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 speculations.
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 for.
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 PROPORTION TO THE COMPLETENESS OF THE RECORD
IS THE UNEQUIVOCAL CHARACTER OF ITS TESTIMONY TO THE TRUTH OF THE
EVOLUTIONARY THEORY.
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.