_THE APPARITION AND SUCCESSION OF ANIMAL FORMS._
Time was when naturalists were content to take nature as they found
it, without any over-curious inquiries as to the origin of its several
parts, or the changes of which they might be susceptible. Geology
first removed this pleasant state of repose, by showing that all our
present species had a beginning, and were preceded by others, and these
again by others. Geologists were, however, too much occupied with the
facts of the succession to speculate on the ultimate causes of the
appearance and disappearance of species, and it remained for zoologists
and botanists, or as some prefer to call themselves, biologists, to
construct hypotheses or theories to account for the ascertained fact
that successive dynasties of species have succeeded each other in
time. I do not propose in this paper so much to deal with the various
doctrines as to derivation and development now current, as to ask the
question, What do we actually know as to the origin and history of life
on our planet?
This great question, confessedly accompanied with many difficulties and
still waiting for its full solution, has points of intense interest
both for the Geologist and the Biologist. "If," says the great
founder of the uniformitarian School of Geology, "the past duration
of the earth be finite, then the aggregate of geological epochs,
however numerous, must constitute a mere moment of the past, a mere
infinitesimal portion of eternity." Yet to our limited vision, the
origin of life fades away in the almost illimitable depths of past
time, and we are ready to despair of ever reaching, by any process of
discovery, to its first steps of progress. At what time did life begin?
In what form did dead matter first assume or receive those mysterious
functions of growth, reproduction and sensation? Only when we picture
to ourselves an absolutely lifeless world, destitute of any germ of
life or organization, can we realize the magnitude of these questions,
and perceive how necessary it is to limit their scope if we would hope
for any satisfactory answer.
We may here dismiss altogether that form in which these questions
present themselves to the biologist, when he experiments as to the
evolution of living forms from dead liquids or solids attacking the
unsolved problem of spontaneous generation. Nor need we enter on the
vast field of discussion as to modern animals and plants opened up by
Darwin and others. I shall confine myself altogether to that historical
or palæontological aspect in which life presents itself when we study
the fossil remains entombed in the sediments of the earth's crust, and
which will enable me at least to show why some students of fossils
hesitate to give in their adhesion to any of the current notions as to
the origin of species. It will also be desirable to avoid, as far as
possible, the use of the term "evolution," as this has recently been
employed in so many senses, whether of development or causation, as to
have become nearly useless for any scientific purpose; and that when I
speak of creation of species, the term is to be understood not in the
arbitrary sense forced on it by some modern writers, but as indicating
the continuous introduction of new forms of life under definite laws,
but by a power not emanating from within themselves, nor from the
inanimate nature surrounding them.[68]
[68] The terms Derivation, Development and Causation have clear and
definite meanings, and it is preferable, wherever possible, to use one
or other of these.
If we were to follow the guidance of those curious analogies which
present themselves when we consider the growth of the individual
plant or animal from the spore or the ovum, and the development of
vegetable and animal life in geological time--analogies which, however,
it must be borne in mind can have no scientific value whatever,
inasmuch as that similarity of conditions which alone can give force
to reasoning from analogy in matters of science, is wholly wanting--we
should expect to find in the oldest rocks embryonic forms alone, but
of course embryonic forms suited to exist and reproduce themselves
independently.[69]
[69] I may be pardoned for taking an example of the confusion of
thought which this mode of reasoning has introduced into Biology
from a clever article in the _Contemporary_ written by a very able
and much-esteemed biologist. He says: "The morphological distance
between a newly hatched frog's tadpole and the adult frog is almost
as great as that between the adult lancelet and the newly hatched
larvæ of the lamprey." The "morphological distance" truly, but what
of the physiological distance between the young and adult of the same
animal and two adult animals between which is placed the great gulf of
specific and generic diversity which within human experience neither
has been known to pass?
I need not say to palæontologists that this is not what we actually
find in the primordial rocks. I need but to remind them of the early
and remarkable development of such forms as the Trilobites, the
Lingulidæ and the Pteropods, all of them highly complex and specialized
types, and remote from the embryonic stages of the groups to which
they severally belong. In the case of the Trilobites, one need merely
consider the beautiful symmetry of their parts, both transversely and
longitudinally, their division into distinct regions, the necessary
complexity of their muscular and nervous systems, their highly complex
visual organs, the superficial ornamentation and microscopic structure
of their crusts, their advanced position among Crustaceans, indicated
by their strong affinity with the Arachnidans or spiders and scorpions.
(See figures prefixed.)
All these characters give them an aspect far from embryonic, while, as
Barrande has pointed out, this advanced position of the group has its
significance greatly strengthened by the fact that in early primordial
times we have to deal not with one species, but with a vast and highly
differentiated group, embracing forms of many and varied subordinate
types. As we shall see, these and other early animals may be regarded
as of generalized types, but not as embryonic. Here, then, meets us at
the outset the fact that in as far as the great groups of annulose and
molluscous animals are concerned, we can trace these back no farther
than to a period in which they appear already highly advanced, much
specialized and represented by many diverse forms. Either, therefore,
these great groups came in on this high initial plane, or we have
scarcely reached half way back in the life-history of our planet.
We have, here, however, by this one consideration, attained at once to
two great and dominant laws regulating the history of life. First, the
law of continuity, whereby new forms come in successively, throughout
geological time, though, as we shall see, with periods of greater or
less frequency. Secondly, the law of specialization of types, whereby
generalized forms are succeeded by those more special, and this
probably connected with the growing specialization of the inorganic
world. It is this second law which causes the parallelism between the
history of successive species and that of the embryo.
We have already considered the claims which Eozoon and its
contemporaries may urge to recognition, as beginnings of life; but
when we ascend from the Laurentian beds, we find ourselves in a barren
series of conglomerates, sandstones, and other rocks, indicating shore
rather than sea conditions, and remarkably destitute of indications of
life. These are the Huronian beds, and possibly other series associated
with them. They have afforded spicules of sponges, casts of burrows
of worms, obscure forms, which may represent crustaceans or mollusks,
markings of unknown origin, and some laminated forms which may perhaps
represent remains of Eozoon, though their structures are imperfectly
preserved. These are sufficient to show that marine life continued in
some forms, and to encourage the hope that a rich pre-Cambrian fauna
may yet be discovered.
But let us leave for the present the somewhat isolated case of Eozoon,
and the few scattered forms of the Huronian, and go on farther to
the early Cambrian fauna. This is graphically presented to us in
the sections in South Wales, as described by Hicks. Here we find a
nucleus of ancient rocks, supposed to be Laurentian, though in mineral
character more nearly akin to the Huronian, but which have hitherto
afforded no trace of fossils. Resting unconformably on these is a
series of slates and sandstones, regarded as Lower Cambrian, the
Caerfai group of Hicks, and which are the earliest holding organic
remains. The lowest bed which contains indications of life is a red
shale near the base of the series, which holds a few organic remains.
The species are a _Lingulella_, worm burrows and a Trilobite.[70]
Supposing these to be all, it is remarkable that we have no Protozoa
or Corals or Echinoderms, and that the types of Brachiopods and
Crustaceans are of comparatively modern affinities. Passing upward
through 1,000 feet of barren sandstone and shale, we reach a zone in
which many Trilobites of at least five genera are found, along with
Pteropods, Brachiopods and Sponges. Thus it is that life comes in at
the base of the Cambrian in Wales, and it may be regarded as a fair
specimen of the facts as they appear in the earlier fossiliferous beds
succeeding the Laurentian. Taking the first of these groups of fossils,
we may recognise in the worms representatives of those that still
haunt our shores, in the Trilobite a Crustacean or Arachnoid of no
mean grade. The _Lingulellæ_, whether we regard them as molluscoids,
or, with Professor Morse, as singularly specialized worms, represent a
peculiar and distinct type, handed down, through all the vicissitudes
of the geological ages, to the present day. Had the Primordial life
begun with species altogether inscrutable and unexampled in succeeding
ages, this would no doubt have been mysterious; but next to this is
the mystery of the oldest forms of life being also among the newest.
One great fact shines here with the clearness of noon-day. Whatever
the origin of these creatures, they represent families which have
endured till now in the struggle for existence without either elevation
or degradation. Here, again, we may formulate another creative law.
In every great group there are some forms much more capable of long
continuance than others. Lingula among the Brachiopods is a marked
instance.
[70] Probably of the genus Olenellus.
But when, with Hicks, we surmount the mass of barren beds underlying
these remains, which from its unfossiliferous character is probably a
somewhat rapid deposit of Arctic mud, like that which in all geological
time has constituted the rough filling of our continental formations,
and have suddenly sprung upon us many genera of Trilobites, including
the fewest-jointed and most many-jointed, the smallest and the largest
of their race, our astonishment must increase, till we recognise the
fact that we are now in the presence of another great law of creation,
which provides that every new type shall be rapidly extended to the
extreme limits of its power of adaptation.
That this is not merely local is evidenced by the researches of
Matthew and Walcott in the oldest Cambrian of America, where a similar
succession occurs, but with this difference, that in the wider area
presented by the American continent we find a greater variety of
forms of life. Walcott records up to 1892 no less than 67 genera
and 165 species in the oldest Cambrian of America. These include
representatives of the Sponges, Hydroids, Corals, Echinoderms, Worms,
Brachiopods, Bivalve and Univalve Mollusks and Crustaceans, or in
other words, all the leading groups of invertebrate animals that
we find in the sea at present. Of these the dominant group is the
Crustaceans, including Trilobites, numbering one-third of the whole;
and these with the univalve Mollusks and the Brachiopods constitute
the majority, the other groups having comparatively few species. What
a marvellous incoming of life is here! Walcott may well say that on
the theory of gradual development we must suppose that life existed at
a period far before the Cambrian--as far, indeed, as the Cambrian is
before our own time. But this would mean that we know only half of the
history of life; and perhaps it is more reasonable to suppose that when
the conditions became favourable, it came in with a rush.
Before considering the other laws that may be inferred from these
facts, however, let us in imagination transfer ourselves back to the
Primordial age, and suppose that we have in our hands a living specimen
of one of the larger Trilobites, recently taken from the sea, flapping
vigorously its great tail, and full of life and energy; an animal
larger and heavier than the modern king-crab of our shores, furnished
with all the complexity of external parts for which the crustaceans are
so remarkable, and no doubt with instincts and feelings and modes of
action as pronounced as those of its modern allies, and, if Woodward's
views are correct, on a higher plane of rank than the king-crab itself,
inasmuch as it is a composite type connecting Limuli with Isopods,
and even with scorpions. We have obviously here, in the appearance of
this great Crustacean or Arachnoid, a repetition of the facts which
we met with in Eozoon; but how vast the interval between them in
geological time, and in zoological rank! Standing in the presence of
this testimony, I think it is only right to say that we possess no
causal solution of the appearance of these early forms of life; but in
tracing them and their successors upward through the succeeding ages,
we may hope at least to reach some expressions of the laws of their
succession, in possession of which we may return to attack the mystery
of their origin.
First, it must strike every observer that there is a great sameness
of plan throughout the whole history of marine invertebrate life. If
we turn over the pages of an illustrated text-book of geology, or
examine the cases or drawers of a collection of fossils, we shall find
extending through every succeeding formation, representative forms of
Crustaceans, Mollusks, Corals, etc., in such a manner as to indicate
that in each successive period there has been a reproduction of the
same type with modifications; and if the series is not continuous, this
appears to be due rather to abrupt physical changes; since sometimes,
where two formations pass into each other, we find a gradual change in
the fossils by the dropping out and introduction of species one by one.
Thus, in the whole of the great Palæozoic Period, both in its Fauna and
Flora, we have a continuity and similarity of a most marked character.
It is evident that there is presented to us in this similarity of the
forms of successive faunas and floras, a phenomenon which deserves very
careful sifting as to the question of identity or diversity of species.
The data for its comprehension must be obtained by careful study of
the series of closely allied forms occurring in successive formations,
and the great and undisturbed areas of the older rocks in America seem
to give special facilities for this, which should be worked, not in
the direction of constituting new species for every slightly divergent
form, but in striving to group these forms into large specific
types.[71]
[71] The Rynchonellæ of the type of _R. plena_, the Orthids, of
the type of _O. testudinaria_, the Strophomenæ of the types of _S.
alternata_ and _S. Rhomboidalis_, the Atrypæ of the type of _A.
reticularis_, furnish cases in point among the Brachiopods.
There is nothing to preclude the supposition that some of the groups
mentioned in the note are really specific types, with numerous race
modifications. My own provisional conclusion, based on the study of
Palæozoic plants, is that the general law will be found to be the
existence of distinct specific types, independent of each other, but
liable in geological time to a great many modifications, which have
often been regarded as distinct species.[72]
[72] "Geological History of Plants."
While this unity of successive faunæ at first sight presents an
appearance of hereditary succession, it loses much of this character
when we consider the number of new types introduced without apparent
predecessors, the necessity that there should be similarity of type
in successive faunæ on any hypothesis of a continuous plan; and above
all, the fact that the recurrence of representative species or races
in large proportion marks times of decadence rather than of expansion
in the types to which they belong. To turn to another period, this is
very manifest in that singular resemblance which obtains between the
modern mammals of South America and Australia, and their immediate
fossil predecessors--the phenomenon being here manifestly that of
decadence of large and abundant species into a few depauperated
representatives. This will be found to be a very general law, elevation
being accompanied by the apparent abrupt appearance of new types and
decadence by the apparent continuation of old species, or modifications
of them.
This resemblance with difference in successive faunas also connects
itself very directly with the successive elevations and depressions
of our continental plateaus in geological time. Every great Palæozoic
limestone, for example, indicates a depression with succeeding
elevation. On each elevation marine animals were driven back into the
ocean, and on each depression swarmed in over the land, reinforced
by new species, either then introduced, or derived by migration from
other localities. In like manner, on every depression, land plants and
animals were driven in upon insular areas, and on re-elevation, again
spread themselves widely. Now I think it will be found to be a law here
that periods of expansion were eminently those of introduction of new
specific types, and periods of contraction those of extinction, and
also of continuance of old types under new varietal forms.
It must also be noticed that all the leading types of invertebrate
life were early introduced, that change within these was necessarily
limited, and that elevation could take place mainly by the introduction
of the vertebrate orders. So in plants, Cryptogams early attained
their maximum as well as Gymnosperms, and elevation occurred in the
introduction of Phænogams, and this not piecemeal, but as we shall see
in a succeeding chapter, in great force at once.
We may further remark the simultaneous appearance of like types of life
in one and the same geological period, over widely separated regions of
the earth's surface. This strikes us especially in the comparatively
simple and homogeneous life-dynasties of the Palæozoic, when, for
example, we find the same types of Silurian Graptolites, Trilobites and
Brachiopods appearing simultaneously in Australia, America and Europe.
Perhaps in no department is it more impressive than in the introduction
of the Devonian and Carboniferous Ages of that grand cryptogamous
and gymnospermous flora which ranges from Brazil to Spitzbergen, and
from Australia to Scotland, accompanied in all by the same groups of
marine invertebrates. Such facts may depend either on that long life of
specific types which gives them ample time to spread to all possible
habitats, before their extinction, or on some general law whereby the
conditions suitable to similar types of life emerge at one time in all
parts of the world. Both causes may be influential, as the one does
not exclude the other, and there is reason to believe that both are
natural facts. Should it be ultimately proved that species allied and
representative, but distinct in origin, come into being simultaneously
everywhere, we shall arrive at one of the laws of creation, and one
probably connected with the gradual change of the physical conditions
of the world.
Another general truth, obvious from the facts which have been already
collected, is the periodicity of introduction of species. They come
in by bursts or flood tides at particular points of time, while these
great life waves are followed and preceded by times of ebb in which
little that is new is being produced. We labour in our investigation of
this matter under the disadvantage that the modern period is evidently
one of the times of pause in the creative work. Had our time been
that of the early Tertiary or early Mesozoic, our views as to the
question of origin of species might have been very different. It is a
striking fact, in illustration of this, that since the glacial age no
new species of mammal, except, possibly, man himself, can be proved to
have originated on our continents, while a great number of large and
conspicuous forms have disappeared. It is possible that the proximate
or secondary causes of the ebb and flow of life production may be in
part at least physical, but other and more important efficient causes
may be behind these. In any case these undulations in the history of
life are in harmony with much that we see in other departments of
nature.
It results from the above and the immediately preceding statement,
that specific and generic types enter on the stage in great force,
and gradually taper off towards extinction. They should so appear in
the geological diagrams made to illustrate the succession of living
beings. This applies even to those forms of life which come in with
fewest species and under the most humble guise. What a remarkable
swarming, for example, there must have been of Marsupial Mammals in
the early Mesozoic, and in the Coal formation the only known Pulmonate
snails, five or six in number, belong to four generic types, while the
Myriapods and Amphibians alike appear in a crowd of generic forms.
I have already referred to the permanence of species in geological
time. We may now place this in connection with the law of rapid
origination and more or less continuous transmission of varietal forms.
A good illustration will be afforded by a group of species with which
I am very familiar, that which came into our seas at the beginning of
the Glacial age, and still exists. With regard to their permanence, it
can be affirmed that the shells now elevated in Wales to 1,200, and in
Canada to 600 feet above the sea, and which lived before the last great
revolution of our continents a period very remote as compared with
human history--differ in no tittle from their modern successors after
hundreds or thousands of generations. It can also be affirmed that the
more variable species appear under precisely the same varietal forms
then as now, though these varieties have changed much in their local
distribution. The real import of these statements, which might also
be made with regard to other groups, well known to palæontologists,
is of so great significance that it can be realized only after we
have thought of the vast time and numerous changes through which
these humble creatures have survived. I may call in evidence here a
familiar New England animal, the common sand clam, _Mya arenaria_, and
its relative _Mya truncata_, the short sand clam, which now inhabit
together all the northern seas; for the Pacific specimens, from Japan
and California, though differently named, are undoubtedly the same.
_Mya truncata_ appears in Europe in the Coralline Crag, and was
followed by _M. arenaria_ in the Red Crag. Both shells occur in the
Pleistocene of America, and their several varietal forms had already
developed themselves in the Crag, and remain the same to-day; so that
these humble mollusks, littoral in their habits, and subjected to a
great variety of conditions, have continued for a very long period
to construct their shells precisely as at present; while in many
places, as on the Lower St. Lawrence, we find them living together on
the same banks, and yet preserving their distinctness.[73] Nor are
there any indications of a transition between the two species. I might
make similar statements with regard to the Astartes, Buccinums and
Tellinæ of the drift, and could illustrate them by extensive series of
specimens from my own collections.
[73] Paper in _Record of Science_, on Shells at Little Metis.
Another curious illustration is that presented by the Tertiary and
modern faunæ of some oceanic islands far separated from the continents.
In Madeira and Porto Santo, for example, according to Lyell, we have
fifty-six species of land shells in the former, and forty-two in the
latter, only twelve being common to the two, though these islands are
only thirty miles apart. Now in the Pliocene strata of Madeira and
Porto Santo we find thirty-six species in the former, and thirty-five
in the latter, of which only eight per cent, are extinct, and yet
only eight are common to the two islands. Further, there seem to be
no transitional forms connecting the species, and of some of them the
same varieties existed in the Pliocene as now. The main difference
in time is the extinction of some species and the introduction of
others without known connecting links, and the fact that some species,
plentiful in the Pliocene, are rare now, and _vice versâ_. All these
shells differ from those of modern Europe, but some of them are allied
to Miocene species of that continent. Here we have a case of continued
existence of the same forms, and in circumstances which, the more we
think of them, the more do they defy all our existing theories as to
specific origins.
Perhaps some of the most remarkable facts in connection with the
permanence of varietal forms of species are those furnished by that
magnificent flora which burst in all its majesty on the American
continent in the Cretaceous period, and still survives among us, even
in some of its specific types. I say survives; for we have but a
remnant of its forms living, and comparatively little that is new has
probably been added since. The confusion which has obtained as to the
age of this flora, and its mistaken reference to the Miocene Tertiary,
have arisen in part from the fact that this modern flora was in its
earlier times contemporary with Cretaceous animals, and survived the
gradual change from the animal life of the Cretaceous down to that of
the Eocene, and even of the Miocene. In collections of these plants,
from what may be termed beds of transition from the Cretaceous to the
Tertiary, we find many plants of modern species, or so closely related
that they may be mere varietal forms. Some of these will be mentioned
in the next paper, and they show that modern plants, some of them
small and insignificant, others of gigantic size, reach back to a time
when the Mesozoic Dinosaurs were becoming extinct, and the earliest
Placental mammals being introduced. Shall we say that these plants
have propagated themselves unchanged for half a million of years, or
more?[74]
[74] Among these are living species of ferns, one of them our common
"Sensitive Fern," of Eastern America, two species of Hazel still
extant, and Sequoias or giant pines, like those now surviving in
California.
Take from the western Mesozoic a contrasting yet illustrative fact. In
the lowest Cretaceous rocks of Queen Charlotte's Island, Mr. Richardson
and Dr. G. M. Dawson find Ammonites and allied Cephalopods similar
in many respects to those discovered farther south by the California
Survey, and Mr. Whiteaves finds that some of them are apparently
not distinct from species described by the Palæontologists of the
Geological Survey of British India. On both sides of the Pacific these
shells lie entombed in solid rock, and the Pacific rolls between, as
of yore. Yet these species, genera, and even families are all extinct
why, no man can tell, while land plants that must have come in while
the survivors of these Cephalopods still lived, reach down to the
present. How mysterious is all this, and how strongly does it show the
independence in some sense of merely physical agencies on the part of
the manifestations of life!
We have naturally been occupied hitherto with the lower tribes of
animals and with plant life, because these are predominant in the
early ages of the earth. Let us turn now to the history of vertebrate
or back-boned animals, which presents some peculiarities special to
itself. Many years ago Pander[75] described and figured from the
Cambro-Silurian of Russia, a number of minute teeth, some conical and
some comb-like, which he referred to fishes, and to that low form
of the fish type represented by the modern lampreys. Much doubt was
thrown on this determination, more especially as the teeth seemed
to be composed not of bone earth, but of carbonate of lime, and it
was suggested that they may have belonged to marine worms, or to the
lingual ribbons of Gastropod mollusks. Some confirmatory evidence seems
to have been supplied by the discovery of great numbers of similar
forms in the shales of the coal formation of Ohio, by the late Dr.
Newberry. I have had an opportunity to examine these, and find that
they consist of calcium phosphate,[76] or bone earth, and that their
microscopic structure is not dissimilar from that of the teeth of some
of the smaller sharks (Diplodus) found with them. I have therefore
been inclined to believe that there may have already been, even in the
Cambrian or Lower Silurian seas, true fishes, related partly to the
lampreys and partly to sharks; so that the history of the back-boned
animals may have gone nearly as far back as that of their humbler
relations. This conjecture has recently received further support
from the discovery in rocks of Lower Silurian age, in Colorado of a
veritable bone bed, rich in fragmentary remains of fishes. They are
unfortunately so comminuted as to resemble the _débris_ of the food
of some larger animal; but in so far as I can judge from specimens
kindly given to me,[77] they resemble the bony coverings of some of
the familiar fishes of the Devonian. Thus they would indicate, with
Pander's and Rohan's specimens, already two distinct types of fishes as
existing almost as early as the higher invertebrates of the sea.
[75] More recently Rohan has described conical teeth (St. Petersburg
Academy, 1889), but I have not seen his paper.
[76] Analysis of Dr. B. J. Harrington.
[77] By Mr. F. D. Adams and Dr. Walcott.
In the Silurian (Upper Silurian of Murchison) we have undoubted
evidence of the same kind, on both sides of the Atlantic, in teeth and
spines of sharks, and the plates which protected the heads and bodies
of the plate-covered fishes (Placo-ganoids). But it is in the Devonian
that these types appear to culminate, and we have added to them that
remarkable type of "lung fish," as the Germans call them, represented
in our modern world only by the curious and exceptional Burramunda
of Australia, and the mud fishes of Africa and South America,[78]
creatures which show, as do some of the mailed fishes, or ganoids, of
equally great age, the intermediate stages between a swimming bladder
and a lung, and thus approach nearer to the air-breathing animals than
any other fishes.
[78] Ceratodus, Lipidosiren, Protopterus.
Many years ago, in "Acadian Geology," I referred to the probability
that the mailed and lung fishes of the Devonian and Carboniferous
possessed air bladders so constructed as to enable them to breathe air,
as is the case with their modern representatives. In the modern species
this, no doubt, enables them to haunt badly aërated waters, in swamps
and sluggish streams, and in some cases even to survive when the water
in which they live is dried up. In the Carboniferous and Devonian it
may have served a similar purpose, fitting them to inhabit the lagoons
and creeks of the coal swamps, the water of which must often have
been badly aërated. It makes against this that some sharks followed
them into these waters, and the modern sharks have no swim-bladders.
Possibly, however, the sharks habitually haunted the open sea, and only
made occasional raids on the dangerous waters tenanted by the ganoids.
It is also true that only certain genera of sharks are found to be
represented in the carbonaceous shales, and they may have differed
in this respect from the ordinary forms of the order. It has been
suggested that only a small change would be necessary to enable some
of these lung fishes to become Batrachians, and no doubt this is the
nearest approach of the fish to the reptile; but we have not yet found
connecting links sufficient to bridge over the whole distance.
[Illustration: Two Primitive Vertebrates, _Palæospondylus_
(enlarged) and _Pterichthys_ (reduced), (After Woodward, with some
modifications.)]
The plate-bearing ganoids of the Silurian and Devonian, at one time
supposed to be allied to Crustaceans, but whose dignity as "Forerunners
of the back-boned animals" is now generally admitted,[79] are clearly
true fishes, and of somewhat high rank, their strange bony armour being
evidently a special protection against the attacks of contemporary
sharks and gigantic crustaceans; and if we may judge by the Colorado
specimens, their existence dates back almost to the close of the
Cambrian, and they were probably contemporary with small sharks; while
as early as the Silurian and Devonian, if we regard the scaly ganoids
as a distinct type, we have already four types of fishes, and these
akin to those which in modern time we must regard as the highest of
their class.
[79] A. Smith Woodward, "Natural Science," 1892, and _Annals and Maga.
Nat. Hist._, October, 1890. This able naturalist, in introducing his
subject, remarks, from the point of view of an evolutionist:--"Whether
some form of 'worm' gave origin to the forerunners of the great
back-boned race, or whether a primeval relative of the King-crab
turned upside down and rearranged limbs and head these are questions
still admitting of endless discussion, no doubt fruitless in their
main object, but desirable from the new lines of investigation they
continually suggest."
One very little fish of the Devonian, of which specimens have been
kindly sent me by a friend in Scotland,[80] the Palæospondylus of
Traquair, may raise still higher hopes for the early vertebrates. It is
a little creature, an inch to two inches in length, destitute or nearly
destitute of bony covering, having a head which suggests the presence
of external gills, large eyes, and even elongated nasal bones,[81] a
long vertebral column composed of separate bony rings, more than fifty
in number, with possible indications of ribs in front and distinct
neural and haemal processes behind. One cannot look at it without the
suggestion occurring of some of the smaller snake-like Batrachians of
the Carboniferous and Permian; and I should not be surprised if it
should come to be regarded either as a forerunner of the Batrachians or
as a primitive tadpole.
[80] James Reed, Esq., of Allan House, Blairgowrie.
[81] I am aware that Woodward regards these parts differently.
However this may be, the upper part of the Devonian, though rich in
fishes and plants, has afforded no higher vertebrates than its lower
parts, and in the lowest Carboniferous beds we suddenly find ourselves
in the presence of Batrachians with well-developed limbs and characters
which ally them to the Lizards. True lizard-like reptiles appear in
the Permian, and then we enter on that marvellous reign of reptiles,
in which this class assumed so many great and remarkable forms, and
asserted itself in a manner of which the now degraded reptilian class
can afford no conception.
The mammals and birds make their first appearance quietly in small and
humble forms in the reign of reptiles, in which there was little place
left for them by the latter; but the mammals burst upon us in all their
number and magnitude in the Eocene and Miocene, in which quadrupedal
mammalian life may be said to have culminated in grandeur, variety, and
geographical distribution; far excelling in these respects the time in
which we live.
The development in time of the back-boned animals thus stands in
some degree by itself; but it illustrates the same laws of early
generalised types, and sudden and wide introduction of new forms, which
we have seen in the case of the invertebrates and the plants.
Such facts as those to which I have referred, and many others, which
want of space prevents me from noticing, are in one respect eminently
unsatisfactory, for they show us how difficult must be any attempts
to explain the origin and succession of life. For this reason they
are quietly put aside or explained away in most of the current
hypotheses on the subject. But we must, as men of science, face these
difficulties, and be content to search for facts and laws, even if they
should prove fatal to preconceived views.
A group of new laws, indeed, here breaks upon us. (1) The great
vitality and rapid extension and variation of new specific types. (2)
The law of spontaneous decay and mortality of species in time. (3) The
law of periodicity and of simultaneous appearance of many allied forms.
(4) The abrupt entrance and slow decay of groups of species. (5) The
extremely long duration of some species in time. (6) The grand march of
new forms landwards, and upwards in rank. Such general truths deeply
impress us at least with the conclusion that we are tracing, not a
fortuitous succession, but the action of power working by law.
I have thus far said nothing of the bearing of the prevalent ideas of
descent with modification on this wonderful procession of life. None
of these, of course, can be expected to take us back to the origin of
living beings; but they also fail to explain why so vast numbers of
highly organized species struggle into existence simultaneously in one
age and disappear in another, why no continuous chain of succession in
time can be found gradually blending species into each other, and why,
in the natural succession of things, degradation under the influence
of external conditions and final extinction seem to be laws of organic
existence. It is useless here to appeal to the imperfection of the
record, or to the movements or migrations of species. The record is
now, in many important parts, too complete, and the simultaneousness
of the entrance of the faunas and floras too certainly established,
and moving species from place to place only evades the difficulty. The
truth is that such hypotheses are at present premature, and that we
require to have larger collections of facts. Independently of this,
however, it appears to me that from a philosophical point of view it
is extremely probable that all theories of evolution, as at present
applied to life, are fundamentally defective in being too partial in
their character; and perhaps I cannot better group the remainder of the
facts to which I wish to refer than by using them to illustrate this
feature of most of our attempts at generalization on this subject.
First, then, these hypotheses are too partial, in their tendency to
refer numerous and complex phenomena to one cause, or to a few causes
only, when all trustworthy analogy would indicate that they must result
from many concurrent forces and determinations of force. We have all,
no doubt, read those ingenious, not to say amusing, speculations in
which some entomologists and botanists have indulged with reference to
the mutual relations of flowers and suctorial insects. Geologically
the facts oblige us to begin with Cryptogamous plants and chewing
insects, and out of the desire of insects for non-existent honey,
and the adaptations of plants to the requirements of non-existent
suctorial apparatus, we have to evolve the marvellous complexity of
floral form and colouring, and the exquisitely delicate apparatus
of the mouths of haustellate insects. Now, when it is borne in mind
that this theory implies a mental confusion on our part precisely
similar to that which, in the department of mechanics, actuates the
seekers for perpetual motion, that we have not the smallest tittle of
evidence that the changes required have actually occurred in any one
case, and that the thousands of other structures and relations of the
plant and the insect have to be worked out by a series of concurrent
developments so complex and absolutely incalculable in the aggregate,
that the cycles and epicycles of the Ptolemaic astronomy were child's
play in comparison, we need not wonder that the common sense of mankind
revolts against such fancies, and that we are accused of attempting to
construct the universe by methods that would baffle Omnipotence itself,
because they are simply absurd. In this aspect of them, indeed, such
speculations are necessarily futile, because no mind can grasp all the
complexities of even any one case, and it is useless to follow out an
imaginary line of development which unexplained facts must contradict
at every step. This is also, no doubt, the reason why all recent
attempts at constructing "Phylogenies" are so changeable, and why no
two experts can agree about almost any of them.
A second aspect in which such speculations are too partial, is in
the unwarranted use which they make of analogy. It is not unusual to
find such analogies as that between the embryonic development of the
individual animal and the succession of animals in geological time
placed on a level with that reasoning from analogy by which geologists
apply modern causes to explain geological formations. No claim could
be more unfounded. When the geologist studies ancient limestones built
up of the remains of corals, and then applies the phenomena of modern
coral reefs to explain their origin, he brings the latter to bear
on the former by an analogy which includes not merely the apparent
results, but the causes at work, and the conditions of their action,
and it is on this that the validity of his comparison depends, in so
far as it relates to similarity of mode of formation. But when we
compare the development of an animal from an embryo cell with the
progress of animals in time, though we have a curious analogy as to the
steps of the process, the conditions and causes at work are known to
be altogether dissimilar, and therefore we have no evidence whatever
as to identity of cause, and our reasoning becomes at once the most
transparent of fallacies. Further, we have no right here to overlook
the fact that the conditions of the embryo are determined by those of
a previous adult, and that no sooner does this hereditary potentiality
produce a new adult animal, than the terrible external agencies of the
physical world, in presence of which all life exists, begin to tell on
the organism, and after a struggle of longer or shorter duration it
succumbs to death, and its substance returns into inorganic nature, a
law from which even the longer life of the species does not seem to
exempt it. All this is so plain and manifest that it is extraordinary
that evolutionists will continue to use such partial and imperfect
arguments. Another illustration may be taken from that application
of the doctrine of natural selection to explain the introduction of
species in geological time, which is so elaborately discussed by Sir
C. Lyell in the last edition of his "Principles of Geology." The great
geologist evidently leans strongly to the theory, and claims for it
the "highest degree of probability," yet he perceives that there is a
serious gap in it; since no modern fact has ever proved the origin of a
new species by modification. Such a gap, if it existed in those grand
analogies by which he explained geological formations through modern
causes, would be admitted to be fatal.
A third illustration of the partial character of these hypotheses may
be taken from the use made of the theory deduced from modern physical
discoveries, that life must be merely a product of the continuous
operation of physical laws. The assumption for it is nothing more
that the phenomena of life are produced merely by some arrangement
of physical forces, even if it be admitted to be true, gives only a
partial explanation of the possible origin of life. It does not account
for the fact that life, as a force, or combination of forces, is set in
antagonism to all other forces. It does not account for the marvellous
connection of life with organization. It does not account for the
determination and arrangement of forces implied in life. A very simple
illustration may make this plain. If the problem to be solved were the
origin of the mariner's compass, one might assert that it is wholly a
physical arrangement, both as to matter and force. Another might assert
that it involves mind and intelligence in addition. In some sense
both would be right. The properties of magnetic force and of iron or
steel are purely physical, and it might even be within the bounds of
possibility that somewhere in the universe a mass of natural lodestone
may have been so balanced as to swing in harmony with the earth's
magnetism. Yet we would surely be regarded as very credulous if we
could be induced to believe that the mariner's compass has originated
in that way. This argument applies with a thousandfold greater force to
the origin of life, which involves even in its simplest forms so many
more adjustments of force and so much more complex machinery.
Fourthly, these hypotheses are partial, inasmuch as they fail to
account for the vastly varied and correlated interdependencies of
natural things and forces, and for the unity of plan which pervades the
whole. These can be explained only by taking into the account another
element from without. Even when it professes to admit the existence
of a God, the evolutionist reasoning of our day contents itself
altogether with the physical or visible universe, and leaves entirely
out of sight the power of the unseen and spiritual, as if this were
something with which science has nothing to do, but which belongs only
to imagination or sentiment. So much has this been the case, that when
recently a few physicists and naturalists have referred to the "Unseen
Universe," they have seemed to be teaching new and startling truths,
though only reviving some of the oldest and most permanent ideas of
our race. From the dawn of human thought it has been the conclusion
alike of philosophers, theologians, and the common sense of mankind,
that the seen can be explained only by reference to the unseen, and
that any merely physical theory of the world is necessarily partial.
This, too, is the position of our sacred Scriptures, and is broadly
stated in their opening verse, and indeed it lies alike at the basis
of all true religion and all sound philosophy, for it must necessarily
be that "the things that are seen are temporal, the things that are
unseen, eternal." With reference to the primal aggregation of energy in
the visible universe, with reference to the introduction of life, with
reference to the soul of man, with reference to the heavenly gifts of
genius and prophecy, with reference to the introduction of the Saviour
Himself into the world, and with reference to the spiritual gifts and
graces of God's people, all these spring, not from sporadic acts of
intervention, but from the continuous action of God and the unseen
world; and this, we must never forget, is the true ideal of creation in
Scripture and in sound theology. Only in such exceptional and little
influential philosophies as that of Democritus, and in the speculations
of a few men carried off their balance by the brilliant physical
discoveries of our age, has this necessarily partial and imperfect view
been adopted. Never, indeed, was its imperfection more clear than in
the light of modern science.
Geology, by tracing back all present things to their origin, was the
first science to establish on a basis of observed facts the necessity
of a beginning and end of the world. But even physical science now
teaches us that the visible universe is a vast machine for the
dissipation of energy; that the processes going on in it must have had
a beginning in time, and that all things tend to a final and helpless
equilibrium. This necessity implies an unseen power, an invisible
universe, in which the visible universe must have originated, and to
which its energy is ever returning. The hiatus between the seen and the
unseen may be bridged over by the conceptions of atomic vortices of
force, and by the universal and continuous ether; but whether or not,
it has become clear that the conception of the unseen, as existing,
has become necessary to our belief in the possible existence of the
physical universe itself, even without taking life into account.
It is in the domain of life, however, that this necessity becomes most
apparent; and it is in the plant that we first clearly perceive a
visible testimony to that unseen which is the counterpart of the seen.
Life in the plant opposes the outward rush of force in our system,
arrests a part of it on its way, fixes it as potential energy, and
thus, forming a mere eddy, so to speak, in the process of dissipation
of energy, it accumulates that on which animal life and man himself may
subsist, and assert for a time supremacy over the seen and temporal on
behalf of the unseen and eternal. I say, for a time, because life is,
in the visible universe, as at present constituted, but a temporary
exception, introduced from that unseen world where it is no longer
the exception but the eternal rule. In a still higher sense, then,
than that in which matter and force testify to a Creator, organization
and life, whether in the plant, the animal, or man, bear the same
testimony, and exist as outposts put forth in the succession of ages
from that higher heaven that surrounds the visible universe. In them,
too, Almighty power is no doubt conditioned or limited by law; yet they
bear more distinctly upon them the impress of their Maker, and, while
all explanations of the physical universe which refuse to recognise its
spiritual and unseen origin must necessarily be partial and in the end
incomprehensible, this destiny falls more quickly and surely on the
attempt to account for life and its succession on merely materialistic
principles.
Here again, however, we must bear in mind that creation, as maintained
against such materialistic evolution, whether by theology, philosophy,
or Holy Scripture, is necessarily a continuous, nay, an eternal,
influence, not an intervention of disconnected acts. It is the true
continuity, which includes and binds together all other continuity.
It is here that natural science meets with theology, not as an
antagonist, but as a friend and ally in its time of greatest need;
and I must here record my belief that neither men of science nor
theologians have a right to separate what God in Holy Scripture has
joined together, or to build up a wall between nature and religion, and
write upon it, "no thoroughfare." The science that does this must be
impotent to explain nature, and without hold on the higher sentiments
of man. The theology that does this must sink into mere superstition.
In conclusion, can we formulate a few of the general laws, or perhaps I
had better call them the general conclusions, respecting life, in which
all Palæontologists may agree. Perhaps it is not possible to do this at
present satisfactorily, but the attempt may do no harm. We may, then, I
think, make the following affirmations:--
1. The existence of life and organization on the earth is not eternal,
or even coeval with the beginning of the physical universe, but may
possibly date from Laurentian or immediately pre-Laurentian ages.
2. The introduction of new species of animals and plants has been a
continuous process, not necessarily in the sense of derivation of
one species from another, but in the higher sense of the continued
operation of the cause or causes which introduced life at first. This,
as already stated, I take to be the true theological or Scriptural as
well as scientific idea of what we ordinarily and somewhat loosely term
creation.
3. Though thus continuous, the process has not been uniform; but
periods of rapid production of species have alternated with others in
which many disappeared and few were introduced. This may have been an
effect of physical cycles reacting on the progress of life.
4. Species, like individuals, have greater energy and vitality in
their younger stages, and rapidly assume all their varietal forms, and
extend themselves as widely as external circumstances will permit. Like
individuals also, they have their periods of old age and decay, though
the life of some species has been of enormous duration in comparison
with that of others; the difference appearing to be connected with
degrees of adaptation to different conditions of life.
5. Many allied species, constituting groups of animals and plants, have
made their appearance at once in various parts of the earth, and these
groups have obeyed the same laws with the individual and the species in
culminating rapidly, and then slowly diminishing, though a large group
once introduced has rarely disappeared altogether.
6. Groups of species, as genera and orders, do not usually begin with
their highest or lowest forms, but with intermediate and generalized
types, and they show a capacity for both elevation and degradation in
their subsequent history.
7. The history of life presents a progress from the lower to the
higher, and from the simpler to the more complex, and from the more
generalized to the more specialized. In this progress new types are
introduced, and take the place of the older ones, which sink to a
relatively subordinate place, and become thus degraded. But the
physical and organic changes have been so correlated and adjusted
that life has not only always maintained its existence, but has been
enabled to assume more complex forms, and thus older forms have been
made to prepare the way for newer, so that there has been, on the
whole, a steady elevation culminating in man himself. Elevation and
specialization have, however, been secured at the expense of vital
energy and range of adaptation, until the new element of a rational and
inventive nature was introduced only in the case of man.
8. In regard to the larger and more distinct types, we cannot find
evidence that they have, in their introduction, been preceded by
similar forms connecting them with previous groups; but there is reason
to believe that many supposed representative species in successive
formations are really only races or varieties.
9. In so far as we can trace their history, specific types are
permanent in their characters from their introduction to their
extinction, and their earlier varietal forms are similar to their later
ones.
10. Palæontology furnishes no direct evidence, perhaps never can
furnish any, as to the actual transformation of one species into
another, or as to the actual circumstances of creation of a species;
but the drift of its testimony is to show that species come in _per
saltum_, rather than by any slow and gradual process.
11. The origin and history of life cannot, any more than the origin
and determination of matter and force, be explained on purely material
grounds, but involve the consideration of power referable to the unseen
and spiritual world.
Different minds may state these principles in different ways, but I
believe that in so far as palæontology is concerned, in substance they
must hold good, at least as steps to higher truths. And now allow me
to say that we should be thankful that it is given to us to deal with
so great questions, and that in doing so, deep humiliation, earnest
seeking for truth, patient collection of all facts, self-denying
abstinence from hasty generalizations, forbearance and generous
estimation with regard to our fellow labourers, and reliance on that
Divine Spirit which has breathed into us our intelligent life, and is
the source of all true wisdom, are the qualities which best become us.
But while the principles noted above may be said to be known laws of
the apparition of new forms of life, they do not reach to the secondary
efficient causes of the introduction of new species. What these may
ultimately prove to be, to what extent they can be known by us, and to
what extent they may include processes of derivation, it is impossible
now to say. At present we must recognise in the prevailing theories on
the subject merely the natural tendency of the human mind to grasp the
whole mass of the unknown under some grand general hypothesis, which,
though perhaps little else than a figure of speech, satisfies for the
moment. We are dealing with the origin of species precisely as the
alchemists did with chemistry, and as the Plutonists and Neptunists
did with geology; but the hypotheses of to-day may be the parents
of investigations which will become real science to-morrow. In the
meantime it is safe to affirm that whatever amount of truth there may
be in the several hypotheses which have engaged our attention, there is
a creative force above and beyond them, and to the threshold of which
we shall inevitably be brought, after all their capabilities have been
exhausted by rigid investigation of facts. It is also consolatory to
know that species, in so far as the Modern period, or any one past
geological period may be concerned, are so fixed that for all practical
purposes they may be regarded as unchanging. They are to us what the
planets in their orbits are to the astronomer, and speculations as
to the origin of species are merely our nebular hypotheses as to the
possible origin of worlds and systems.
References:--Address as Vice-President of American Association at
Detroit, 1875. "The Chain of Life in Geological Time," London,
1879. Addresses to Natural History Society of Montreal, published
in _Canadian Naturalist_, "Apparition of Animal Forms," _Princeton
Review_.
_THE GENESIS AND MIGRATIONS OF PLANTS._
DEDICATED TO THE MEMORY OF
DR. OSWALD HEER,
The Able and Successful Student of the later Floras
of the Northern Hemisphere.
Geological Periods as Related to Plants--Arctic Origin of Floras--The
Devonian Flora--Arctic Climates of the Past--History of Some Modern
Forms--Laws of the Succession
[Illustration: Vegetation of the Middle Devonian or Erian, restored
from actual specimens (p. 202).]