spermophytes the structure is very much simplified, and this holds good
even in the Coniferae, where there is no countervailing complication of
ovary and stigma.
Reduction, in fact, is not always, or even generally, the same thing as
degeneration. Simplification of parts is one of the most usual means of
advance for the organism as a whole. A large proportion of the higher
plants are microphyllous in comparison with the highly megaphyllous
fern-like forms from which they appear to have been derived.
Darwin treated the general question of advance in organisation with much
caution, saying: "The geological record... does not extend far enough
back, to show with unmistakeable clearness that within the known history
of the world organisation has largely advanced." ("Origin of Species",
page 308.) Further on (Ibid. page 309.) he gives two standards by which
advance may be measured: "We ought not solely to compare the highest
members of a class at any two periods... but we ought to compare all the
members, high and low, at the two periods." Judged by either standard
the Horsetails and Club Mosses of the Carboniferous were higher than
those of our own day, and the same is true of the Mesozoic Cycads. There
is a general advance in the succession of classes, but not within each
class.
Darwin's argument that "the inhabitants of the world at each successive
period in its history have beaten their predecessors in the race for
life, and are, in so far, higher in the scale" ("Origin of Species",
page 315.) is unanswerable, but we must remember that "higher in the
scale" only means "better adapted to the existing conditions." Darwin
points out (Ibid. page 279.) that species have remained unchanged for
long periods, probably longer than the periods of modification, and only
underwent change when the conditions of their life were altered. Higher
organisation, judged by the test of success, is thus purely relative to
the changing conditions, a fact of which we have a striking illustration
in the sudden incoming of the Angiosperms with all their wonderful
floral adaptations to fertilisation by the higher families of Insects.