Darwin and Modern Science
A.C. Seward

Part 5 out of 14

differences between the adults, but who is prepared to affirm that the
presence of a cephalic coelom and of cranial segments, of external gills,
of six gill slits, of the kidney tubes opening into the muscle-plate
coelom, of an enormous yolk-sac, of a neurenteric canal, and the absence of
any trace of an amnion, of an allantois and of a primitive streak are not
morphological facts of as high an import as those implied by the
differences between the adults? The generalisation undoubtedly had its
origin in the fact that there is what may be called a family resemblance
between embryos and larvae, but this resemblance, which is by no means
exact, is largely superficial and does not extend to anatomical detail.

It is useless to say, as Weismann has stated ("The Evolution Theory", by A.
Weismann, English Translation, Vol. II. page 176, London, 1904.), that "it
cannot be disputed that the rudiments [vestiges his translator means] of
gill-arches and gill-clefts, which are peculiar to one stage of human
ontogeny, give us every ground for concluding that we possessed fish-like
ancestors." The question at issue is: did the pharyngeal arches and
clefts of mammalian embryos ever discharge a branchial function in an adult
ancestor of the mammalia? We cannot therefore, without begging the
question at issue in the grossest manner, apply to them the terms "gill-
arches" and "gill-clefts". That they are homologous with the "gill-arches"
and "gill-clefts" of fishes is true; but there is no evidence to show that
they ever discharged a branchial function. Until such evidence is
forthcoming, it is beside the point to say that it "cannot be disputed"
that they are evidence of a piscine ancestry.

It must, therefore, be admitted that one outcome of the progress of
embryological and palaeontological research for the last 50 years is
negative. The recapitulation theory originated as a deduction from the
evolution theory and as a deduction it still remains.

Let us before leaving the subject apply another test. If the evolution
theory and the recapitulation theory are both true, how is it that living
birds are not only without teeth but have no rudiments of teeth at any
stage of their existence? How is it that the missing digits in birds and
mammals, the missing or reduced limb of snakes and whales, the reduced
mandibulo-hyoid cleft of elasmobranch fishes are not present or relatively
more highly developed in the embryo than in the adult? How is it that when
a marked variation, such as an extra digit, or a reduced limb, or an extra
segment, makes its appearance, it is not confined to the adult but can be
seen all through the development? All the clear evidence we can get tends
to show that marked variations, whether of reduction or increase, of organs
are manifest during the whole of the development of the organ and do not
merely affect the adult. And on reflection we see that it could hardly be
otherwise. All such evidence is distinctly at variance with the theory of
recapitulation, at least as applied to embryos. In the case of larvae of
course the case will be different, for in them the organs are functional,
and reduction in the adult will not be accompanied by reduction in the
larva unless a change in the conditions of life of the larva enables it to

If after 50 years of research and close examination of the facts of
embryology the recapitulation theory is still without satisfactory proof,
it seems desirable to take a wider sweep and to inquire whether the facts
of embryology cannot be included in a larger category.

As has been pointed out by Huxley, development and life are co-extensive,
and it is impossible to point to any period in the life of an organism when
the developmental changes cease. It is true that these changes take place
more rapidly at the commencement of life, but they are never wholly absent,
and those which occur in the later or so-called adult stages of life do not
differ in their essence, however much they may differ in their degree, from
those which occur during the embryonic and larval periods. This
consideration at once brings the changes of the embryonic period into the
same category as those of the adult and suggests that an explanation which
will account for the one will account for the other. What then is the
problem we are dealing with? Surely it is this: Why does an organism as
soon as it is established at the fertilisation of the ovum enter upon a
cycle of transformations which never cease until death puts an end to them?
In other words what is the meaning of that cycle of changes which all
organisms present in a greater or less degree and which constitute the very
essence of life? It is impossible to give an answer to this question so
long as we remain within the precincts of Biology--and it is not my present
purpose to penetrate beyond those precincts into the realms of philosophy.
We have to do with an ultimate biological fact, with a fundamental property
of living matter, which governs and includes all its other properties. How
may this property be stated? Thus: it is a property of living matter to
react in a remarkable way to external forces without undergoing
destruction. The life-cycle, of which the embryonic and larval periods are
a part, consists of the orderly interaction between the organism and its
environment. The action of the environment produces certain morphological
changes in the organism. These changes enable the organism to come into
relation with new external forces, to move into what is practically a new
environment, which in its turn produces further structural changes in the
organism. These in their turn enable, indeed necessitate, the organism to
move again into a new environment, and so the process continues until the
structural changes are of such a nature that the organism is unable to
adapt itself to the environment in which it finds itself. The essential
condition of success in this process is that the organism should always
shift into the environment to which its new structure is suited--any
failure in this leading to the impairment of the organism. In most cases
the shifting of the environment is a very gradual process (whether
consisting in the very slight and gradual alteration in the relation of the
embryo as a whole to the egg-shell or uterine wall, or in the relations of
its parts to each other, or in the successive phases of adult life), and
the morphological changes in connection with each step of it are but
slight. But in some cases jumps are made such as we find in the phenomena
known as hatching, birth, and metamorphosis.

This property of reacting to the environment without undergoing destruction
is, as has been stated, a fundamental property of organisms. It is
impossible to conceive of any matter, to which the term living could be
applied, being without it. And with this property of reacting to the
environment goes the further property of undergoing a change which alters
the relation of the organism to the old environment and places it in a new
environment. If this reasoning is correct, it necessarily follows that
this property must have been possessed by living matter at its first
appearance on the earth. In other words living matter must always have
presented a life-cycle, and the question arises what kind of modification
has that cycle undergone? Has it increased or diminished in duration and
complexity since organisms first appeared on the earth? The current view
is that the cycle was at first very short and that it has increased in
length by the evolutionary creation of new adult phases, that these new
phases are in addition to those already existing and that each of them as
it appears takes over from the preceding adult phase the functional
condition of the reproductive organs. According to the same view the old
adult phases are not obliterated but persist in a more or less modified
form as larval stages. It is further supposed that as the life-history
lengthens at one end by the addition of new adult phases, it is shortened
at the other by the abbreviation of embryonic development and by the
absorption of some of the early larval stages into the embryonic period;
but on the whole the lengthening process has exceeded that of shortening,
so that the whole life-history has, with the progress of evolution, become
longer and more complicated.

Now there can be no doubt that the life-history of organisms has been
shortened in the way above suggested, for cases are known in which this can
practically be seen to occur at the present day. But the process of
lengthening by the creation of new stages at the other end of the life-
cycle is more difficult to conceive and moreover there is no evidence for
its having occurred. This, indeed, may have occurred, as is suggested
below, but the evidence we have seems to indicate that evolutionary
modification has proceeded by ALTERING and not by SUPERSEDING: that is to
say that each stage in the life-history, as we see it to-day, has proceeded
from a corresponding stage in a former era by the modification of that
stage and not by the creation of a new one. Let me, at the risk of
repetition, explain my meaning more fully by taking a concrete
illustration. The mandibulo-hyoid cleft (spiracle) of the elasmobranch
fishes, the lateral digits of the pig's foot, the hind-limbs of whales, the
enlarged digit of the ostrich's foot are supposed to be organs which have
been recently modified. This modification is not confined to the final
adult stage of the life-history but characterises them throughout the whole
of their development. A stage with a reduced spiracle does not proceed in
development from a preceding stage in which the spiracle shows no
reduction: it is reduced at its first appearance. The same statement may
be made of organs which have entirely disappeared in the adult, such as
bird's teeth and snake's fore-limbs: the adult stage in which they have
disappeared is not preceded by embryonic stages in which the teeth and
limbs or rudiments of them are present. In fact the evidence indicates
that adult variations of any part are accompanied by precedent variations
in the same direction in the embryo. The evidence seems to show, not that
a stage is added on at the end of the life-history, but only that some of
the stages in the life-history are modified. Indeed, on the wider view of
development taken in this essay, a view which makes it coincident with
life, one would not expect often to find, even if new stages are added in
the course of evolution, that they are added at the end of the series when
the organism has passed through its reproductive period. It is possible of
course that new stages have been intercalated in the course of the life-
history, though it is difficult to see how this has occurred. It is much
more likely, if we may judge from available evidence, that every stage has
had its counterpart in the ancestral form from which it has been derived by
descent with modification. Just as the adult phase of the living form
differs, owing to evolutionary modification, from the adult phase of the
ancestor from which it has proceeded, so each larval phase will differ for
the same reason from the corresponding larval phase in the life-history of
the ancestor. Inasmuch as the organism is variable at every stage of its
independent existence and is exposed to the action of natural selection
there is no reason why it should escape modification at any stage.

If there is any truth in these considerations it would seem to follow that
at the dawn of life the life-cycle must have been, either in posse or in
esse, at least as long as it is at the present time, and that the
peculiarity of passing through a series of stages in which new characters
are successively evolved is a primordial quality of living matter.

Before leaving this part of the subject, it is necessary to touch upon
another aspect of it. What are these variations in structure which succeed
one another in the life-history of an organism? I am conscious that I am
here on the threshold of a chamber which contains the clue to some of our
difficulties, and that I cannot enter it. Looked at from one point of view
they belong to the class of genetic variations, which depend upon the
structure or constitution of the protoplasm; but instead of appearing in
different zygotes (A zygote is a fertilised ovum, i.e. a new organism
resulting from the fusion of an ovum and a spermatozoon.), they are present
in the same zygote though at different times in its life-history. They are
of the same order as the mutational variations of the modern biologist upon
which the appearance of a new character depends. What is a genetic or
mutational variation? It is a genetic character which was not present in
either of the parents. But these "growth variations" were present in the
parents, and in this they differ from mutational variations. But what are
genetic characters? They are characters which must appear if any
development occurs. They are usually contrasted with "acquired
characters," using the expression "acquired character" in the Lamarckian
sense. But strictly speaking they ARE acquired characters, for the zygote
at first has none of the characters which it subsequently acquires, but
only the power of acquiring them in response to the action of the
environment. But the characters so acquired are not what we technically
understand and what Lamarck meant by "acquired characters." They are
genetic characters, as defined above. What then are Lamarck's "acquired
characters"? They are variations in genetic characters caused in a
particular way. There are, in fact, two kinds of variation in genetic
characters depending on the mode of causation. Firstly, there are those
variations consequent upon a variation in the constitution of the
protoplasm of a particular zygote, and independent of the environment in
which the organism develops, save in so far as this simply calls them
forth: these are the so-called genetic or mutational variations.
Secondly, there are those variations which occur in zygotes of similar
germinal constitution and which are caused solely by differences in the
environment to which the individuals are respectively exposed: these are
the "acquired characters" of Lamarck and of authors generally. In
consequence of this double sense in which the term "acquired characters"
may be used, great confusion may and does occur. If the protoplasm be
compared to a machine, and the external conditions to the hand that works
the machine, then it may be said that, as the machine can only work in one
way, it can only produce one kind of result (genetic character), but the
particular form or quality (Lamarckian "acquired character") of the result
will depend upon the hand that works the machine (environment), just as the
quality of the sound produced by a fiddle depends entirely upon the hand
which plays upon it. It would be improper to apply the term "mutation" to
those genetic characters which are not new characters or new variants of
old characters, but such genetic characters are of the same nature as those
characters to which the term mutation has been applied. It may be noticed
in passing that it is very questionable if the modern biologist has acted
in the real interests of science in applying the term mutation in the sense
in which he has applied it. The genetic characters of organisms come from
one of two sources: either they are old characters and are due to the
action of what we call inheritance or they are new and are due to what we
call variation. If the term mutation is applied to the actual alteration
of the machinery of the protoplasm, no objection can be felt to its use;
but if it be applied, as it is, to the product of the action of the altered
machine, viz. to the new genetic character, it leads to confusion.
Inheritance is the persistence of the structure of the machine; characters
are the products of the working of the machine; variation in genetic
characters is due to the alteration (mutation) in the arrangement of the
machinery, while variation in acquired characters (Lamarckian) is due to
differences in the mode of working the machinery. The machinery when it
starts (in the new zygote) has the power of grinding out certain results,
which we call the characters of the organism. These appear at successive
intervals of time, and the orderly manifestation of them is what we call
the life-history of the organism. This brings us back to the question with
which we started this discussion, viz. what is the relation of these
variations in structure, which successively appear in an organism and
constitute its life-history, to the mutational variations which appear in
different organisms of the same brood or species. The question is brought
home to us when we ask what is a bud-sport, such as a nectarine appearing
on a peach-tree? From one point of view, it is simply a mutation appearing
in asexual reproduction; from another it is one of these successional
characters ("growth variations") which constitute the life-history of the
zygote, for it appears in the same zygote which first produces a peach.
Here our analogy of a machine which only works in one way seems to fail us,
for these bud-sports do not appear in all parts of the organism, only in
certain buds or parts of it, so that one part of the zygotic machine would
appear to work differently to another. To discuss this question further
would take us too far from our subject. Suffice it to say that we cannot
answer it, any more than we can this further question of burning interest
at the present day, viz. to what extent and in what manner is the machine
itself altered by the particular way in which it is worked. In connection
with this question we can only submit one consideration: the zygotic
machine can, by its nature, only work once, so that any alteration in it
can only be ascertained by studying the replicas of it which are produced
in the reproductive organs.

It is a peculiarity that the result which we call the ripening of the
generative organs nearly always appears among the final products of the
action of the zygotic machine. It is remarkable that this should be the
case. What is the reason of it? The late appearance of functional
reproductive organs is almost a universal law, and the explanation of it is
suggested by expressing the law in another way, viz. that the machine is
almost always so constituted that it ceases to work efficiently soon after
the reproductive organs have sufficiently discharged their function. Why
this should occur we cannot explain: it is an ultimate fact of nature, and
cannot be included in any wider category. The period during which the
reproductive organs can act may be short as in ephemerids or long as in man
and trees, and there is no reason to suppose that their action damages the
vital machinery, though sometimes, as in the case of annual plants
(Metschnikoff), it may incidentally do so; but, long or short, the
cessation of their actions is always a prelude to the end. When they and
their action are impaired, the organism ceases to react with precision to
the environment, and the organism as a whole undergoes retrogressive

It has been pointed out above that there is reason to believe that at the
dawn of life the life-cycle was, EITHER IN ESSE OR IN POSSE, at least as
long as it is at the present time. The qualification implied by the words
in italics is necessary, for it is clearly possible that the external
conditions then existing were not suitable for the production of all the
stages of the potential life-history, and that what we call organic
evolution has consisted in a gradual evolution of new environments to which
the organism's innate capacity of change has enabled it to adapt itself.
We have warrant for this possibility in the case of the Axolotl and in
other similar cases of neoteny. And these cases further bring home to us
the fact, to which I have already referred, that the full development of
the functional reproductive organs is nearly always associated with the
final stages of the life-history.

On this view of the succession of characters in the life-history of
organisms, how shall we explain the undoubted fact that the development of
buds hardly ever presents any phenomena corresponding to the embryonic and
larval changes? The reason is clearly this, that budding usually occurs
after the embryonic stage is past; when the characters of embryonic life
have been worked out by the machine. When it takes place at an early stage
in embryonic life, as it does in cases of so-called embryonic fission, the
product shows, either partly or entirely, phenomena similar to those of
embryonic development. The only case known to me in which budding by the
adult is accompanied by morphological features similar to those displayed
by embryos is furnished by the budding of the medusiform spore-sacs of
hydrozoon polyps. But this case is exceptional, for here we have to do
with an attempt, which fails, to form a free-swimming organism, the medusa;
and the vestiges which appear in the buds are the umbrella-cavity, marginal
tentacles, circular canal, etc., of the medusa arrested in development.

But the question still remains, are there no cases in which, as implied by
the recapitulation theory, variations in any organ are confined to the
period in which the organ is functional and do not affect it in the
embryonic stages? The teeth of the whalebone whales may be cited as a case
in which this is said to occur; but here the teeth are only imperfectly
developed in the embryo and are soon absorbed. They have been affected by
the change which has produced their disappearance in the adult, but not to
complete extinction. Nor are they now likely to be extinguished, for
having become exclusively embryonic they are largely protected from the
action of natural selection. This consideration brings up a most important
aspect of the question, so far as disappearing organs are concerned. Every
organ is laid down at a certain period in the embryo and undergoes a
certain course of growth until it obtains full functional development.
When for any cause reduction begins, it is affected at all stages of its
growth, unless it has functional importance in the larva, and in some cases
its life is shortened at one or both ends. In cases, as in that of the
whale's teeth, in which it entirely disappears in the adult, the latter
part of its life is cut off; in others, the beginning of its life may be
deferred. This happens, for instance, with the spiracle of many
Elasmobranchs, which makes its appearance after the hyobranchial cleft, not
before it as it should do, being anterior to it in position, and as it does
in the Amniota in which it shows no reduction in size as compared with the
other pharyngeal clefts. In those Elasmobranchs in which it is absent in
the adult but present in the embryo (e.g. Carcharias) its life is shortened
at both ends. Many more instances of organs, of which the beginning and
end have been cut off, might be mentioned; e.g. the muscle-plate coelom of
Aves, the primitive streak and the neurenteric canal of amniote
blastoderms. In yet other cases in which the reduced organ is almost on
the verge of disappearance, it may appear for a moment and disappear more
than once in the course of development. As an instance of this striking
phenomenon I may mention the neurenteric canal of avine embryos, and the
anterior neuropore of Ascidians. Lastly the reduced organ may disappear in
the developing stages before it does so in the adult. As an instance of
this may be mentioned the mandibular palp of those Crustacea with zoaea
larvae. This structure disappears in the larva only to reappear in a
reduced form in later stages. In all these cases we are dealing with an
organ which, we imagine, attained a fuller functional development at some
previous stage in race-history, but in most of them we have no proof that
it did so. It may be, and the possibility must not be lost sight of, that
these organs never were anything else than functionless and that though
they have been got rid of in the adult by elimination in the course of
time, they have been able to persist in embryonic stages which are
protected from the full action of natural selection. There is no reason to
suppose that living matter at its first appearance differed from non-living
matter in possessing only properties conducive to its well-being and
prolonged existence. No one thinks that the properties of the various
forms of inorganic matter are all strictly related to external conditions.
Of what use to the diamond is its high specific gravity and high
refrangibility, and to gold of its yellow colour and great weight? These
substances continue to exist in virtue of other properties than these. It
is impossible to suppose that the properties of living matter at its first
appearance were all useful to it, for even now after aeons of elimination
we find that it possesses many useless organs and that many of its
relations to the external world are capable of considerable improvement.

In writing this essay I have purposely refrained from taking a definite
position with regard to the problems touched. My desire has been to write
a chapter showing the influence of Darwin's work so far as Embryology is
concerned, and the various points which come up for consideration in
discussing his views. Darwin was the last man who would have claimed
finality for any of his doctrines, but he might fairly have claimed to have
set going a process of intellectual fermentation which is still very far
from completion.



Professor of Geology in the University of Princeton, U.S.A.

To no branch of science did the publication of "The Origin of Species"
prove to be a more vivifying and transforming influence than to
Palaeontology. This science had suffered, and to some extent, still
suffers from its rather anomalous position between geology and biology,
each of which makes claim to its territory, and it was held in strict
bondage to the Linnean and Cuvierian dogma that species were immutable
entities. There is, however, reason to maintain that this strict bondage
to a dogma now abandoned, was not without its good side, and served the
purpose of keeping the infant science in leading-strings until it was able
to walk alone, and preventing a flood of premature generalisations and

As Zittel has said: "Two directions were from the first apparent in
palaeontological research--a stratigraphical and a biological.
Stratigraphers wished from palaeontology mainly confirmation regarding the
true order or relative age of zones of rock-deposits in the field.
Biologists had, theoretically at least, the more genuine interest in fossil
organisms as individual forms of life." (Zittel, "History of Geology and
Palaeontology", page 363, London, 1901.) The geological or stratigraphical
direction of the science was given by the work of William Smith, "the
father of historical geology," in the closing decade of the eighteenth
century. Smith was the first to make a systematic use of fossils in
determining the order of succession of the rocks which make up the
accessible crust of the earth, and this use has continued, without
essential change, to the present day. It is true that the theory of
evolution has greatly modified our conceptions concerning the introduction
of new species and the manner in which palaeontological data are to be
interpreted in terms of stratigraphy, but, broadly speaking, the method
remains fundamentally the same as that introduced by Smith.

The biological direction of palaeontology was due to Cuvier and his
associates, who first showed that fossils were not merely varieties of
existing organisms, but belonged to extinct species and genera, an
altogether revolutionary conception, which startled the scientific world.
Cuvier made careful studies, especially of fossil vertebrates, from the
standpoint of zoology and was thus the founder of palaeontology as a
biological science. His great work on "Ossements Fossiles" (Paris, 1821)
has never been surpassed as a masterpiece of the comparative method of
anatomical investigation, and has furnished to the palaeontologist the
indispensable implements of research.

On the other hand, Cuvier's theoretical views regarding the history of the
earth and its successive faunas and floras are such as no one believes to-
day. He held that the earth had been repeatedly devastated by great
cataclysms, which destroyed every living thing, necessitating an entirely
new creation, thus regarding the geological periods as sharply demarcated
and strictly contemporaneous for the whole earth, and each species of
animal and plant as confined to a single period. Cuvier's immense
authority and his commanding personality dominated scientific thought for
more than a generation and marked out the line which the development of
palaeontology was to follow. The work was enthusiastically taken up by
many very able men in the various European countries and in the United
States, but, controlled as it was by the belief in the fixity of species,
it remained almost entirely descriptive and consisted in the description
and classification of the different groups of fossil organisms. As already
intimated, this narrowness of view had its compensations, for it deferred
generalisations until some adequate foundations for these had been laid.

Dominant as it was, Cuvier's authority was slowly undermined by the
progress of knowledge and the way was prepared for the introduction of more
rational conceptions. The theory of "Catastrophism" was attacked by
several geologists, most effectively by Sir Charles Lyell, who greatly
amplified the principles enunciated by Hutton and Playfair in the preceding
century, and inaugurated a new era in geology. Lyell's uniformitarian
views of the earth's history and of the agencies which had wrought its
changes, had undoubted effect in educating men's minds for the acceptance
of essentially similar views regarding the organic world. In palaeontology
too the doctrine of the immutability of species, though vehemently
maintained and reasserted, was gradually weakening. In reviewing long
series of fossils, relations were observed which pointed to genetic
connections and yet were interpreted as purely ideal. Agassiz, for
example, who never accepted the evolutionary theory, drew attention to
facts which could be satisfactorily interpreted only in terms of that
theory. Among the fossils he indicated "progressive," "synthetic,"
"prophetic," and "embryonic" types, and pointed out the parallelism which
obtains between the geological succession of ancient animals and the
ontogenetic development of recent forms. In Darwin's words: "This view
accords admirably well with our theory." ("Origin of Species" (6th
edition), page 310.) Of similar import were Owen's views on "generalised
types" and "archetypes."

The appearance of "The Origin of Species" in 1859 revolutionised all the
biological sciences. From the very nature of the case, Darwin was
compelled to give careful consideration to the palaeontological evidence;
indeed, it was the palaeontology and modern distribution of animals in
South America which first led him to reflect upon the great problem. In
his own words: "I had been deeply impressed by discovering in the Pampean
formation great fossil animals covered with armour like that on the
existing armadillos; secondly, by the manner in which closely allied
animals replace one another in proceeding southward over the Continent; and
thirdly, by the South American character of most of the productions of the
Galapagos archipelago, and more especially by the manner in which they
differ slightly on each island of the group." ("Life and Letters of
Charles Darwin", I. page 82.) In the famous tenth and eleventh chapters of
the "Origin", the palaeontological evidence is examined at length and the
imperfection of the geological record is strongly emphasised. The
conclusion is reached, that, in view of this extreme imperfection,
palaeontology could not reasonably be expected to yield complete and
convincing proof of the evolutionary theory. "I look at the geological
record as a history of the world imperfectly kept, and written in a
changing dialect; of this history we possess the last volume alone,
relating only to two or three countries. Of this volume, only here and
there a short chapter has been preserved; and of each page, only here and
there a few lines." ("Origin of Species", page 289.) Yet, aside from
these inevitable difficulties, he concludes, that "the other great leading
facts in palaeontology agree admirably with the theory of descent with
modification through variation and natural selection." (Ibid. page 313.)

Darwin's theory gave an entirely new significance and importance to
palaeontology. Cuvier's conception of the science had been a limited,
though a lofty one. "How glorious it would be if we could arrange the
organised products of the universe in their chronological order!...The
chronological succession of organised forms, the exact determination of
those types which appeared first, the simultaneous origin of certain
species and their gradual decay, would perhaps teach us as much about the
mysteries of organisation as we can possibly learn through experiments with
living organisms." (Zittel op. cit. page 140.) This, however, was rather
the expression of a hope for the distant future than an account of what was
attainable, and in practice the science remained almost purely descriptive,
until Darwin gave it a new standpoint, new problems and an altogether fresh
interest and charm. The revolution thus accomplished is comparable only to
that produced by the Copernican astronomy.

From the first it was obvious that one of the most searching tests of the
evolutionary theory would be given by the advance of palaeontological
discovery. However imperfect the geological record might be, its
ascertained facts would necessarily be consistent, under any reasonable
interpretation, with the demands of a true theory; otherwise the theory
would eventually be overwhelmed by the mass of irreconcilable data. A very
great stimulus was thus given to geological investigation and to the
exploration of new lands. In the last forty years, the examination of
North and South America, of Africa and Asia has brought to light many
chapters in the history of life, which are astonishingly full and complete.
The flood of new material continues to accumulate at such a rate that it is
impossible to keep abreast of it, and the very wealth of the collections is
a source of difficulty and embarrassment. In modern palaeontology
phylogenetic questions and problems occupy a foremost place and, as a
result of the labours of many eminent investigators in many lands, it may
be said that this science has proved to be one of the most solid supports
of Darwin's theory. True, there are very many unsolved problems, and the
discouraged worker is often tempted to believe that the fossils raise more
questions than they answer. Yet, on the other hand, the whole trend of the
evidence is so strongly in favour of the evolutionary doctrine, that no
other interpretation seems at all rational.

To present any adequate account of the palaeontological record from the
evolutionary standpoint, would require a large volume and a singularly
unequal, broken and disjointed history it would be. Here the record is
scanty, interrupted, even unintelligible, while there it is crowded with
embarrassing wealth of material, but too often these full chapters are
separated by such stretches of unrecorded time, that it is difficult to
connect them. It will be more profitable to present a few illustrative
examples than to attempt an outline of the whole history.

At the outset, the reader should be cautioned not to expect too much, for
the task of determining phylogenies fairly bristles with difficulties and
encounters many unanswered questions. Even when the evidence seems to be
as copious and as complete as could be wished, different observers will put
different interpretations upon it, as in the notorious case of the
Steinheim shells. (In the Miocene beds of Steinheim, Wurtemberg, occur
countless fresh-water shells, which show numerous lines of modification,
but these have been very differently interpreted by different writers.)
The ludicrous discrepances which often appear between the phylogenetic
"trees" of various writers have cast an undue discredit upon the science
and have led many zoologists to ignore palaeontology altogether as unworthy
of serious attention. One principal cause of these discrepant and often
contradictory results is our ignorance concerning the exact modes of
developmental change. What one writer postulates as almost axiomatic,
another will reject as impossible and absurd. Few will be found to agree
as to how far a given resemblance is offset by a given unlikeness, and so
long as the question is one of weighing evidence and balancing
probabilities, complete harmony is not to be looked for. These formidable
difficulties confront us even in attempting to work out from abundant
material a brief chapter in the phylogenetic history of some small and
clearly limited group, and they become disproportionately greater, when we
extend our view over vast periods of time and undertake to determine the
mutual relationships of classes and types. If the evidence were complete
and available, we should hardly be able to unravel its infinite complexity,
or to find a clue through the mazes of the labyrinth. "Our ideas of the
course of descent must of necessity be diagrammatic." (D.H. Scott,
"Studies in Fossil Botany", page 524. London, 1900.)

Some of the most complete and convincing examples of descent with
modification are to be found among the mammals, and nowhere more abundantly
than in North America, where the series of continental formations, running
through the whole Tertiary period, is remarkably full. Most of these
formations contain a marvellous wealth of mammalian remains and in an
unusual state of preservation. The oldest Eocene (Paleocene) has yielded a
mammalian fauna which is still of prevailingly Mesozoic character, and
contains but few forms which can be regarded as ancestral to those of later
times. The succeeding fauna of the lower Eocene proper (Wasatch stage) is
radically different and, while a few forms continue over from the
Paleocene, the majority are evidently recent immigrants from some region
not yet identified. From the Wasatch onward, the development of many phyla
may be traced in almost unbroken continuity, though from time to time the
record is somewhat obscured by migrations from the Old World and South
America. As a rule, however, it is easy to distinguish between the
immigrant and the indigenous elements of the fauna.

From their gregarious habits and individual abundance, the history of many
hoofed animals is preserved with especial clearness. So well known as to
have become a commonplace, is the phylogeny of the horses, which, contrary
to all that would have been expected, ran the greater part of its course in
North America. So far as it has yet been traced, the line begins in the
lower Eocene with the genus Eohippus, a little creature not much larger
than a cat, which has a short neck, relatively short limbs, and in
particular, short feet, with four functional digits and a splint-like
rudiment in the fore-foot, three functional digits and a rudiment in the
hind-foot. The forearm bones (ulna and radius) are complete and separate,
as are also the bones of the lower leg (fibula and tibia). The skull has a
short face, with the orbit, or eye-socket, incompletely enclosed with bone,
and the brain-case is slender and of small capacity. The teeth are short-
crowned, the incisors without "mark," or enamel pit, on the cutting edge;
the premolars are all smaller and simpler than the molars. The pattern of
the upper molars is so entirely different from that seen in the modern
horses that, without the intermediate connecting steps, no one would have
ventured to derive the later from the earlier plan. This pattern is
quadritubercular, with four principal, conical cusps arranged in two
transverse pairs, forming a square, and two minute cuspules between each
transverse pair, a tooth which is much more pig-like than horse-like. In
the lower molars the cusps have already united to form two crescents, one
behind the other, forming a pattern which is extremely common in the early
representatives of many different families, both of the Perissodactyla and
the Artiodactyla. In spite of the manifold differences in all parts of the
skeleton between Eohippus and the recent horses, the former has stamped
upon it an equine character which is unmistakable, though it can hardly be
expressed in words.

Each one of the different Eocene and Oligocene horizons has its
characteristic genus of horses, showing a slow, steady progress in a
definite direction, all parts of the structure participating in the
advance. It is not necessary to follow each of these successive steps of
change, but it should be emphasised that the changes are gradual and
uninterrupted. The genus Mesohippus, of the middle Oligocene, may be
selected as a kind of half-way stage in the long progression. Comparing
Mesohippus with Eohippus, we observe that the former is much larger, some
species attaining the size of a sheep, and has a relatively longer neck,
longer limbs and much more elongate feet, which are tridactyl, and the
middle toe is so enlarged that it bears most of the weight, while the
lateral digits are very much more slender. The fore-arm bones have begun
to co-ossify and the ulna is greatly reduced, while the fibula, though
still complete, is hardly more than a thread of bone. The skull has a
longer face and a nearly enclosed orbit, and the brain-case is fuller and
more capacious, the internal cast of which shows that the brain was richly
convoluted. The teeth are still very short-crowned, but the upper incisors
plainly show the beginning of the "mark"; the premolars have assumed the
molar form, and the upper molars, though plainly derived from those of
Eohippus, have made a long stride toward the horse pattern, in that the
separate cusps have united to form a continuous outer wall and two
transverse crests.

In the lower Miocene the interesting genus Desmatippus shows a further
advance in the development of the teeth, which are beginning to assume the
long-crowned shape, delaying the formation of roots; a thin layer of cement
covers the crowns, and the transverse crests of the upper grinding teeth
display an incipient degree of their modern complexity. This tooth-pattern
is strictly intermediate between the recent type and the ancient type seen
in Mesohippus and its predecessors. The upper Miocene genera, Protohippus
and Hipparion are, to all intents and purposes, modern in character, but
their smaller size, tridactyl feet and somewhat shorter-crowned teeth are
reminiscences of their ancestry.

From time to time, when a land-connection between North America and Eurasia
was established, some of the successive equine genera migrated to the Old
World, but they do not seem to have gained a permanent footing there until
the end of the Miocene or beginning of the Pliocene, eventually
diversifying into the horses, asses, and zebras of Africa, Asia and Europe.
At about the same period, the family extended its range to South America
and there gave rise to a number of species and genera, some of them
extremely peculiar. For some unknown reason, all the horse tribe had
become extinct in the western hemisphere before the European discovery, but
not until after the native race of man had peopled the continents.

In addition to the main stem of equine descent, briefly considered in the
foregoing paragraphs, several side-branches were given off at successive
levels of the stem. Most of these branches were short-lived, but some of
them flourished for a considerable period and ramified into many species.

Apparently related to the horses and derived from the same root-stock is
the family of the Palaeotheres, confined to the Eocene and Oligocene of
Europe, dying out without descendants. In the earlier attempts to work out
the history of the horses, as in the famous essay of Kowalevsky ("Sur
l'Anchitherium aurelianense Cuv. et sur l'histoire paleontologique des
Chevaux", "Mem. de l'Acad. Imp. des Sc. de St Petersbourg", XX. no. 5,
1873.), the Palaeotheres were placed in the direct line, because the number
of adequately known Eocene mammals was then so small, that Cuvier's types
were forced into various incongruous positions, to serve as ancestors for
unrelated series.

The American family of the Titanotheres may also be distantly related to
the horses, but passed through an entirely different course of development.
From the lower Eocene to the lower sub-stage of the middle Oligocene the
series is complete, beginning with small and rather lightly built animals.
Gradually the stature and massiveness increase, a transverse pair of nasal
horns make their appearance and, as these increase in size, the canine
tusks and incisors diminish correspondingly. Already in the oldest known
genus the number of digits had been reduced to four in the fore-foot and
three in the hind, but there the reduction stops, for the increasing body-
weight made necessary the development of broad and heavy feet. The final
members of the series comprise only large, almost elephantine animals, with
immensely developed and very various nasal horns, huge and massive heads,
and altogether a grotesque appearance. The growth of the brain did not at
all keep pace with the increase of the head and body, and the ludicrously
small brain may will have been one of the factors which determined the
startlingly sudden disappearance and extinction of the group.

Less completely known, but of unusual interest, is the genealogy of the
rhinoceros family, which probably, though not certainly, was likewise of
American origin. The group in North America at least, comprised three
divisions, or sub-families, of very different proportions, appearance and
habits, representing three divergent lines from the same stem. Though the
relationship between the three lines seems hardly open to question, yet the
form ancestral to all of them has not yet been identified. This is because
of our still very incomplete knowledge of several perissodactyl genera of
the Eocene, any one of which may eventually prove to be the ancestor sought

The first sub-family is the entirely extinct group of Hyracodonts, which
may be traced in successive modifications through the upper Eocene, lower
and middle Oligocene, then disappearing altogether. As yet, the
hyracodonts have been found only in North America, and the last genus of
the series, Hyracodon, was a cursorial animal. Very briefly stated, the
modifications consist in a gradual increase in size, with greater
slenderness of proportions, accompanied by elongation of the neck, limbs,
and feet, which become tridactyl and very narrow. The grinding teeth have
assumed the rhinoceros-like pattern and the premolars resemble the molars
in form; on the other hand, the front teeth, incisors and canines, have
become very small and are useless as weapons. As the animal had no horns,
it was quite defenceless and must have found its safety in its swift
running, for Hyracodon displays many superficial resemblances to the
contemporary Oligocene horses, and was evidently adapted for speed. It may
well have been the competition of the horses which led to the extinction of
these cursorial rhinoceroses.

The second sub-family, that of the Amynodonts, followed a totally different
course of development, becoming short-legged and short-footed, massive
animals, the proportions of which suggest aquatic habits; they retained
four digits in the front foot. The animal was well provided with weapons
in the large canine tusks, but was without horns. Some members of this
group extended their range to the Old World, but they all died out in the
middle Oligocene, leaving no successors.

The sub-family of the true rhinoceroses cannot yet be certainly traced
farther back than to the base of the middle Oligocene, though some
fragmentary remains found in the lower Oligocene are probably also
referable to it. The most ancient and most primitive member of this series
yet discovered, the genus Trigonias, is unmistakably a rhinoceros, yet much
less massive, having more the proportions of a tapir; it had four toes in
the front foot, three in the hind, and had a full complement of teeth,
except for the lower canines, though the upper canines are about to
disappear, and the peculiar modification of the incisors, characteristic of
the true rhinoceroses, is already apparent; the skull is hornless.
Representatives of this sub-family continue through the Oligocene and
Miocene of North America, becoming rare and localised in the Pliocene and
then disappearing altogether. In the Old World, on the other hand, where
the line appeared almost as early as it did in America, this group
underwent a great expansion and ramification, giving rise not only to the
Asiatic and African forms, but also to several extinct series.

Turning now to the Artiodactyla, we find still another group of mammals,
that of the camels and llamas, which has long vanished from North America,
yet took its rise and ran the greater part of its course in that continent.
From the lower Eocene onward the history of this series is substantially
complete, though much remains to be learned concerning the earlier members
of the family. The story is very like that of the horses, to which in many
respects it runs curiously parallel. Beginning with very small, five-toed
animals, we observe in the successive genera a gradual transformation in
all parts of the skeleton, an elongation of the neck, limbs and feet, a
reduction of the digits from five to two, and eventually the coalescence of
the remaining two digits into a "cannon-bone." The grinding teeth, by
equally gradual steps, take on the ruminant pattern. In the upper Miocene
the line divides into the two branches of the camels and llamas, the former
migrating to Eurasia and the latter to South America, though
representatives of both lines persisted in North America until a very late
period. Interesting side-branches of this line have also been found, one
of which ended in the upper Miocene in animals which had almost the
proportions of the giraffes and must have resembled them in appearance.

The American Tertiary has yielded several other groups of ruminant-like
animals, some of which form beautifully complete evolutionary series, but
space forbids more than this passing mention of them.

It was in Europe that the Artiodactyla had their principal development, and
the upper Eocene, Oligocene and Miocene are crowded with such an
overwhelming number and variety of forms that it is hardly possible to
marshal them in orderly array and determine their mutual relationships.
Yet in this chaotic exuberance of life, certain important facts stand out
clearly, among these none is of greater interest and importance than the
genealogy of the true Ruminants, or Pecora, which may be traced from the
upper Eocene onward. The steps of modification and change are very similar
to those through which the camel phylum passed in North America, but it is
instructive to note that, despite their many resemblances, the two series
can be connected only in their far distant beginnings. The pecoran stock
became vastly more expanded and diversified than did the camel line and was
evidently more plastic and adaptable, spreading eventually over all the
continents except Australia, and forming to-day one of the dominant types
of mammals, while the camels are on the decline and not far from
extinction. The Pecora successively ramified into the deer, antelopes,
sheep, goats and oxen, and did not reach North America till the Miocene,
when they were already far advanced in specialisation. To this invasion of
the Pecora, or true ruminants, it seems probable that the decline and
eventual disappearance of the camels is to be ascribed.

Recent discoveries in Egypt have thrown much light upon a problem which
long baffled the palaeontologist, namely, the origin of the elephants.
(C.W. Andrews, "On the Evolution of the Proboscidea", "Phil. Trans. Roy.
Soc." London, Vol. 196, 1904, page 99.) Early representatives of this
order, Mastodons, had appeared almost simultaneously (in the geological
sense of that word) in the upper Miocene of Europe and North America, but
in neither continent was any more ancient type known which could plausibly
be regarded as ancestral to them. Evidently, these problematical animals
had reached the northern continents by migrating from some other region,
but no one could say where that region lay. The Eocene and Oligocene beds
of the Fayoum show us that the region sought for is Africa, and that the
elephants form just such a series of gradual modifications as we have found
among other hoofed animals. The later steps of the transformation, by
which the mastodons lost their lower tusks, and their relatively small and
simple grinding teeth acquired the great size and highly complex structure
of the true elephants, may be followed in the uppermost Miocene and
Pliocene fossils of India and southern Europe.

Egypt has also of late furnished some very welcome material which
contributes to the solution of another unsolved problem which had quite
eluded research, the origin of the whales. The toothed-whales may be
traced back in several more or less parallel lines as far as the lower
Miocene, but their predecessors in the Oligocene are still so incompletely
known that safe conclusions can hardly be drawn from them. In the middle
Eocene of Egypt, however, has been found a small, whale-like animal
(Protocetus), which shows what the ancestral toothed-whale was like, and at
the same time seems to connect these thoroughly marine mammals with land-
animals. Though already entirely adapted to an aquatic mode of life, the
teeth, skull and backbone of Protocetus display so many differences from
those of the later whales and so many approximations to those of primitive,
carnivorous land-mammals, as, in a large degree, to bridge over the gap
between the two groups. Thus one of the most puzzling of palaeontological
questions is in a fair way to receive a satisfactory answer. The origin of
the whalebone-whales and their relations to the toothed-whales cannot yet
be determined, since the necessary fossils have not been discovered.

Among the carnivorous mammals, phylogenetic series are not so clear and
distinct as among the hoofed animals, chiefly because the carnivores are
individually much less abundant, and well-preserved skeletons are among the
prizes of the collector. Nevertheless, much has already been learned
concerning the mutual relations of the carnivorous families, and several
phylogenetic series, notably that of the dogs, are quite complete. It has
been made extremely probable that the primitive dogs of the Eocene
represent the central stock, from which nearly or quite all the other
families branched off, though the origin and descent of the cats have not
yet been determined.

It should be clearly understood that the foregoing account of mammalian
descent is merely a selection of a few representative cases and might be
almost indefinitely extended. Nothing has been said, for example, of the
wonderful museum of ancient mammalian life which is entombed in the rocks
of South America, especially of Patagonia, and which opens a world so
entirely different from that of the northern continents, yet exemplifying
the same laws of "descent with modification." Very beautiful phylogenetic
series have already been established among these most interesting and
marvellously preserved fossils, but lack of space forbids a consideration
of them.

The origin of the mammalia, as a class, offers a problem of which
palaeontology can as yet present no definitive solution. Many
morphologists regard the early amphibia as the ancestral group from which
the mammals were derived, while most palaeontologists believe that the
mammals are descended from the reptiles. The most ancient known mammals,
those from the upper Triassic of Europe and North America, are so extremely
rare and so very imperfectly known, that they give little help in
determining the descent of the class, but, on the other hand, certain
reptilian orders of the Permian period, especially well represented in
South Africa, display so many and such close approximations to mammalian
structure, as strongly to suggest a genetic relationship. It is difficult
to believe that all those likenesses should have been independently
acquired and are without phylogenetic significance.

Birds are comparatively rare as fossils and we should therefore look in
vain among them for any such long and closely knit series as the mammals
display in abundance. Nevertheless, a few extremely fortunate discoveries
have made it practically certain that birds are descended from reptiles, of
which they represent a highly specialised branch. The most ancient
representative of this class is the extraordinary genus Archaeopteryx from
the upper Jurassic of Bavaria, which, though an unmistakable bird, retains
so many reptilian structures and characteristics as to make its derivation
plain. Not to linger over anatomical minutiae, it may suffice to mention
the absence of a horny beak, which is replaced by numerous true teeth, and
the long lizard-like tail, which is made up of numerous distinct vertebrae,
each with a pair of quill-like feathers attached to it. Birds with teeth
are also found in the Cretaceous, though in most other respects the birds
of that period had attained a substantially modern structure. Concerning
the interrelations of the various orders and families of birds,
palaeontology has as yet little to tell us.

The life of the Mesozoic era was characterised by an astonishing number and
variety of reptiles, which were adapted to every mode of life, and
dominated the air, the sea and the land, and many of which were of colossal
proportions. Owing to the conditions of preservation which obtained during
the Mesozoic period, the history of the reptiles is a broken and
interrupted one, so that we can make out many short series, rather than any
one of considerable length. While the relations of several reptilian
orders can be satisfactorily determined, others still baffle us entirely,
making their first known appearance in a fully differentiated state. We
can trace the descent of the sea-dragons, the Ichthyosaurs and Plesiosaurs,
from terrestrial ancestors, but the most ancient turtles yet discovered
show us no closer approximation to any other order than do the recent
turtles; and the oldest known Pterosaurs, the flying dragons of the
Jurassic, are already fully differentiated. There is, however, no ground
for discouragement in this, for the progress of discovery has been so rapid
of late years, and our knowledge of Mesozoic life has increased with such
leaps and bounds, that there is every reason to expect a solution of many
of the outstanding problems in the near future.

Passing over the lower vertebrates, for lack of space to give them any
adequate consideration, we may briefly take up the record of invertebrate
life. From the overwhelming mass of material it is difficult to make a
representative selection and even more difficult to state the facts
intelligibly without the use of unduly technical language and without the
aid of illustrations.

Several groups of the Mollusca, or shell-fish, yield very full and
convincing evidence of their descent from earlier and simpler forms, and of
these none is of greater interest than the Ammonites, an extinct order of
the cephalopoda. The nearest living ally of the ammonites is the pearly
nautilus, the other existing cephalopods, such as the squids, cuttle-fish,
octopus, etc., are much more distantly related. Like the nautilus, the
ammonites all possess a coiled and chambered shell, but their especial
characteristic is the complexity of the "sutures." By sutures is meant the
edges of the transverse partitions, or septa, where these join the shell-
wall, and their complexity in the fully developed genera is extraordinary,
forming patterns like the most elaborate oak-leaf embroidery, while in the
nautiloids the sutures form simple curves. In the rocks of the Mesozoic
era, wherever conditions of preservation are favourable, these beautiful
shells are stored in countless multitudes, of an incredible variety of
form, size and ornamentation, as is shown by the fact that nearly 5000
species have already been described. The ammonites are particularly well
adapted for phylogenetic studies, because, by removing the successive
whorls of the coiled shell, the individual development may be followed back
in inverse order, to the microscopic "protoconch," or embryonic shell,
which lies concealed in the middle of the coil. Thus the valuable aid of
embryology is obtained in determining relationships.

The descent of the ammonites, taken as a group, is simple and clear; they
arose as a branch of the nautiloids in the lower Devonian, the shells known
as goniatites having zigzag, angulated sutures. Late in the succeeding
Carboniferous period appear shells with a truly ammonoid complexity of
sutures, and in the Permian their number and variety cause them to form a
striking element of the marine faunas. It is in the Mesozoic era, however,
that these shells attain their full development; increasing enormously in
the Triassic, they culminate in the Jurassic in the number of families,
genera and species, in the complexity of the sutures, and in the variety of
shell-ornamentation. A slow decline begins in the Cretaceous, ending in
the complete extinction of the whole group at the end of that period. As a
final phase in the history of the ammonites, there appear many so-called
"abnormal" genera, in which the shell is irregularly coiled, or more or
less uncoiled, in some forms becoming actually straight. It is interesting
to observe that some of these genera are not natural groups, but are
"polyphyletic," i.e. are each derived from several distinct ancestral
genera, which have undergone a similar kind of degeneration.

In the huge assembly of ammonites it is not yet possible to arrange all the
forms in a truly natural classification, which shall express the various
interrelations of the genera, yet several beautiful series have already
been determined. In these series the individual development of the later
general shows transitory stages which are permanent in antecedent genera.
To give a mere catalogue of names without figures would not make these
series more intelligible.

The Brachiopoda, or "lamp-shells," are a phylum of which comparatively few
survive to the present day; their shells have a superficial likeness to
those of the bivalved Mollusca, but are not homologous with the latter, and
the phylum is really very distinct from the molluscs. While greatly
reduced now, these animals were incredibly abundant throughout the
Palaeozoic era, great masses of limestone being often composed almost
exclusively of their shells, and their variety is in keeping with their
individual abundance. As in the case of the ammonites, the problem is to
arrange this great multitude of forms in an orderly array that shall
express the ramifications of the group according to a genetic system. For
many brachiopods, both recent and fossil, the individual development, or
ontogeny, has been worked out and has proved to be of great assistance in
the problems of classification and phylogeny. Already very encouraging
progress has been made in the solution of these problems. All brachiopods
form first a tiny, embryonic shell, called the protegulum, which is
believed to represent the ancestral form of the whole group, and in the
more advanced genera the developmental stages clearly indicate the
ancestral genera of the series, the succession of adult forms in time
corresponding to the order of the ontogenetic stages. The transformation
of the delicate calcareous supports of the arms, often exquisitely
preserved, are extremely interesting. Many of the Palaeozoic genera had
these supports coiled like a pair of spiral springs, and it has been shown
that these genera were derived from types in which the supports were simply
shelly loops.

The long extinct class of crustacea known as the Trilobites are likewise
very favourable subjects for phylogenetic studies. So far as the known
record can inform us, the trilobites are exclusively Palaeozoic in
distribution, but their course must have begun long before that era, as is
shown by the number of distinct types among the genera of the lower
Cambrian. The group reached the acme of abundance and relative importance
in the Cambrian and Ordovician; then followed a long, slow decline, ending
in complete and final disappearance before the end of the Permian. The
newly-hatched and tiny trilobite larva, known as the protaspis, is very
near to the primitive larval form of all the crustacea. By the aid of the
correlated ontogenetic stages and the succession of the adult forms in the
rocks, many phylogenetic series have been established and a basis for the
natural arrangement of the whole class has been laid.

Very instructive series may also be observed among the Echinoderms and,
what is very rare, we are able in this sub-kingdom to demonstrate the
derivation of one class from another. Indeed, there is much reason to
believe that the extinct class Cystidea of the Cambrian is the ancestral
group, from which all the other Echinoderms, star-fishes, brittle-stars,
sea-urchins, feather-stars, etc., are descended.

The foregoing sketch of the palaeontological record is, of necessity,
extremely meagre, and does not represent even an outline of the evidence,
but merely a few illustrative examples, selected almost at random from an
immense body of material. However, it will perhaps suffice to show that
the geological record is not so hopelessly incomplete as Darwin believed it
to be. Since "The Origin of Species" was written, our knowledge of that
record has been enormously extended and we now possess, no complete
volumes, it is true, but some remarkably full and illuminating chapters.
The main significance of the whole lies in the fact, that JUST IN

The test of a true, as distinguished from a false, theory is the manner in
which newly discovered and unanticipated facts arrange themselves under it.
No more striking illustration of this can be found than in the contrasted
fates of Cuvier's theory and of that of Darwin. Even before Cuvier's death
his views had been undermined and the progress of discovery soon laid them
in irreparable ruin, while the activity of half-a-century in many different
lines of inquiry has established the theory of evolution upon a foundation
of ever growing solidity. It is Darwin's imperishable glory that he
prescribed the lines along which all the biological sciences were to
advance to conquests not dreamed of when he wrote.



President of the Linnean Society.

There are several points of view from which the subject of the present
essay may be regarded. We may consider the fossil record of plants in its
bearing: I. on the truth of the doctrine of Evolution; II. on Phylogeny,
or the course of Evolution; III. on the theory of Natural Selection. The
remarks which follow, illustrating certain aspects only of an extensive
subject, may conveniently be grouped under these three headings.


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.


The question of phylogeny is really inseparable from that of the truth of
the doctrine of evolution, for we cannot have historical evidence that
evolution has actually taken place without at the same time having evidence
of the course it has followed.

As already pointed out, the progress hitherto made has been rather in the
way of joining up the great classes of plants than in tracing the descent
of particular species or genera of the recent flora. There appears to be a
difference in this respect from the Animal record, which tells us so much
about the descent of living species, such as the elephant or the horse.
The reason for this difference is no doubt to be found in the fact that the
later part of the palaeontological record is the most satisfactory in the
case of animals and the least so in the case of plants. The Tertiary
plant-remains, in the great majority of instances, are impressions of
leaves, the conclusions to be drawn from which are highly precarious; until
the whole subject of Angiospermous palaeobotany has been reinvestigated, it
would be rash to venture on any statements as to the descent of the
families of Dicotyledons or Monocotyledons.

Our attention will be concentrated on the following questions, all relating
to the phylogeny of main groups of plants: i. The Origin of the
Angiosperms. ii. The Origin of the Seed-plants. iii. The Origin of the
different classes of the Higher Cryptogamia.


The first of these questions has long been the great crux of botanical
phylogeny, and until quite recently no light had been thrown upon the
difficulty. The Angiosperms are the Flowering Plants, par excellence, and
form, beyond comparison, the dominant sub-kingdom in the flora of our own
age, including, apart from a few Conifers and Ferns, all the most familiar
plants of our fields and gardens, and practically all plants of service to
man. All recent work has tended to separate the Angiosperms more widely
from the other seed-plants now living, the Gymnosperms. Vast as is the
range of organisation presented by the great modern sub-kingdom, embracing
forms adapted to every environment, there is yet a marked uniformity in
certain points of structure, as in the development of the embryo-sac and
its contents, the pollination through the intervention of a stigma, the
strange phenomenon of double fertilisation (One sperm fertilising the egg,
while the other unites with the embryo-sac nucleus, itself the product of a
nuclear fusion, to give rise to a nutritive tissue, the endosperm.), the
structure of the stamens, and the arrangement of the parts of the flower.
All these points are common to Monocotyledons and Dicotyledons, and
separate the Angiosperms collectively from all other plants.

In geological history the Angiosperms first appear in the Lower Cretaceous,
and by Upper Cretaceous times had already swamped all other vegetation and
seized the dominant position which they still hold. Thus they are isolated
structurally from the rest of the Vegetable Kingdom, while historically
they suddenly appear, almost in full force, and apparently without
intermediaries with other groups. To quote Darwin's vigorous words: "The
rapid development, as far as we can judge, of all the higher plants within
recent geological times is an abominable mystery." ("More Letters of
Charles Darwin", Vol. II. page 20, letter to J.D. Hooker, 1879.) A couple
of years later he made a bold suggestion (which he only called an "idle
thought") to meet this difficulty. He says: "I have been so astonished at
the apparently sudden coming in of the higher phanerogams, that I have
sometimes fancied that development might have slowly gone on for an immense
period in some isolated continent or large island, perhaps near the South
Pole." (Ibid, page 26, letter to Hooker, 1881.) This idea of an
Angiospermous invasion from some lost southern land has sometimes been
revived since, but has not, so far as the writer is aware, been supported
by evidence. Light on the problem has come from a different direction.

The immense development of plants with the habit of Cycads, during the
Mesozoic Period up to the Lower Cretaceous, has long been known. The
existing Order Cycadaceae is a small family, with 9 genera and perhaps 100
species, occurring in the tropical and sub-tropical zones of both the Old
and New World, but nowhere forming a dominant feature in the vegetation.
Some few attain the stature of small trees, while in the majority the stem
is short, though often living to a great age. The large pinnate or rarely
bipinnate leaves give the Cycads a superficial resemblance in habit to
Palms. Recent Cycads are dioecious; throughout the family the male
fructification is in the form of a cone, each scale of the cone
representing a stamen, and bearing on its lower surface numerous pollen-
sacs, grouped in sori like the sporangia of Ferns. In all the genera,
except Cycas itself, the female fructifications are likewise cones, each
carpel bearing two ovules on its margin. In Cycas, however, no female cone
is produced, but the leaf-like carpels, bearing from two to six ovules
each, are borne directly on the main stem of the plant in rosettes
alternating with those of the ordinary leaves--the most primitive
arrangement known in any living seed-plant. The whole Order is relatively
primitive, as shown most strikingly in its cryptogamic mode of
fertilisation, by means of spermatozoids, which it shares with the
maidenhair tree alone, among recent seed-plants.

In all the older Mesozoic rocks, from the Trias to the Lower Cretaceous,
plants of the Cycad class (Cycadophyta, to use Nathorst's comprehensive
name) are extraordinarily abundant in all parts of the world; in fact they
were almost as prominent in the flora of those ages as the Dicotyledons are
in that of our own day. In habit and to a great extent in anatomy, the
Mesozoic Cycadophyta for the most part much resemble the recent Cycadaceae.
But, strange to say, it is only in the rarest cases that the fructification
has proved to be of the simple type characteristic of the recent family;
the vast majority of the abundant fertile specimens yielded by the Mesozoic
rocks possess a type of reproductive apparatus far more elaborate than
anything known in Cycadaceae or other Gymnosperms. The predominant
Mesozoic family, characterised by this advanced reproductive organisation,
is known as the Bennettiteae; in habit these plants resembled the more
stunted Cycads of the recent flora, but differed from them in the presence
of numerous lateral fructifications, like large buds, borne on the stem
among the crowded bases of the leaves. The organisation of these
fructifications was first worked out on European specimens by Carruthers,
Solms-Laubach, Lignier and others, but these observers had only more or
less ripe fruits to deal with; the complete structure of the flower has
only been elucidated within the last few years by the researches of Wieland
on the magnificent American material, derived from the Upper Jurassic and
Lower Cretaceous beds of Maryland, Dakota and Wyoming. (G.R. Wieland,
"American Fossil Cycads", Carnegie Institution, Washington, 1906.) The
word "flower" is used deliberately, for reasons which will be apparent from
the following brief description, based on Wieland's observations.

The fructification is attached to the stem by a thick stalk, which, in its
upper part, bears a large number of spirally arranged bracts, forming
collectively a kind of perianth and completely enclosing the essential
organs of reproduction. The latter consist of a whorl of stamens, of
extremely elaborate structure, surrounding a central cone or receptacle
bearing numerous ovules. The stamens resemble the fertile fronds of a
fern; they are of a compound, pinnate form, and bear very large numbers of
pollen-sacs, each of which is itself a compound structure consisting of a
number of compartments in which the pollen was formed. In their lower part
the stamens are fused together by their stalks, like the "monadelphous"
stamens of a mallow. The numerous ovules borne on the central receptacle
are stalked, and are intermixed with sterile scales; the latter are
expanded at their outer ends, which are united to form a kind of pericarp
or ovary-wall, only interrupted by the protruding micropyles of the ovules.
There is thus an approach to the closed pistil of an Angiosperm, but it is
evident that the ovules received the pollen directly. The whole
fructification is of large size; in the case of Cycadeoidea dacotensis, one
of the species investigated by Wieland, the total length, in the bud
condition, is about 12 cm., half of which belongs to the peduncle.

The general arrangement of the organs is manifestly the same as in a
typical Angiospermous flower, with a central pistil, a surrounding whorl of
stamens and an enveloping perianth; there is, as we have seen, some
approach to the closed ovary of an Angiosperm; another point, first
discovered nearly 20 years ago by Solms-Laubach in his investigation of a
British species, is that the seed was practically "exalbuminous," its
cavity being filled by the large, dicotyledonous embryo, whereas in all
known Gymnosperms a large part of the sac is occupied by a nutritive
tissue, the prothallus or endosperm; here also we have a condition only met
with elsewhere among the higher Flowering Plants.

Taking all the characters into account, the indications of affinity between
the Mesozoic Cycadophyta and the Angiosperms appear extremely significant,
as was recognised by Wieland when he first discovered the hermaphrodite
nature of the Bennettitean flower. The Angiosperm with which he specially
compared the fossil type was the Tulip tree (Liriodendron) and certainly
there is a remarkable analogy with the Magnoliaceous flowers, and with
those of related orders such as Ranunculaceae and the Water-lilies. It
cannot, of course, be maintained that the Bennettiteae, or any other
Mesozoic Cycadophyta at present known, were on the direct line of descent
of the Angiosperms, for there are some important points of difference, as,
for example, in the great complexity of the stamens, and in the fact that
the ovary-wall or pericarp was not formed by the carpels themselves, but by
the accompanying sterile scale-leaves. Botanists, since the discovery of
the bisexual flowers of the Bennettiteae, have expressed different views as
to the nearness of their relation to the higher Flowering Plants, but the
points of agreement are so many that it is difficult to resist the
conviction that a real relation exists, and that the ancestry of the
Angiosperms, so long shrouded in complete obscurity, is to be sought among
the great plexus of Cycad-like plants which dominated the flora of the
world in Mesozoic times. (On this subject see, in addition to Wieland's
great work above cited, F.W. Oliver, "Pteridosperms and Angiosperms", "New
Phytologist", Vol. V. 1906; D.H. Scott, "The Flowering Plants of the
Mesozoic Age in the Light of Recent Discoveries", "Journal R. Microscop.
Soc." 1907, and especially E.A.N. Arber and J. Parkin, "On the Origin of
Angiosperms", "Journal Linn. Soc." (Bot.) Vol. XXXVIII. page 29, 1907.)

The great complexity of the Bennettitean flower, the earliest known
fructification to which the word "flower" can be applied without forcing
the sense, renders it probable, as Wieland and others have pointed out,
that the evolution of the flower in Angiosperms has consisted essentially
in a process of reduction, and that the simplest forms of flower are not to
be regarded as the most primitive. The older morphologists generally took
the view that such simple flowers were to be explained as reductions from a
more perfect type, and this opinion, though abandoned by many later
writers, appears likely to be true when we consider the elaboration of
floral structure attained among the Mesozoic Cycadophyta, which preceded
the Angiosperms in evolution.

If, as now seems probable, the Angiosperms were derived from ancestors
allied to the Cycads, it would naturally follow that the Dicotyledons were
first evolved, for their structure has most in common with that of the
Cycadophyta. We should then have to regard the Monocotyledons as a side-
line, diverging probably at a very early stage from the main dicotyledonous
stock, a view which many botanists have maintained, of late, on other
grounds. (See especially Ethel Sargant, "The Reconstruction of a Race of
Primitive Angiosperms", "Annals of Botany", Vol. XXII. page 121, 1908.) So
far, however, as the palaeontological record shows, the Monocotyledons were
little if at all later in their appearance than the Dicotyledons, though
always subordinate in numbers. The typical and beautifully preserved Palm-
wood from Cretaceous rocks is striking evidence of the early evolution of a
characteristic monocotyledonous family. It must be admitted that the whole
question of the evolution of Monocotyledons remains to be solved.

Accepting, provisionally, the theory of the cycadophytic origin of
Angiosperms, it is interesting to see to what further conclusions we are
led. The Bennettiteae, at any rate, were still at the gymnospermous level
as regards their pollination, for the exposed micropyles of the ovules were
in a position to receive the pollen directly, without the intervention of a
stigma. It is thus indicated that the Angiosperms sprang from a
gymnospermous source, and that the two great phyla of Seed-plants have not
been distinct from the first, though no doubt the great majority of known
Gymnosperms, especially the Coniferae, represent branch-lines of their own.

The stamens of the Bennettiteae are arranged precisely as in an
angiospermous flower, but in form and structure they are like the fertile
fronds of a Fern, in fact the compound pollen-sacs, or synangia as they are
technically called, almost exactly agree with the spore-sacs of a
particular family of Ferns--the Marattiaceae, a limited group, now mainly
tropical, which was probably more prominent in the later Palaeozoic times
than at present. The scaly hairs, or ramenta, which clothe every part of
the plant, are also like those of Ferns.

It is not likely that the characters in which the Bennettiteae resemble the
Ferns came to them directly from ancestors belonging to that class; an
extensive group of Seed-plants, the Pteridospermeae, existed in Palaeozoic
times and bear evident marks of affinity with the Fern phylum. The fern-
like characters so remarkably persistent in the highly organised
Cycadophyta of the Mesozoic were in all likelihood derived through the
Pteridosperms, plants which show an unmistakable approach to the
cycadophytic type.

The family Bennettiteae thus presents an extraordinary association of
characters, exhibiting, side by side, features which belong to the
Angiosperms, the Gymnosperms and the Ferns.


The general relation of the gymnospermous Seed-plants to the Higher
Cryptogamia was cleared up, independently of fossil evidence, by the
brilliant researches of Hofmeister, dating from the middle of the past
century. (W. Hofmeister, "On the Germination, Development and
Fructification of the Higher Cryptogamia", Ray Society, London, 1862. The
original German treatise appeared in 1851.) He showed that "the embryo-sac
of the Coniferae may be looked upon as a spore remaining enclosed in its
sporangium; the prothallium which it forms does not come to the light."
(Ibid. page 438.) He thus determined the homologies on the female side.
Recognising, as some previous observers had already done, that the
microspores of those Cryptogams in which two kinds of spore are developed,
are equivalent to the pollen-grains of the higher plants, he further
pointed out that fertilisation "in the Rhizocarpeae and Selaginellae takes
place by free spermatozoa, and in the Coniferae by a pollen-tube, in the
interior of which spermatozoa are probably formed"--a remarkable instance
of prescience, for though spermatozoids have not been found in the Conifers
proper, they were demonstrated in the allied groups Cycadaceae and Ginkgo,
in 1896, by the Japanese botanists Ikeno and Hirase. A new link was thus
established between the Gymnosperms and the Cryptogams.

It remained uncertain, however, from which line of Cryptogams the
gymnospermous Seed-plants had sprung. The great point of morphological
comparison was the presence of two kinds of spore, and this was known to
occur in the recent Lycopods and Water-ferns (Rhizocarpeae) and was also
found in fossil representatives of the third phylum, that of the
Horsetails. As a matter of fact all the three great Cryptogamic classes
have found champions to maintain their claim to the ancestry of the Seed-
plants, and in every case fossil evidence was called in. For a long time
the Lycopods were the favourites, while the Ferns found the least support.
The writer remembers, however, in the year 1881, hearing the late Prof.
Sachs maintain, in a lecture to his class, that the descent of the Cycads
could be traced, not merely from Ferns, but from a definite family of
Ferns, the Marattiaceae, a view which, though in a somewhat crude form,
anticipated more modern ideas.

Williamson appears to have been the first to recognise the presence, in the
Carboniferous flora, of plants combining the characters of Ferns and
Cycads. (See especially his "Organisation of the Fossil Plants of the
Coal-Measures", Part XIII. "Phil. Trans. Royal Soc." 1887 B. page 299.)
This conclusion was first reached in the case of the genera Heterangium and
Lyginodendron, plants, which with a wholly fern-like habit, were found to
unite an anatomical structure holding the balance between that of Ferns and
Cycads, Heterangium inclining more to the former and Lyginodendron to the
latter. Later researches placed Williamson's original suggestion on a
firmer basis, and clearly proved the intermediate nature of these genera,
and of a number of others, so far as their vegetative organs were
concerned. This stage in our knowledge was marked by the institution of
the class Cycadofilices by Potonie in 1897.

Nothing, however, was known of the organs of reproduction of the
Cycadofilices, until F.W. Oliver, in 1903, identified a fossil seed,
Lagenostoma Lomaxi, as belonging to Lyginodendron, the identification
depending, in the first instance, on the recognition of an identical form
of gland, of very characteristic structure, on the vegetative organs of
Lyginodendron and on the cupule enveloping the seed. This evidence was
supported by the discovery of a close anatomical agreement in other
respects, as well as by constant association between the seed and the
plant. (F.W. Oliver and D.H. Scott, "On the Structure of the Palaeozoic
Seed, Lagenostoma Lomaxi, etc." "Phil. Trans. Royal Soc." Vol. 197 B.
1904.) The structure of the seed of Lyginodendron, proved to be of the
same general type as that of the Cycads, as shown especially by the
presence of a pollen-chamber or special cavity for the reception of the
pollen-grains, an organ only known in the Cycads and Ginkgo among recent

Within a few months after the discovery of the seed of Lyginodendron,
Kidston found the large, nut-like seed of a Neuropteris, another fern-like
Carboniferous plant, in actual connection with the pinnules of the frond,
and since then seeds have been observed on the frond in species of
Aneimites and Pecopteris, and a vast body of evidence, direct or indirect,
has accumulated, showing that a large proportion of the Palaeozoic plants
formerly classed as Ferns were in reality reproduced by seeds of the same
type as those of recent Cycadaceae. (A summary of the evidence will be
found in the writer's article "On the present position of Palaeozoic
Botany", "Progressus Rei Botanicae", 1907, page 139, and "Studies in Fossil
Botany", Vol. II. (2nd edition) London, 1909.) At the same time, the
anatomical structure, where it is open to investigation, confirms the
suggestion given by the habit, and shows that these early seed-bearing
plants had a real affinity with Ferns. This conclusion received strong
corroboration when Kidston, in 1905, discovered the male organs of
Lyginodendron, and showed that they were identical with a fructification of
the genus Crossotheca, hitherto regarded as belonging to Marattiaceous
Ferns. (Kidston, "On the Microsporangia of the Pteridospermeae, etc."
"Phil. Trans. Royal Soc." Vol. 198, B. 1906.)

The general conclusion which follows from the various observations alluded
to, is that in Palaeozoic times there was a great body of plants
(including, as it appears, a large majority of the fossils previously
regarded as Ferns) which had attained the rank of Spermophyta, bearing
seeds of a Cycadean type on fronds scarcely differing from the vegetative
foliage, and in other respects, namely anatomy, habit and the structure of
the pollen-bearing organs, retaining many of the characters of Ferns. From
this extensive class of plants, to which the name Pteridospermeae has been
given, it can scarcely be doubted that the abundant Cycadophyta, of the
succeeding Mesozoic period, were derived. This conclusion is of far-
reaching significance, for we have already found reason to think that the
Angiosperms themselves sprang, in later times, from the Cycadophytic stock;
it thus appears that the Fern-phylum, taken in a broad sense, ultimately
represents the source from which the main line of descent of the
Phanerogams took its rise.

It must further be borne in mind that in the Palaeozoic period there
existed another group of seed-bearing plants, the Cordaiteae, far more
advanced than the Pteridospermeae, and in many respects approaching the
Coniferae, which themselves begin to appear in the latest Palaeozoic rocks.
The Cordaiteae, while wholly different in habit from the contemporary fern-
like Seed-plants, show unmistakable signs of a common origin with them.
Not only is there a whole series of forms connecting the anatomical
structure of the Cordaiteae with that of the Lyginodendreae among
Pteridosperms, but a still more important point is that the seeds of the
Cordaiteae, which have long been known, are of the same Cycadean type as
those of the Pteridosperms, so that it is not always possible, as yet, to
discriminate between the seeds of the two groups. These facts indicate
that the same fern-like stock which gave rise to the Cycadophyta and
through them, as appears probable, to the Angiosperms, was also the source
of the Cordaiteae, which in their turn show manifest affinity with some at
least of the Coniferae. Unless the latter are an artificial group, a view
which does not commend itself to the writer, it would appear probable that
the Gymnosperms generally, as well as the Angiosperms, were derived from an
ancient race of Cryptogams, most nearly related to the Ferns. (Some
botanists, however, believe that the Coniferae, or some of them, are
probably more nearly related to the Lycopods. See Seward and Ford, "The
Araucarieae, Recent and Extinct", "Phil. Trans. Royal Soc." Vol. 198 B.

It may be mentioned here that the small gymnospermous group Gnetales
(including the extraordinary West African plant Welwitschia) which were
formerly regarded by some authorities as akin to the Equisetales, have
recently been referred, on better grounds, to a common origin with the
Angiosperms, from the Mesozoic Cycadophyta.

The tendency, therefore, of modern work on the palaeontological record of
the Seed-plants has been to exalt the importance of the Fern-phylum, which,
on present evidence, appears to be that from which the great majority,
possibly the whole, of the Spermophyta have been derived.

One word of caution, however, is necessary. The Seed-plants are of
enormous antiquity; both the Pteridosperms and the more highly organised
family Cordaiteae, go back as far in geological history (namely to the
Devonian) as the Ferns themselves or any other Vascular Cryptogams. It
must therefore be understood that in speaking of the derivation of the
Spermophyta from the Fern-phylum, we refer to that phylum at a very early
stage, probably earlier than the most ancient period to which our record of
land-plants extends. The affinity between the oldest Seed-plants and the
Ferns, in the widest sense, seems established, but the common stock from
which they actually arose is still unknown; though no doubt nearer to the
Ferns than to any other group, it must have differed widely from the Ferns
as we now know them, or perhaps even from any which the fossil record has
yet revealed to us.


The Sub-kingdom of the higher Spore-plants, the Cryptogamia possessing a
vascular system, was more prominent in early geological periods than at
present. It is true that the dominance of the Pteridophyta in Palaeozoic
times has been much exaggerated owing to the assumption that everything
which looked like a Fern really was a Fern. But, allowing for the fact,
now established, that most of the Palaeozoic fern-like plants were already
Spermophyta, there remains a vast mass of Cryptogamic forms of that period,
and the familiar statement that they formed the main constituent of the
Coal-forests still holds good. The three classes, Ferns (Filicales),
Horsetails (Equisetales) and Club-mosses (Lycopodiales), under which we now
group the Vascular Cryptogams, all extend back in geological history as far
as we have any record of the flora of the land; in the Palaeozoic, however,
a fourth class, the Sphenophyllales, was present.

As regards the early history of the Ferns, which are of special interest
from their relation to the Seed-plants, it is impossible to speak quite
positively, owing to the difficulty of discriminating between true fossil
Ferns and the Pteridosperms which so closely simulated them. The
difficulty especially affects the question of the position of Marattiaceous
Ferns in the Palaeozoic Floras. This family, now so restricted, was until
recently believed to have been one of the most important groups of
Palaeozoic plants, especially during later Carboniferous and Permian times.
Evidence both from anatomy and from sporangial characters appeared to
establish this conclusion. Of late, however, doubts have arisen, owing to
the discovery that some supposed members of the Marattiaceae bore seeds,
and that a form of fructification previously referred to that family
(Crossotheca) was really the pollen-bearing apparatus of a Pteridosperm
(Lyginodendron). The question presents much difficulty; though it seems
certain that our ideas of the extent of the family in Palaeozoic times will
have to be restricted, there is still a decided balance of evidence in
favour of the view that a considerable body of Marattiaceous Ferns actually
existed. The plants in question were of large size (often arborescent) and
highly organised--they represent, in fact, one of the highest developments
of the Fern-stock, rather than a primitive type of the class.

There was, however, in the Palaeozoic period, a considerable group of
comparatively simple Ferns (for which Arber has proposed the collective
name Primofilices); the best known of these are referred to the family
Botryopterideae, consisting of plants of small or moderate dimensions,
with, on the whole, a simple anatomical structure, in certain cases
actually simpler than that of any recent Ferns. On the other hand the
sporangia of these plants were usually borne on special fertile fronds, a
mark of rather high differentiation. This group goes back to the Devonian
and includes some of the earliest types of Fern with which we are
acquainted. It is probable that the Primofilices (though not the
particular family Botryopterideae) represent the stock from which the
various families of modern Ferns, already developed in the Mesozoic period,
may have sprung.

None of the early Ferns show any clear approach to other classes of
Vascular Cryptogams; so far as the fossil record affords any evidence,
Ferns have always been plants with relatively large and usually compound
leaves. There is no indication of their derivation from a microphyllous
ancestry, though, as we shall see, there is some slight evidence for the
converse hypothesis. Whatever the origin of the Ferns may have been it is
hidden in the older rocks.

It has, however, been held that certain other Cryptogamic phyla had a
common origin with the Ferns. The Equisetales are at present a well-
defined group; even in the rich Palaeozoic floras the habit, anatomy and
reproductive characters usually render the members of this class
unmistakable, in spite of the great development and stature which they then
attained. It is interesting, however, to find that in the oldest known
representatives of the Equisetales the leaves were highly developed and
dichotomously divided, thus differing greatly from the mere scale-leaves of
the recent Horsetails, or even from the simple linear leaves of the later
Calamites. The early members of the class, in their forked leaves, and in
anatomical characters, show an approximation to the Sphenophyllales, which
are chiefly represented by the large genus Sphenophyllum, ranging through
the Palaeozoic from the Middle Devonian onwards. These were plants with
rather slender, ribbed stems, bearing whorls of wedge-shaped or deeply
forked leaves, six being the typical number in each whorl. From their weak
habit it has been conjectured, with much probability, that they may have
been climbing plants, like the scrambling Bedstraws of our hedgerows. The
anatomy of the stem is simple and root-like; the cones are remarkable for
the fact that each scale or sporophyll is a double structure, consisting of
a lower, usually sterile lobe and one or more upper lobes bearing the
sporangia; in one species both parts of the sporophyll were fertile.
Sphenophyllum was evidently much specialised; the only other known genus is
based on an isolated cone, Cheirostrobus, of Lower Carboniferous age, with
an extraordinarily complex structure. In this genus especially, but also
in the entire group, there is an evident relation to the Equisetales; hence
it is of great interest that Nathorst has described, from the Devonian of
Bear Island in the Arctic regions, a new genus Pseudobornia, consisting of
large plants, remarkable for their highly compound leaves which, when found
detached, were taken for the fronds of a Fern. The whorled arrangement of
the leaves, and the habit of the plant, suggest affinities either with the
Equisetales or the Sphenophyllales; Nathorst makes the genus the type of a
new class, the Pseudoborniales. (A.G. Nathorst, "Zur Oberdevonischen Flora
der Baren-Insel", "Kongl. Svenska Vetenskaps-Akademiens Handlingar" Bd. 36,
No. 3, Stockholm, 1902.)

The available data, though still very fragmentary, certainly suggest that
both Equisetales and Sphenophyllales may have sprung from a common stock
having certain fern-like characters. On the other hand the Sphenophylls,
and especially the peculiar genus Cheirostrobus, have in their anatomy a
good deal in common with the Lycopods, and of late years they have been
regarded as the derivatives of a stock common to that class and the
Equisetales. At any rate the characters of the Sphenophyllales and of the
new group Pseudoborniales suggest the existence, at a very early period, of
a synthetic race of plants, combining the characters of various phyla of
the Vascular Cryptogams. It may further be mentioned that the Psilotaceae,
an isolated epiphytic family hitherto referred to the Lycopods, have been
regarded by several recent authors as the last survivors of the
Sphenophyllales, which they resemble both in their anatomy and in the
position of their sporangia.

The Lycopods, so far as their early history is known, are remarkable rather
for their high development in Palaeozoic times than for any indications of
a more primitive ancestry. In the recent Flora, two of the four living
genera (Excluding Psilotaceae.) (Selaginella and Isoetes) have spores of
two kinds, while the other two (Lycopodium and Phylloglossum) are
homosporous. Curiously enough, no certain instance of a homosporous
Palaeozoic Lycopod has yet been discovered, though well-preserved
fructifications are numerous. Wherever the facts have been definitely
ascertained, we find two kinds of spore, differentiated quite as sharply as
in any living members of the group. Some of the Palaeozoic Lycopods, in
fact, went further, and produced bodies of the nature of seeds, some of
which were actually regarded, for many years, as the seeds of Gymnosperms.
This specially advanced form of fructification goes back at least as far as
the Lower Carboniferous, while the oldest known genus of Lycopods,
Bothrodendron, which is found in the Devonian, though not seed-bearing, was
typically heterosporous, if we may judge from the Coal-measure species. No
doubt homosporous Lycopods existed, but the great prevalence of the higher
mode of reproduction in days which to us appear ancient, shows how long a
course of evolution must have already been passed through before the oldest
known members of the group came into being. The other characters of the
Palaeozoic Lycopods tell the same tale; most of them attained the stature
of trees, with a corresponding elaboration of anatomical structure, and
even the herbaceous forms show no special simplicity. It appears from
recent work that herbaceous Lycopods, indistinguishable from our recent
Selaginellas, already existed in the time of the Coal-measures, while one
herbaceous form (Miadesmia) is known to have borne seeds.

The utmost that can be said for primitiveness of character in Palaeozoic
Lycopods is that the anatomy of the stem, in its primary ground-plan, as
distinguished from its secondary growth, was simpler than that of most
Lycopodiums and Selaginellas at the present day. There are also some
peculiarities in the underground organs (Stigmaria) which suggest the
possibility of a somewhat imperfect differentiation between root and stem,
but precisely parallel difficulties are met with in the case of the living
Selaginellas, and in some degree in species of Lycopodium.

In spite of their high development in past ages the Lycopods, recent and
fossil, constitute, on the whole, a homogeneous group, and there is little
at present to connect them with other phyla. Anatomically some relation to
the Sphenophylls is indicated, and perhaps the recent Psilotaceae give some
support to this connection, for while their nearest alliance appears to be
with the Sphenophylls, they approach the Lycopods in anatomy, habit, and
mode of branching.

The typically microphyllous character of the Lycopods, and the simple
relation between sporangium and sporophyll which obtains throughout the
class, have led various botanists to regard them as the most primitive
phylum of the Vascular Cryptogams. There is nothing in the fossil record
to disprove this view, but neither is there anything to support it, for
this class so far as we know is no more ancient than the megaphyllous
Cryptogams, and its earliest representatives show no special simplicity.
If the indications of affinity with Sphenophylls are of any value the
Lycopods are open to suspicion of reduction from a megaphyllous ancestry,
but there is no direct palaeontological evidence for such a history.

The general conclusions to which we are led by a consideration of the
fossil record of the Vascular Cryptogams are still very hypothetical, but
may be provisionally stated as follows:

The Ferns go back to the earliest known period. In Mesozoic times
practically all the existing families had appeared; in the Palaeozoic the
class was less extensive than formerly believed, a majority of the supposed
Ferns of that age having proved to be seed-bearing plants. The oldest
authentic representatives of the Ferns were megaphyllous plants, broadly
speaking, of the same type as those of later epochs, though differing much
in detail. As far back as the record extends they show no sign of becoming
merged with other phyla in any synthetic group.

The Equisetales likewise have a long history, and manifestly attained their
greatest development in Palaeozoic times. Their oldest forms show an
approach to the extinct class Sphenophyllales, which connects them to some
extent, by anatomical characters, with the Lycopods. At the same time the
oldest Equisetales show a somewhat megaphyllous character, which was more
marked in the Devonian Pseudoborniales. Some remote affinity with the
Ferns (which has also been upheld on other grounds) may thus be indicated.
It is possible that in the Sphenophyllales we may have the much-modified
representatives of a very ancient synthetic group.

The Lycopods likewise attained their maximum in the Palaeozoic, and show,
on the whole, a greater elaboration of structure in their early forms than
at any later period, while at the same time maintaining a considerable
degree of uniformity in morphological characters throughout their history.
The Sphenophyllales are the only other class with which they show any
relation; if such a connection existed, the common point of origin must lie
exceedingly far back.

The fossil record, as at present known, cannot, in the nature of things,
throw any direct light on what is perhaps the most disputed question in the
morphology of plants--the origin of the alternating generations of the
higher Cryptogams and the Spermophyta. At the earliest period to which
terrestrial plants have been traced back all the groups of Vascular
Cryptogams were in a highly advanced stage of evolution, while innumerable
Seed-plants--presumably the descendants of Cryptogamic ancestors--were
already flourishing. On the other hand we know practically nothing of
Palaeozoic Bryophyta, and the evidence even for their existence at that
period cannot be termed conclusive. While there are thus no
palaeontological grounds for the hypothesis that the Vascular plants came
of a Bryophytic stock, the question of their actual origin remains


Hitherto we have considered the palaeontological record of plants in
relation to Evolution. The question remains, whether the record throws any
light on the theory of which Darwin and Wallace were the authors--that of
Natural Selection. The subject is clearly one which must be investigated
by other methods than those of the palaeontologist; still there are certain
important points involved, on which the palaeontological record appears to

One of these points is the supposed distinction between morphological and
adaptive characters, on which Nageli, in particular, laid so much stress.
The question is a difficult one; it was discussed by Darwin ("Origin of
Species" (6th edition), pages 170-176.), who, while showing that the
apparent distinction is in part to be explained by our imperfect knowledge
of function, recognised the existence of important morphological characters
which are not adaptations. The following passage expresses his conclusion.
"Thus, as I am inclined to believe, morphological differences, which we
consider as important--such as the arrangement of the leaves, the divisions
of the flower or of the ovarium, the position of the ovules, etc.--first
appeared in many cases as fluctuating variations, which sooner or later
became constant through the nature of the organism and of the surrounding
conditions, as well as through the inter-crossing of distinct individuals,
but not through natural selection; for as these morphological characters do
not affect the welfare of the species, any slight deviations in them could
not have been governed or accumulated through this latter agency." (Ibid.
page 176.)

This is a sufficiently liberal concession; Nageli, however, went much
further when he said: "I do not know among plants a morphological
modification which can be explained on utilitarian principles." (See "More
Letters", Vol. II. page 375 (footnote).) If this were true the field of
Natural Selection would be so seriously restricted, as to leave the theory
only a very limited importance.

It can be shown, as the writer believes, that many typical "morphological
characters," on which the distinction between great classes of plants is
based, were adaptive in origin, and even that their constancy is due to
their functional importance. Only one or two cases will be mentioned,
where the fossil evidence affects the question.

The pollen-tube is one of the most important morphological characters of
the Spermophyta as now existing--in fact the name Siphonogama is used by
Engler in his classification, as expressing a peculiarly constant character
of the Seed-plants. Yet the pollen-tube is a manifest adaptation,
following on the adoption of the seed-habit, and serving first to bring the
spermatozoids with greater precision to their goal, and ultimately to
relieve them of the necessity for independent movement. The pollen-tube is
constant because it has proved to be indispensable.

In the Palaeozoic Seed-plants there are a number of instances in which the
pollen-grains, contained in the pollen-chamber of a seed, are so
beautifully preserved that the presence of a group of cells within the
grain can be demonstrated; sometimes we can even see how the cell-walls
broke down to emit the sperms, and quite lately it is said that the sperms
themselves have been recognised. (F.W. Oliver, "On Physostoma elegans, an
archaic type of seed from the Palaeozoic Rocks", "Annals of Botany",
January, 1909. See also the earlier papers there cited.) In no case,
however, is there as yet any satisfactory evidence for the formation of a
pollen-tube; it is probable that in these early Seed-plants the pollen-
grains remained at about the evolutionary level of the microspores in
Pilularia or Selaginella, and discharged their spermatozoids directly,
leaving them to find their own way to the female cells. It thus appears
that there were once Spermophyta without pollen-tubes. The pollen-tube
method ultimately prevailed, becoming a constant "morphological character,"
for no other reason than because, under the new conditions, it provided a
more perfect mechanism for the accomplishment of the act of fertilisation.
We have still, in the Cycads and Ginkgo, the transitional case, where the
tube remains short, serves mainly as an anchor and water-reservoir, but yet
is able, by its slight growth, to give the spermatozoids a "lift" in the
right direction. In other Seed-plants the sperms are mere passengers,
carried all the way by the pollen-tube; this fact has alone rendered the
Angiospermous method of fertilisation through a stigma possible.

We may next take the seed itself--the very type of a morphological
character. Our fossil record does not go far enough back to tell us the
origin of the seed in the Cycadophyta and Pteridosperms (the main line of
its development) but some interesting sidelights may be obtained from the
Lycopod phylum. In two Palaeozoic genera, as we have seen, seed-like
organs are known to have been developed, resembling true seeds in the
presence of an integument and of a single functional embryo-sac, as well as
in some other points. We will call these organs "seeds" for the sake of
shortness. In one genus (Lepidocarpon) the seeds were borne on a cone
indistinguishable from that of the ordinary cryptogamic Lepidodendreae, the
typical Lycopods of the period, while the seed itself retained much of the
detailed structure of the sporangium of that family. In the second genus,
Miadesmia, the seed-bearing plant was herbaceous, and much like a recent
Selaginella. (See Margaret Benson, "Miadesmia membranacea, a new
Palaeozoic Lycopod with a seed-like structure", "Phil. Trans. Royal Soc.
Vol. 199, B. 1908.) The seeds of the two genera are differently
constructed, and evidently had an independent origin. Here, then, we have
seeds arising casually, as it were, at different points among plants which
otherwise retain all the characters of their cryptogamic fellows; the seed
is not yet a morphological character of importance. To suppose that in
these isolated cases the seed sprang into being in obedience to a Law of
Advance ("Vervollkommungsprincip"), from which other contemporary Lycopods
were exempt, involves us in unnecessary mysticism. On the other hand it is
not difficult to see how these seeds may have arisen, as adaptive
structures, under the influence of Natural Selection. The seed-like
structure afforded protection to the prothallus, and may have enabled the
embryo to be launched on the world in greater security. There was further,
as we may suppose, a gain in certainty of fertilisation. As the writer has
pointed out elsewhere, the chances against the necessary association of the
small male with the large female spores must have been enormously great
when the cones were borne high up on tall trees. The same difficulty may
have existed in the case of the herbaceous Miadesmia, if, as Miss Benson
conjectures, it was an epiphyte. One way of solving the problem was for
pollination to take place while the megaspore was still on the parent
plant, and this is just what the formation of an ovule or seed was likely
to secure.

The seeds of the Pteridosperms, unlike those of the Lycopod stock, have not
yet been found in statu nascendi--in all known cases they were already
highly developed organs and far removed from the cryptogamic sporangium.
But in two respects we find that these seeds, or some of them, had not yet
realised their possibilities. In the seed of Lyginodendron and other cases
the micropyle, or orifice of the integument, was not the passage through
which the pollen entered; the open neck of the pollen-chamber protruded
through the micropyle and itself received the pollen. We have met with an
analogous case, at a more advanced stage of evolution, in the Bennettiteae,
where the wall of the gynaecium, though otherwise closed, did not provide a
stigma to catch the pollen, but allowed the micropyles of the ovules to
protrude and receive the pollen in the old gymnospermous fashion. The
integument in the one case and the pistil in the other had not yet assumed
all the functions to which the organ ultimately became adapted. Again, no
Palaeozoic seed has yet been found to contain an embryo, though the
preservation is often good enough for it to have been recognised if
present. It is probable that the nursing of the embryo had not yet come to
be one of the functions of the seed, and that the whole embryonic
development was relegated to the germination stage.

In these two points, the reception of the pollen by the micropyle and the
nursing of the embryo, it appears that many Palaeozoic seeds were
imperfect, as compared with the typical seeds of later times. As evolution
went on, one function was superadded on another, and it appears impossible
to resist the conclusion that the whole differentiation of the seed was a
process of adaptation, and consequently governed by Natural Selection, just
as much as the specialisation of the rostellum in an Orchid, or of the
pappus in a Composite.

Did space allow, other examples might be added. We may venture to maintain
that the glimpses which the fossil record allows us into early stages in
the evolution of organs now of high systematic importance, by no means
justify the belief in any essential distinction between morphological and
adaptive characters.

Another point, closely connected with Darwin's theory, on which the fossil
history of plants has been supposed to have some bearing, is the question
of Mutation, as opposed to indefinite variation. Arber and Parkin, in
their interesting memoir on the Origin of Angiosperms, have suggested
calling in Mutation to explain the apparently sudden transition from the
cycadean to the angiospermous type of foliage, in late Mesozoic times,
though they express themselves with much caution, and point out "a distinct
danger that Mutation may become the last resort of the phylogenetically

The distinguished French palaeobotanists, Grand'Eury (C. Grand'Eury, "Sur
les mutations de quelques Plantes fossiles du Terrain houiller". "Comptes
Rendus", CXLII. page 25, 1906.) and Zeiller (R. Zeiller "Les Vegetaux
fossiles et leurs Enchainements", "Revue du Mois", III. February, 1907.),
are of opinion, to quote the words of the latter writer, that the facts of
fossil Botany are in agreement with the sudden appearance of new forms,
differing by marked characters from those that have given them birth; he
adds that these results give more amplitude to this idea of Mutation,
extending it to groups of a higher order, and even revealing the existence
of discontinuous series between the successive terms of which we yet
recognise bonds of filiation. (Loc. cit. page 23.)

If Zeiller's opinion should be confirmed, it would no doubt be a serious
blow to the Darwinian theory. As Darwin said: "Under a scientific point
of view, and as leading to further investigation, but little advantage is
gained by believing that new forms are suddenly developed in an
inexplicable manner from old and widely different forms, over the old
belief in the creation of species from the dust of the earth." ("Origin of
Species", page 424.)

It most however be pointed out, that such mutations as Zeiller, and to some
extent Arber and Parkin, appear to have in view, bridging the gulf between
different Orders and Classes, bear no relation to any mutations which have
been actually observed, such as the comparatively small changes, of sub-
specific value, described by De Vries in the type-case of Oenothera
Lamarckiana. The results of palaeobotanical research have undoubtedly
tended to fill up gaps in the Natural System of plants--that many such gaps
still persist is not surprising; their presence may well serve as an
incentive to further research but does not, as it seems to the writer,
justify the assumption of changes in the past, wholly without analogy among
living organisms.

As regards the succession of species, there are no greater authorities than
Grand'Eury and Zeiller, and great weight must be attached to their opinion
that the evidence from continuous deposits favours a somewhat sudden change
from one specific form to another. At the same time it will be well to
bear in mind that the subject of the "absence of numerous intermediate
varieties in any single formation" was fully discussed by Darwin. ("Origin
of Species", pages 275-282, and page 312.); the explanation which he gave
may go a long way to account for the facts which recent writers have
regarded as favouring the theory of saltatory mutation.

The rapid sketch given in the present essay can do no more than call
attention to a few salient points, in which the palaeontological records of
plants has an evident bearing on the Darwinian theory. At the present day
the whole subject of palaeobotany is a study in evolution, and derives its
chief inspiration from the ideas of Darwin and Wallace. In return it
contributes something to the verification of their teaching; the recent
progress of the subject, in spite of the immense difficulties which still
remain, has added fresh force to Darwin's statement that "the great leading
facts in palaeontology agree admirably with the theory of descent with
modification through variation and natural selection." (Ibid. page 313.)


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