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of numerous distinct lines in a given group of animals. The
polyphyletic law was early demonstrated among invertebrates
by Neumayr (i88g) when he showed that the ammonite genus
Phylloceras follows not one but five distinct lines of evolu-
tion of unequal duration. The brachiopods, generally classed
collectively as Spirifer mucronatus, follow at least five distinct
lines of evolution in the Middle Devonian of North America,
while more than twenty divergent lines have been observed by
Grabau among the species of the gastropod genus Fusus in
Tertiary and recent times. Vertebrate palaeontologists were
slow to grasp this principle; while the early speculative phylo-
genies of the horse of Huxley and Marsh, for example, were
mostly displayed monophylelically, or in single lines of descent,
it is now recognized that the horses which were placed by Marsh
in a single series are really to be ranged in a great number of
contemporaneous but separate series, each but partially known,
and that the direct phylum which leads to the modern horse has
become a matter of far more difficult search. As early as 1862
Gaudry set forth this very polyphyletic principle in his tabular
phylogenies, but failed to carry it to its logical application. It
is now applied throughout the Vertebrata of both Mesozoic and
Cenozoic times. Among marine Mesozoic reptiles, each of the
groups broadly known as ichthyosaurs, plesiosaurs, mosasaurs
and crocodiles were polyphyletic in a marked degree. Among
land animals striking illustrations of this local polyphyletic law
are found in the existence of seven or eight contemporary series
of rhinoceroses, five or six contemporary series of horses, and
an equally numerous contemporary series of American Miocene
and Pliocene camels; in short, the polyphyletic condition is
the rule rather than the exception. It is displayed to-day among
the antelopes and to a limited degree among the zebras and
rhinoceroses of Africa, a continent which exhibits a survival
of the Miocene and Pliocene conditions of the northern
hemisphere.

6. Development of Analogous Progressive and Retrogressive
Groups. — Because of the repetition of analogous physiographic
and climatic conditions in regions widely separated both in time
and in space, we discover that continental and local adaptive
radiations result in the creation of analogous groups of radii
among aU the vertebrates and invertebrates. Illustrations of
this law were set forth by Cope as early as 1861 (see " Origin of
Genera," reprinted in the Origin of the Fittest, pp. 95-106) in
pointing out the extraordinary parallelisms between unrelated
groups of amphibians, reptiles and mammals. In the Jurassic
period there were no less than six orders of reptiles which
independently abandoned terrestrial life and acquired more or
less perfect adaptation to sea life. Nature, limited in her
resources for adaptation, fashioned so many of these animals in
like form that we have learned only recently to distinguish
similarities of analogous habit from the similitudes of real kinship.
From whatever order of Mammalia or Reptilia an animal may
be derived, prolonged aquatic adaptation will model its outer,
and finally its inner, structure according to certain advantageous
designs. The requirements of an elongate body moving through
the resistant medium of water are met by the evolution of similar
entrant and exit curves, and the bodies of most s-niftly moving
aquatic animals evolve into forms resembling the hulls of modern
sailing yachts (Bashford Dean). We owe especiaUy to Willy
Kukenthal, Eberhard Fraas, S.W. Williston and R. C. Osburn
a summary of those modifications of form to which aquatic life
invariably leads.

The law of analogy also operates in retrogression. A. Smith
Woodward has observed that the decline of many groups of



590



PALAEONTOLOGY



fishes is heralded by the tendency to assume elongate and finally
eel-shaped forms, as seen independently, for example, among
the declining Acanthodians or palaeozoic sharks, among the
modern crossopterygian Polyptcrus and Calamoichthys of the Nile,
in the modern dipneustan Lepidosiren and Protopierus, in the
Triassic chondrostean Belonorhynchus, as well as in the bow-fin
{Amia) and the garpike (Lepidoslcus).

Among invertebrates similar analogous groups also develop.
This is especially marked in retrogressive, though also well-
known in progressive series. The loss of the power to coil,
observed in the terminals of many declining series of gastropods
from the Cambrian to the present time, and the similar loss of
power among Natiloidea and Ammonoidea of many genetic
series, as well as the ostraean form assumed by various declining
series of pelecypods and by some brachiopods, may be cited as
examples.

7. Periods of Gradual Evolution of Groups. — It is certainly a
very striking fact that wherever we have been able to trace
genetic series, either of invertebrates or vertebrates, in closely
sequent geological horizons, or life zones, we find strong proof
of evolution through extremely gradual mutation simultaneously
affecting many parts of each organism, as set forth above. This
proof has been reached quite independently by a very large
number of observers studying a still greater variety of animals.
Such diverse organisms as brachiopods, ammonites, horses and
rhinoceroses absolutely conform to this law in all those rare
localities where we have been able to observe closely sequent
stages. The inference is almost irresistible that the law of gradual
transformation through minute continuous change is by far the
most universal; but many palaeontologists as well as zoologists
and botanists hold a contrary opinion.

8. Periods of Rapid Evolution of Groups. — The above law of
gradual evolution is perfectly consistent with a second principle,
namely, that at certain times evolution is much more rapid
than at others, and that organisms are accelerated or retarded in
development in a manner broadly analogous to the acceleration
or retardation of separate organs. Thus H. S. Williams observes
{Geological Biology, p. 268) that the evolution of those funda-
mental characters which mark differences between separate
classes, orders, sub-orders, and even families of organisms, took
place in relatively short periods of time. Among the brachiopods
the chief expansion of each tj-pe is at a relatively early period in
their life-history. Hyatt (1883) observed of the ammonites that
each group originated suddenly and spread out with great
rapidity. Deperet notes that the genus Neumayria, an ammonite
of the Kimmeridgian, suddenly branches out into an explosion''
of forms. Deperet also observes the contrast between periods
of quiescence and limited variability and periods of sudden
efflorescence. A. Smith Woodward (" Relations of Palaeontology
to Biology," Annals and Mag. Natural Hist., 1906, p. 317) notes
that the fundamental advances in the growth of fish life have
always been sudden, beginning with excessive vigour at the end
of long periods of apparent stagnation; while each advance has
been marked by the fixed and definite acquisition of some new
anatomical character or " expression point," a term first used
by Cope. One of the causes of these sudden advances is un-
doubtedly to be found in the acquisition of a new and extremely
useful character. Thus the perfect jaw and the perfect pair of
lateral fins when first acquired among the fishes favoured a very
rapid and for a time unchecked development. It by no means
follows, however, from this incontrovertible evidence that the
acquisition either of the jaw or of the lateral fins had not been in
itself an extremely gradual process.

Thus both invertebrate and vertebrate palaeontologists have
reached independently the conclusion that the evolution of
groups is not continuously at a uniform rate, but that there are,
especially in the beginnings of new phyla or at the time of
acquisition of new organs, sudden variations in the rate of evolu-
tion which have been termed variously " rhythmic," "pvilsating,"
" efflorescent," "intermittent " and even " explosive " (Deperet).

This varying rate of evolution has (illogically, we believe) been
compared with and advanced in support of the "mutation law



of De Vries,"or the theory of saltatory evolution, which we may-
next consider.

g. Hypothesis of the Sudden Appearance of New Paris or
Organs. — The rarity of really continuous series has naturally
led palaeontologists to support the hypothesis of brusque tran-
sitions of structure. As we have seen, this hypothesis was
fathered by Geoffroy St Hilaire in 1830 from his studies of Meso-
zoic Crocodilia, was sustained by Haldemann, and quite recently
has been revived by such eminent palaeontologists as Louis
DoUo and A. Smith Woodward. The evidence for it is not to be
confused mth that for the law of rapid efflorescence of groups
just considered. It should be remembered that palaeontology
is the most unfavourable field of all for observation and demon-
stration of sudden saltations or mutations of character, because
of the limited materials available for comparison and the rarity
of genetic series. It should be borne in mind, first, that wherever
a new animal suddenly appears or a new character suddenly
arises in a fossil horizon we must consider whether such appear-
ance maybe due to the non-discovery of transitional links with
older forms, or to the sudden invasion of a new type or new organ
which has gradually evolved elsewhere. The rapid variation of
certain groups of animals or the acceleration of certain organs is
also not evidence of the sudden appearance of new adaptive
characters. Such sudden appearances may be demonstrated
possibly in zoology and embryology but never can be demon-
strated by palaeontology, because of the incompleteness of the
geological record.

10. Decline or Senescence of Groups. — Periods of gradual
evolution and of efflorescence may be foUowed by stationary or
senescent conditions. In his history of the Arietidae Hyatt
points out that toward the close of the Cretaceous this entire
group of ammonites appears to have been affected with some
malady; the unroUed forms multiply, the septa are simplified,
the ornamentation becomes heavy, thick, and finally disappears
in the adult ; the entire group ends by dying out and leaving no
descendants. This is not due to environmental conditions
solely, because senescent branches of normal progressive groups
are found in aU geologic horizons, beginning, for gastropods, in
the Lower Cambrian. Among the ammonites the loss of power
to coU the shell is one feature of racial old age, and in others old
age is accompanied by closer coiling and loss of surface orna-
mentation, such as spines, ribs, spirals; while in other forms an
arresting of variability precedes extinction. Thus Williams has
observed that if we find a species breeding perfectly true we can
conceive it to have reached the end of its racial life period.
Brocchi and Daniel Rosa (iSgp) have developed the hypothesis
of the progressive reduction of variability. Such decline is by no
means a universal law of Hfe, however, because among many
of the continental vertebrates at least we observe extinctions
repeatedly occurring during the expression of maximum varia-
bility. Whereas among many ammonites and gastropods smooth
ness of the shell, following upon an ornamental youthful
condition, is generaUy a symptom of decline, among many other
invertebrates and vertebrates, as C. E. Beecher (1856-1905) has
pointed out (1898), many animals possessing hard parts tend
toward the close of their racial history to produce a superfluity
of dead matter, which accumulates in the form of spines among
invertebrates, and of horns among the land vertebrates, reaching
a maximum when the animals are really on the down-grade of
development.

11. The Extinction of Groups. — We have seen that different
lines vary in vitahty and in longevity, that from the earliest
times senescent branches are given off, that different lines vary
in the rate of evolution, that extinction is often heralded by
symptoms of racial old age, which, however, vary widely in
different groups. In general we find an analogy between the
development of groups and of organs; we discover that each
phyletic branch of certain organisms traverses a geologic career
comparable to the life of an individual, that we may often
distinguish, especially among invertebrates, a phase of youth, a
phase of maturity, a phase of senility or degeneration fore-
shadowing the extinction of a type.



PALAEOSPONDYLUS



591



Internal causes of extinction are to be found in exaggeration
of body size, in the hypertrophy or over-specialization of certain
organs, in the irreversibility of evolution, and possibly, although
this has not been demonstrated, in a progressive reduction of
variability. In a full analysis of this problem of internal and
external causes in relation to the Tertiary Mammalia, H. F.
Osborn (" Causes of Extinction of the Mammalia," Amer. Natur-
alist, 1906, pp. 76Q-795, 829-859) finds that foremost in the long
series of causes which lead to extinction are the grander environ-
mental changes, such as physiographic changes, diminished or
contracted land areas, substitution of insular for continental
conditions; changes of climate and secular lowering of temperature
accompanied by deforestation and checking of the food supply;
changes influencing the mating period as well as fertility; changes
causing increased humidity, which in turn favours enemies
among insect life. Similarly secular elevations of temperature,
either accompanied by moisture or desiccation, by increasing
droughts or by disturbance of the balance of nature, have been
followed by great waves of extinction of the Mammalia. In
the sphere of living environment, the varied evolution of plant
life, the periods of forestation and deforestation, the introduction
of deleterious plants simultaneously with harsh conditions of life
and enforced migration, as well as of mechanically dangerous
plants, are among the well-ascertained causes of diminution and
extinction. The evolution of insect life in driving animals from
feeding ranges and in the spread of disease probably has been a
prime cause of extinction. Food competition among mammals,
especially intensified on islands, and the introduction of Carnivora
constitute another class of causes. Great waves of extinction
have followed the long periods of the slow evolution of relatively
inadaptive types of tooth and foot structure, as first demon-
strated by Waldemar Kowalevsky; thus mammals are repeatedly
observed in a cul-de-sac of structure from which there is no escape
in an adaptive direction. Among still other causes are great
bulk, which proves fatal under certain new conditions; rela-
tively slow breeding; extreme specialization and development of
dominant organs, such as horns and tusks, on which for a time
selection centres to the detriment of more useful characters.
Little proof is afforded among the mammals of extinction
through arrested evolution or through the limiting of variation,
although such laws undoubtedly exist. One of the chief
deductions is that there are special dangers in numerical diminu-
tion of herds, which may arise from a chief or original cause
and be followed by a conspiracy of other causes which are cumu-
lative in effect. This survey of the phenomena of extinction in
one great class of animals certainly establishes the existence of an
almost infinite variety of causes, some of which are internal, some
external in origin, operating on animals of different kinds.

VIII. — Underlying Biological Principles as they

APPEAR TO THE PALAEONTOLOGIST

It follows from the above brief summary that palaeontology
affords a distinct and highly suggestive field of purely biological
research; that is, of the causes of evolution underlying the observ-
able modes which we have been describing. The net result
of observation is not favourable to the essentially Darwinian
view that the adaptive arises out of the fortuitous by selection,
but is rather favourable to the hypothesis of the existence of
some quite unknown intrinsic law of hfe which we are at present
totally unable to comprehend or even conceive. We have shown
that the direct observation of the origin of new characters in
palaeontology brings them within that domain of natural law
and order to which the evolution of the physical universe con-
forms. The nature of this law, which, upon the whole, appears
to be purposive or teleological in its operations, is altogether a
mystery which may or may not be illumined by future research.
In other words, the origin, or first appearance of new characters,
which is the essence of evolution, is an orderly process so far as
the vertebrate and invertebrate palaeontologist observes it.
The selection of organisms through the crucial test of fitness and
the shaping of the organic world is an orderly process when
contemplated on a grand scale, but of another kind; here the



test of fitness is supreme. The only inkling of possible underlying
principles in this orderly process is that there appears to be in
respect to certain characters a potentiality or a predisposition
through hereditary kinship to evolve in certain definite directions.
Yet there is strong evidence against the existence of any law in
the nature of an internal perfecting tendency which would
operate independently of external conditions. In other words,
a balance appears to be always sustained between the internal
(hereditary and ontogenetic) and the external (environmental
and selectional) factors of evolution.

BiHLiOGRAPHY. — Among the older works on the history of
palaeontology arc the treatises of Giovanni Batlista Brocchi (1772-
1826), Conchiologia fossile Subappenina . . . Disc, sui progressi
dcllo studio . . . 1S43 (Milan); of £lienne Jules d'Archiac, IJistoire
du progrcs de la gcologie de 1834 i 1862 (Paris, Sac. Giol. de France,
1847-1860); of Charles Lyell in his Principles 0} Geology. A clear
narrative of the work of many of the earlier contributors is found
in Founders of Geology, by Sir Archibald Geikie (London, 1897-
1905). The most comprehensive and up-to-date reference work
on the history of geology and palaeontology is Geschichte der Geologie
itnd Paldontologie, by Karl Alfred von Zittcl (Munich and Leipzig,
1899), the final life-work of this great authority, translated into
English in part by Maria M. Ogilvie-Gordon, entitled " History of
Geology and Palaeontology to the end of the 19th Century." The
succession of life from the earliest times as it was known at the close
of the last century was treated by the same author in his Handbuch
der Paldontologie (5 vols., Munich and Leipzig, 1876-1893). Abbre-
viated editions of this work have appeared from the author, Crund-
ziige der Paldontologie (Palaeozoologie) (Munich and Leipzig, 1895,
2nd ed., 1903), and in English form in Charles R. Eastman's Text-
Book of Palaeontology (1900-1902). A classic but unfinished work
describing the methods of invertebrate palaeontology is Die Sidmme
des Thierreichs (Vienna, 1889), by Melchior Neumayr. In France
admirable recent works are Eliments de Paleontologie, by Felix
Bernard (Paris, 1895), and the still more recent philosophical
treatise by Charles Dep^ret, Les Transformations du monde animal
(Paris, 1907). Huxley's researches, and especially his share in the
development of the philosophy of palaeontology', will be found in
his essays. The Scientific Memoirs of Thomas Henry Huxley (4 vols.,
London, 1898-1902). The whole subject is treated systematically
in Nicholson and Lydekker's A Manual of Palaeontology (2 vols.,
Edinburgh and London, 1889), and A. Smith Woodward's Outlines
of Vertebrate Palaeontology (Cambridge, 1898).

Among American contributions to vertebrate palaeontology, the
development of Cope's theories is to be found in the volumes of
his collected essays, The Origin of the Fittest (New York, 1887),
and The Primary Factors of Organic Evolution (Chicago, 1896). A
brief summary of the rise of vertebrate palaeontology is found in
the address of O. Marsh, entitled " History and Methods of Palaeonto-
logical Discovery " (American Association for the Advancement
of Science, 1879). The chief presentations of the methods of the
American school of invertebrate palaeontologists are to be found in
A. Hyatt's great memoir " Genesis of the Arietidae " {Smithsonian
Contr. to Knowledge, 673, 1889), in Hyatt's " Phylogeny of an
Acquired Characteristic " {Philosophical Soc. Proc, vol. xxxii.
1894), and in Geological Biology, by H.S.Williams (New York, 1895).

In preparing the present article the author has drawn freely on
his own addresses: see H. F. Osborn, " The Rise of the Mammalia
in North America " (Proc. Amer. Assn. Adv. Science, vol. xlii.,
'893), " Ten Years' Progress in the Mammalian Palaeontology of
North America " {Comptes rendus du 6' Congres intern, de zoologie,
session de Bern, 1904), " The Present Problems of Palaeontology "
(Address before Section of Zool. International Congress of Arts
and Science, St Louis, Sept. 1904), " The Causes of Extinction of
Mammalia " {Amer. Naturalist, xl. 769-795, 829-859, 1906).

(H. F. O.)

PALAEOSPONDYLUS, a small fish-like organism, of which
the skeleton is found fossil in the Middle Old Red Sandstone




From British Museum tjuide to Fossil Reptiles and Fishes, by
permission of the Trustees.
Palaeospondylus gunni, restored by Dr R. H. Traquair.
(Nearly twice nat. size.)

of Achanarras, near Thurso, Caithness. It was thus named
(Or. ancient vertebra) by Dr R. H. Traquair in 1890, in allusion
to its well-developed vertebral rings; and its structure was



592



PALAEOTHERIUM—PALAEPHATUS



studied in detail in 1903 by Professor and Miss Sollas, who
succeeded in making enlarged models of the fossO in wax.
The skeleton as preserved is carbonized, and indicates an eel-
shaped animal from 3 to 5 cm., in length. The skuU, which
must have consisted of hardened cartilage, exhibits pairs of
nasal and auditory capsules, with a giU-apparatus below its
hinder part, but no indications of ordinary jaws. The anterior
opening of the brain-case is surrounded by a ring of hard cirri.
A pair of " post-branchial plates " projects backwards from the
head. The vertebral axis shows a series of broad rings, with
distinct neural arches, but no ribs. Towards the end of the body
both neural and haemal arches are continued into forked
radial cartilages, which support a median fin. There are no
traces either of paired fins or of dermal armour. The affinities
of Palacospondylus are doubtful, but it is probably related to
the contemporaneous armoured Ostracoderms.

Referenxes. — R. H. Traquair, paper in Proc. Roy. PJiys. Soc.
Edin., xii. 312, (1894); W. J. Sollas and I. B. J. Sollas, paper in
Phil. Trans. Roy. Soc. (1903 B.). (A. S. Wo.)

PALAEOTHERIUM {i.e. ancient animal), a name applied by
Cuvier to the remains of ungulate mammals recalling tapirs
in general appearance, from the Lower Oligocene gypsum
quarries of Paris. These were the first indications of the




tFrom the Paris gypsum.)

Restoration of Palaeotherium magnum. (About \ nat. size.)

occurrence in the fossil state of perissodactyle ungulates allied
to the horse, although it was long before the relationship was
recognized. The palaeotheres, which range in size from that
of a pig to that of a small rhinoceros, are now regarded as repre-
senting a family, Palaeotheriidac, nearly related to the horse-
tribe, and having, in fact, probably originated from the same
ancestral stock, namely, Hyracolheriiim of the Lower Eocene
(see Equidae). The connecting link with Hyracotherium was
formed by Pachynolophus (Propalacotheriimt), and the line
apparently terminated in Paloplotherium, which is also Ohgocene.
Representatives of the family occur in many parts of Europe,
but the typical genus is unknown in North America, where,
however, other forms occur.

Although palaeotheres resemble tapirs in general appearance,
they differ in having only three toes on the fore as well as on the
hind foot. The dentition normally comprises the typical series
of 44 teeth, although in some instances the first premolar is
wanting. The cheek-teeth are short-crowned, generally with
no cement, the upper molars having a W-shaped outer wall,
from which proceed two oblique transverse crests, while the lower
ones carry two crescents. Unlike the early horses, the later
premolars are as complex as the molars; and although there is a
well-marked gap between the canine and the premolars, there is



Online LibraryHugh ChisholmThe Encyclopædia britannica; a dictionary of arts, sciences, literature and general information (Volume v. 20) → online text (page 211 of 353)