Transcriber Note
Text emphasis displayed as _Italics_ and =Bold=.
RELICS OF PRIMEVAL LIFE
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[Illustration: Cryptozoon Boreale, _Dawson_.]
Two divisions or branches of a large specimen collected by Mr. E. T.
Chambers in the Ordovician of Lake St. John. (See Appendix D.)
[_Frontis._
RELICS OF PRIMEVAL LIFE
BEGINNING OF LIFE IN THE DAWN OF GEOLOGICAL TIME
BY
SIR J. WILLIAM DAWSON LL.D., F.R.S., Etc.
_WITH SIXTY-FIVE ILLUSTRATIONS_
[Illustration]
NEW YORK CHICAGO TORONTO FLEMING H. REVELL COMPANY
1897
_The substance of a Course of Lectures on Pre-Cambrian Fossils
delivered in the Lowell Institute, Boston, in November, 1895._
To
AUGUSTUS LOWELL Esq
_Vice-President of the American Academy of Arts and Sciences
Trustee of the Lowell Institute_
AS THE WISE AND LIBERAL ADMINISTRATOR OF A NOBLE
ENDOWMENT FOR THE ADVANCEMENT AND DIFFUSION
OF KNOWLEDGE
THIS WORK IS DEDICATED
WITH MUCH RESPECT AND ESTEEM
BY THE AUTHOR
PREFACE
IT is now more than thirty-five years since the announcement was made
of the discovery of remains supposed to indicate the existence of
animal life in the oldest rocks known to geologists. It was hailed with
enthusiasm by some as "opening a new era in geological science"; but
was regarded with scepticism by others, in consequence of the condition
and mineral character of the supposed fossil, and because of the great
interval in time between the oldest animal remains previously known and
these new claimants for recognition. Since that time, many new facts
have been learned, and the question has been under almost continuous
discussion and debate, with various fortunes, in different quarters.
The author was associated with the original discovery and description
of these supposed earliest traces of life; and has since, in the
intervals of other work, devoted much time to further exploration and
research, the results of which have been published from time to time in
the form of scientific papers. He has also given attention to the later
discoveries which have tended to fill up the gap between the Laurentian
fossil and its oldest known successors.
In 1875 he endeavoured to sum up in a popular form what was then known,
in a little volume named "The Dawn of Life," which has long been out of
print; and in 1893 the matter was referred to in a chapter of his work
"Salient Points in the Science of the Earth." In 1895 he was invited
to present the subject to a large and intelligent audience in a course
of lectures delivered in the Lowell Institute, Boston; and the success
which attended these lectures has induced him to reproduce them in the
present work, in the hope that inquiries into the Dawn of Life may
prove as fascinating to general readers as to those who prosecute them
as a matter of serious work, and that their presentation in this form
may stimulate further research in a field which is destined in the
coming years to add new and important domains to the knowledge of life
in the early history of the earth.
Hypotheses respecting the introduction and development of life are
sufficiently plentiful; but the most scientific method of dealing
with such questions is that of searching carefully for the earliest
remains of living beings which have been preserved to us in the rocky
storehouses of the earth.
There are many earnest labourers in this difficult field, and it will
be the object of the writer in the following pages to do justice to
their work as far as known to him, as well as to state his own results.
J. W. D.
CONTENTS
I
PAGE
The Chain of Life Traced Backward in Geological Time 3
II
Life in the Early Cambrian 17
III
Pre-Cambrian Life 47
IV
Foundations of the Continents, and their General
Testimony as to Life 79
V
Probabilities as to Laurentian Life, and Conditions
of its Preservation 107
VI
The History of a Discovery 125
VII
The Dawn of Life 147
VIII
Contemporaries of Eozoon 193
IX
Difficulties and Objections 221
X
The Origin of Life 245
XI
Some General Conclusions 281
APPENDIX
A. Geological Relations of Eozoon, etc. 295
B. Organic Remains and Hydrous Silicates 298
C. Affinities of Eozoon, etc. 303
D. Cryptozoon 310
E. Receptaculites and Archæocyathus 315
F. Pre-Geological Evolution 320
G. Controversies respecting Eozoon 324
H. Notes to Appendix, December, 1896 329
LIST OF ILLUSTRATIONS
FIG. PAGE
Cryptozoon Boreale _Frontispiece_
Map xvi
1. Olenellus 20
2. Triarthrus 23
3. Hymenocaris 27
4. Ctenichnites 32
5, 6. Archæocyathus 35
7, 8. Cryptozoon 37, 39
9. Fossils in Lower Cambrian Boulder 41
10. Section Hanford Brook 51
11. Worm Tracks 53
12. Pre-Cambrian Fossils 54
13. Arenicolites and Aspidella 54
14. Cryptozoon 56
15. Worm Burrows 67
16. Casts of Foraminifera 68
17. Tudor Eozoon 69
18. Laurentian America 85
19. Map of Grenville Limestones 88
19A. Attitude of Limestone, Côte St. Pierre 91
20, 21. Disturbed Beds 103
22. Section of Limestone 113
23. Silicification of Coral 113
24. Cast of Polystomella in Glauconite 115
24A. Crinoid and Shell in Glauconite 116
25. Nature-print of Eozoon 121
26, 27. Eozoon from Calumet 130
28, 29. Canals of Eozoon 133
30, 31. Canals and Tubuli 135
32. General Form of Eozoon 149
33, 34. Eozoon with Funnels 152, 153
35. Small Specimen and Structure 155
36. Decalcified Eozoon 157
37. Finest Tubuli filled with Dolomite 158
38. Arrangement of Canals 159
39-41. Finest Tubuli 160-2
42. Canals after Möbius 163
43. Stromatocerium 172
44. Stromatopora 173
45. Cœnostroma 174
46. Recent Protozoa 176
47. Fragmental Eozoon 183
48, 49. Nummulites and Calcarina 186
50, 51. Archæospherinæ 190, 200
52. Acervuline Eozoon 205
53, 54. Archæospherinæ 205, 208
55. Ditto, Finland 212
56. Eozoon Bavaricum 213
57. Archæozoon 215
58. Restoration of Eozoon 230
59. Eozoon in Different States 237
60. Nature-print of Large Specimen _To face_ 296
[Illustration: _Walker & Boutall SCt_
Grenville Series on the Ottawa River (17 miles to an inch).
_From Logan's Original Map of 1865._]
_THE CHAIN OF LIFE TRACED BACKWARD IN GEOLOGICAL TIME_
GEOLOGICAL CHRONOLOGY OF LIFE.
_After Prof. C. A. White._
=Column Key=--_Using First Letter_
----------------------------------------
Invertebrates. Vertebrates. Plants.
-------------- ------------- ---------
Protozoa Ganoid Fishes Algæ
Corals, etc. Telios Fishes Land Cryptogs
Echinoids Batrachians Phænogams
Worms Reptiles
Mollusks Dinosaurs
Arthropods Birds
Insects Marsupials
Land Snails Placentals
Humans
Geological Invertebrates. Vertebrates. Plants.
Systems or
Periods. P C E W M A I L G T B R D B M P H A L P
+------------------------------------------+
Kainozoic. | ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖|
{ Cretaceous ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖|
{ |-‖-‖-‖-‖-‖-‖-‖-‖--‖-‖-‖-‖-‖-‖-‖------‖-‖--|
{ Jurassic ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ |
{ |-‖-‖-‖-‖-‖-‖-‖-‖--‖---‖-‖-‖---‖------‖-‖--|
{ Triassic ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ |
|-‖-‖-‖-‖-‖-‖-‖-‖--‖---‖-‖------------‖-‖--|
Palæozoic. | ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ |
{ Permian ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ |
{ |-‖-‖-‖-‖-‖-‖-‖-‖--‖---‖--------------‖-‖--|
{ Carboniferous ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ |
{ |-‖-‖-‖-‖-‖-‖-‖-‖--‖------------------‖-‖--|
{ Devonian ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ |
{ |-‖-‖-‖-‖-‖-‖-‖----‖------------------‖-‖--|
{ Silurian ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ |
{ |-‖-‖-‖-‖-‖-‖-------------------------‖-‖--|
{ Ordovician ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ |
{ |-‖-‖-‖-‖-‖-‖-------------------------‖----|
{ Cambrian ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ |
{ |-‖-‖-‖-‖-‖-‖-------------------------‖----|
{ Etcheminian ...| ‖ ‖ ‖ ‖ ‖ ‖ |
|-‖-‖-‖-‖-----------------------------‖----|
Eozoic. | ‖ ‖ ‖ ‖ |
{ Huronian ...| ‖ · |
{ |-‖----------------------------------------|
{ Laurentian. | ‖ |
{ Grenvillian ...| ‖ |
{ |------------------------------------------|
{ Archæan ...| |
| |
+------------------------------------------+
Note.--It is not supposed that the Geological Periods were of equal
lengths, as represented in the diagram.
ERRATA.
Where _Cryptozoon prolificum_ occurs in the text, read _Cryptozoon
proliferum_.
[Transcriber Note: Errata Corrections HAVE BEEN applied to text!]
I
_THE CHAIN OF LIFE TRACED BACKWARD IN GEOLOGICAL TIME_
In infancy we have little conception of the perspective of time. To us
the objects around us and even our seniors in age seem to have always
been, and to have had no origin or childhood. It is only as we advance
in knowledge and experience that we learn to recognise distinctions
of age in beings older than ourselves. In thinking of this, it seems
at first sight an anomaly, or at least contrary to analogy, that the
oldest literature and philosophy deal so much with doctrines as to the
origins of things. In this respect primitive men do not seem to have
resembled children; and the fact that our own sacred records begin with
answers to such questions, and that these appear in the oldest literary
remains of so many ancient nations, and even in the folk-lore of
barbarous tribes, might be used as an additional argument in favour of
an early Divine revelation on such subjects, as a means of awakening
primitive men to the comprehension of their own place in the universe.
However this may be, it is certain that modern science at first took a
different stand.
The constancy of the motions of the heavenly bodies, our great
time-keepers, and of the changes on the earth depending upon them, and
the resolution of apparent perturbations into cycles of greater or less
length, impressed astronomers and physicists with the permanence of the
arrangements of the heavens and their eternal circling round without
any change. In like manner, on the rise of geology, the succession of
changes recorded in the earth seemed interminable, and Hutton could
say that in the geological chronology he could see "no vestige of a
beginning, no prospect of an end."
But the progress of investigation has changed all this, and has brought
physical and natural science back to a position nearer to that of
the old cosmogonies. Physical astronomy has shown that the constant
emission of heat and light from the sun and other stars must have had
a beginning, and is hurrying on toward an end, that the earth and
its satellite the moon are receding from each other, and that even
the spinning of our globe on its axis is diminishing in rapidity.
In summing up these and other changes, Lord Kelvin says: "To hold
the doctrine of the eternity of the universe would be to maintain a
stupendous miracle, and one contrary to the fundamental laws of matter
and force."
So, on our earth itself, we can now assign to their relative ages
those great mountain chains which have been emblems of eternity. We
can transfer ourselves in imagination back to a time when man and his
companion animals of to-day did not exist, when our continents and
seas had not assumed their present forms, and even when the earth was
an incandescent mass with all its volatile materials suspended in its
atmosphere. It is true that in all the changes which our earth has
undergone the same properties of matter and the same natural laws
have prevailed; but the interactions of these properties and laws
have been tending to continuous changes in definite directions, and
not infrequently to accumulations of tension leading to paroxysmal
vicissitudes.
If all this is true of the earth itself, it is especially applicable
to its living inhabitants. Successive dynasties of animals and plants
have occupied the earth in the course of geological time; and as we go
back in the record of the rocks, first man himself and, in succession,
all the higher animals disappear, until at length in the oldest
fossiliferous beds only a portion of the more humble inhabitants of the
sea can be found. In the time of the formation of the oldest of these
rocks, or perhaps somewhat earlier, must have been the first beginning
of life on our planet.
Just as we can trace every individual animal to a microscopic germ in
which all its parts were potentially present, so we can trace species,
genera, and larger groups of animals to their commencement at different
points of the earth's history, and can endeavour to follow the lines
of creation or descent back to the first beings in which vital powers
manifested themselves. All such beginnings must end in mystery, for
as yet we do not know how either a germ or a perfect animal could
originate from inanimate matter; but we may hope at least to make some
approximation to the date of the origin of life and to a knowledge of
the conditions under which it began to exist, confining ourselves for
the present principally to the Animal Kingdom.
As preliminary to the consideration of this subject, we may shortly
notice the grades of animals at present existing, and then the evidence
which we have of their successive appearance in different periods of
geological time, in order that we may eliminate all those of more
recent origin, in so far as the knowledge at present available will
permit, and restrict our consideration to forms which seem to have been
the earliest. In attempting this, we may use for reference the table
of geological periods and animal types presented in the diagram facing
this chapter, which is based on one prepared by Prof. Charles A. White,
of the United States Geological Survey, with modifications to adapt it
to our present purpose. In this table the leading groups of animals are
represented by lines stretching downward in the geological column of
formations as far as they have yet been traced. Such a table, it must
be observed, is always liable to the possibility of one or more of its
lines being extended farther downward by new discoveries.
The broadest general division of the Animal Kingdom is into back-boned
animals (Vertebrates) and those which have no back-bone or equivalent
structure (Invertebrates).[1] The former includes, besides man
himself, the familiar groups of Beasts, Birds, Reptiles, and Fishes.
The latter consists of the great swarms of creatures included under
the terms Insects, Crustaceans, Worms, Cuttle-fishes, Snails, Bivalve
Mollusks, Star-fishes, Sea-urchins, Coral Animals, Sea-jellies.
Sponges, and Animalcules. This mixed multitude of animals, mostly of
low grade and aquatic. Includes a vast variety of forms, which, though
comparatively little known to ordinary observers, are vastly numerous,
of great interest to naturalists, and, as we shall find, greatly older
in geological date than the higher animals.
[Footnote 1: The twofold primary division now sometimes used, into
Metazoa and Protozoa, seems more arbitrary and unequal, and therefore
of less practical value.]
It will be seen by a glance at the diagram that the higher vertebrates
are of most recent origin, man himself coming in as one of the newest
of all. Only the lower reptiles or batrachians and the fishes extend
very far back in geological time. None of the other vertebrate groups
reach, so far as yet known, farther back than the middle of the
geological scale--probably in point of time very much less than this.
Those of the invertebrates that breathe air reach no farther back than
the fishes, possibly not so far. On the other hand, all the leading
groups of marine invertebrates run without interruption back to the
Lower Cambrian, and some of them still farther. Thus it would appear
that for long ages before the introduction of land or air-breathing
animals of any kind, the sea swarmed with animal life, which was almost
as varied as that which now inhabits it. The reasons of this would seem
to be that the better support given by the water makes less demands
upon organs for mechanical strength, that the water preserves a more
uniform temperature than the air, and that arrangements for respiration
in water are less elaborate than those necessary in air. Hence the
conditions of life are, so to speak, easier in water than in air, more
especially for creatures of simple structure and low vital energy.
Besides this, the waters occupy two-thirds of the surface of the earth,
and in earlier periods probably covered a still greater area.
We are now in a position to understand that the Animal Kingdom had
not one but many beginnings, its leading types arriving in succession
throughout geological time. Thus the special beginning of any one line
of life, or those of different lines, might form special subjects
of inquiry; but our present object is to inquire as to the first or
earliest introduction of life in our planet, and in what form or forms
it appeared. We may, therefore, neglect all the vertebrate animals and
the air-breathing invertebrates, and may restrict our inquiries to
marine invertebrates.
In relation to these, six of the larger divisions or provinces of the
Animal Kingdom may suffice to include all the lower inhabitants of the
ocean, whether now or in some of the oldest fossiliferous rocks.[2]
[Footnote 2: Some modern zoologists, having perhaps, like some of the
old Greeks, lost the idea of the unity of nature, or at least that of
one presiding divinity, prefer for the larger divisions of animals
the term _phylum_ or _phylon_, implying merely a stock, race or kind,
without reference to a definite place in an ordered kosmos.]
Looking more in detail at our diagram, we observe that the higher
vertebrates nearest to man in structure extend back but a little
way, or, with a few minor exceptions, only as far as the beginning
of the Kainozoic or Tertiary Period, in the later part of which
we still exist. Other air-breathing vertebrates, the birds and
the true reptiles, extend considerably farther, to the beginning
of the previous or Mesozoic Period. The amphibians, or frog-like
reptiles, reach somewhat farther, and the fishes and the air-breathing
arthropods farther still. On the other hand, our six great groups of
marine invertebrates run back for a vast length of time, without any
companions, to the lowest Palæozoic, and this applies to their higher
types, the cuttles and their allies, and the crustaceans, as well as
to the lower tribes. Turning now again to our table, we find that
these creatures extend in unbroken lines back to the Lower Cambrian,
the oldest beds in which we find any considerable number of organic
remains, and leave all the other members of the Animal Kingdom far
behind.
If now we endeavour to arrange the leading groups of these persistent
invertebrates under a few general names, we may use the following,
beginning with those highest in rank:--
(1) _Insects_ and _Crustaceans_ (Arthropoda).
(2) _Cuttles, univalve and bivalve Shell-fishes_ (Mollusca).
(3) _Worms_ (Annelida).
(4) _Sea-urchins_ and _Sea-stars_ (Echinodermata).
(5) _Coral Animals_, _Sea-anemones_, and _Sea-jellies_ (Cœlenterata).
(6) _Sponges_, _Foraminifera_ and _Animalcules_ of simple organization
(Protozoa).
There are, it is true, some animals allied to the mollusks and worms,
which might be entitled to form separate groups, though of minor
importance The position of the sponges is doubtful, and the great mass
of Protozoa may admit of subdivision; but for our present purpose these
six great groups or provinces of the Animal Kingdom may be held to
include all the humbler forms of aquatic life, and they keep company
with each other as far as the Early Cambrian. If, in accordance with
the previous statements, we choose to divide the earth's history by
the development of animal life rather than by rock formations, and to
regard each period as presided over by dominant animal forms, we shall
thus have an age of man, an age of mammals, an age of reptiles and
birds, an age of amphibians and fishes, and an age of crustaceans and
mollusks.
It is only within recent years that the researches more especially of
Barrande, Hicks, Lapworth, Linarrson, Brögger, and others in Europe,
and of Matthew, Ford and Walcott in America, have enlarged the known
animals of the Lower Cambrian to nearly 200 species, and below this
we know as yet very little of animal life. We may therefore take the
Lower Cambrian, or "Olenellus Zone" as it has been called from one of
its more important crustaceans,[3] as our starting-point for plunging
into the depths below. In doing so, we may remark on the orderly and
symmetrical nature of the chain of life, and on the strange fact that
for so long ages animal life seems to have been confined to the waters,
and to have undergone little development toward its higher forms. It
is like a tree with a tall branchless stem bearing all its leaves and
verdure at the top, or like some obscure tribe of men long living in
isolation and unknown to fame, and then, under some hidden impulse
or opportunity, becoming a great conquering and dominant nation. Or
to compare it with higher things, it is like the Christian religion,
for ages confined to a small and comparatively unimportant people,
and developing slowly its faith and hopes, and then suddenly, under
the personal influence of Christ and His apostles, spreading itself
over the world, and in a few centuries becoming the ruling power in
its greatest empire, surviving the fall of this and permeating all
the great nations that sprang from its ruins. God's plans in nature,
in history, and in grace seem to us very slow in their growth and
maturity, but they are very sure.
[Footnote 3: See figure, p. 20.]
_LIFE IN THE EARLY CAMBRIAN_
II
_LIFE IN THE EARLY CAMBRIAN_
In the old Chaldean fable of the descent of Ishtar into Hades, to
recover her lost Tammuz, at each successive gate of the lower regions
she is stripped of some of her ornaments and garments, till at length
she has to appear naked and unadorned in the presence of the lord of
the Nether World. So in our descent from the surface on which men
live, through the successive rocky layers of the earth's crust, we
leave behind, one by one, all the higher forms of life with which we
are familiar; but there still remain to us our six groups of aquatic
invertebrates, in the guise, it is true, of species and genera now
unknown in a living state, yet well represented as far down as the
lower part of the Cambrian. Let us now suppose that we take our stand
on the shores of the Cambrian sea, or cast our dredge into its waters
in search of these old animals; though we can only actually do so by
painfully hammering and chiselling them out of their rocky tombs, and
this often in fragments which must be put together before we can fully
realize the forms and structures of the animals to which they belonged.
We may pause here, however, to remark that neither the geographical
nor climatal conditions of the earth at this early time were similar
to these with which we are now familiar. The marine animals of the
Cambrian have left their remains in beds of sediment, which now
constitute rocks forming parts of our continents remote from the sea,
and much elevated above its level, showing that large areas, then under
the ocean, are now dry land; while there is no good evidence that
the sea and land have changed places. The facts rather indicate that
the continents have extended their area at the expense of the ocean,
which has, however, probably increased in depth. In evidence of these
statements, I need only mention that some of the oldest rocks in the
Scottish and Welsh hills, in Scandinavia, in Russia and in Bohemia, are
rich in Cambrian marine fossils.
[Illustration: Fig. 1.--_Olenellus Thompsoni_, Hall.
A characteristic Trilobite of the Lower Cambrian in North America.
After Walcott and specimen in Peter Redpath Museum.]
In America, in like manner, such rocks are found on the flanks of the
Appalachians, in New Brunswick, and in Newfoundland, in the table-land
of Colorado and in the Rocky Mountains. In point of fact, a map of
the Northern Hemisphere at this period would show only a limited
circumpolar continent with some outlying islands to the south of it,
and shallows stretching across the northern part of the areas of the
present Atlantic and Pacific Oceans. The great ocean, however, thus
extending over most of the temperate and tropical parts of the Northern
Hemisphere, was probably also more muddy and shallow than that of
modern times. The surface temperature of this vast ocean was also,
it is probable, more uniform than that of the modern sea, while even
its profounder depths or abysses would have more earth-heat than at
present. Thus we may, without hesitation, affirm that in this early
age the conditions for the introduction of swarming marine life of low
grade, and its extension over the whole earth, were at a maximum.
Let us inquire, then, what these old Cambrian seas actually produced,
more especially in the early portions of that ancient and probably
protracted time.
The most highly organized type of which we have any certain evidence
is that of the Crustacea, the group to which our modern lobsters and
crabs belong, and its most prominent representatives are the trilobites
(Figs. 1, 2), so called from the three lobes into which the body is
divided. These creatures are indeed remarkable for the twofold property
of bilateral symmetry, and fore and aft jointed structure, both based
on the number three. From front to rear we have a large head, usually
with well-developed eyes and oral organs, a middle or thoracic part
composed of a series of movable segments, and a tail-piece sometimes
small, sometimes nearly as large as the head. Transversely, the body
is divided into a central and two lateral lobes, which can be seen in
the head, the thorax, and usually in the tail as well. The organization
of these animals must have been as complex as that of most existing
Crustaceans. Their nerve system must have been well developed; a vast
number of muscles were required to move the different parts of the
trunk, and the numerous and complex limbs which have been observed
in some of the species, and no doubt were possessed by all. Their
digestive and circulatory organs must have been in proportion to the
complexity of their locomotive organs.
[Illustration: Fig. 2.--_Triarthrus Becki_, Green.
A Trilobite of primitive type, showing its limbs and antennæ. (After
Beecher.)]
Figure 2, borrowed from Beecher,[4] shows the limbs of a species,
not of the Lower Cambrian, but of a somewhat later formation. There
can be no doubt, however, that those of earlier species were equally
perfect, more especially as Triarthrus is an animal of an old type
approaching to extinction in the age succeeding the Cambrian, and its
representatives in the earlier and palmy days of the family could not
have been inferior in organization. These creatures swarmed in every
sea in the Cambrian period, and were represented by a great number of
species, some of them of large size, others very small; some
many-jointed, others few-jointed, and with a great variety of tubercles,
spines, and other ornamental and protective parts. If we ask for their
affinities and place in the great group of Crustacea, the answer must
be that, while in some points allied to the higher forms, they approach
most nearly to those which occupy a medium position in the class, and
are, in fact, a composite type, presenting points of structure now
distributed among different groups. If we ask for affinities with lower
groups, we have to reply that their nearest allies in this direction
are the bristle-footed marine worms; but there is a vast gap, both
in the Cambrian and Modern seas, between any of these worms and the
Crustacea, which, either as embryos or as adults, have any resemblance
to them.
[Footnote 4: _American Journal of Science_, 1896.]
The Trilobites, after appearing in a great variety of generic and
specific forms, and playing a most important part in their time,
were not destined to continue beyond the Carboniferous period, and
before that time they were beginning to give place to the Limuli,
King-crabs, or Horseshoe-crabs, a few species of which continue on our
coasts until the present time. In this limited duration the Trilobites
present a strange contrast to certain shrimp-like Crustaceans, their
contemporaries (the Phyllopods), which very closely resemble some
still extant, and the same remark applies to swarms of little bivalve
Crustaceans (Ostracods), which are still represented by hosts of modern
species both in the sea and in the fresh waters. There is, however,
a remarkable group of shrimp-like Crustaceans, represented in the
modern world by only a few small species, which in the Cambrian age
attained greater size, and constitute a very generalized type combining
characters now found in lower and higher groups of Crustacea.
_Hymenocaris vermicauda_ of Salter (Fig. 3) may serve to illustrate one
of these primitive forms.
[Illustration: Fig. 3.--_Hymenocaris vermicauda_, Salter.
A Lower Cambrian Shrimp of generalized type. (After Salter.)]
In point of fact, as Dr. Henry Woodward has shown in an able
presidential address delivered to the Geological Society in 1895, at
the base of the Lower Cambrian we still have several distinct groups
of Crustacea; and if with some we were to hold them as traceable to
one original form or to a worm-like ancestor, we must seek for this
far back in those pre-Cambrian rocks in which we find no Crustaceans
whatever. There is, it is true, no good reason to demand this; for
whatever the cause, secondary or final, which produced any form of
Crustacean in the Lower Cambrian, it might just as well have produced
several distinct forms. Evolutionists seem to be somewhat unreasonable
in demands of this kind, for any cause capable of originating a new
form of living being, might have been operative at the same time in
different localities and under somewhat diverse conditions, and may
also have acted at different times. All imaginary lines of descent of
animals are more or less subject to this contingency; and this may
partly account for the great diversity in the lines of affiliation
presented to us by evolutionists, which may in part have a basis in
fact in so far as distinct varietal and racial forms are concerned, but
may just as likely be entirely fallacious in the case of true species.
In any case, in the lowest rocks into which we can trace Crustacea, we
have already probably five of the orders into which their successors
in the modern seas are divided by zoologists; and this is certainly a
singular and suggestive fact, the significance of which we shall be
better prepared to understand at a later stage of our investigation.
Allied in some respects to the Crustacea, though much lower in grade,
are the marine Worms--a great and varied host--usually inhabiting the
shallower parts of the ocean; though the 330 species collected by the
_Challenger_ expedition show that they also abound in those greater
depths to which voyagers have only recently had access. Sea-worms seem
thus to be able to live in all depths, as well as in all climates; and
in accordance with this they abound in the oldest rocks, which are
often riddled with the holes caused by their burrowing, or abundantly
marked on the surfaces of the beds with their trails.
The great province of the Mollusca, in which, for our present purpose,
we may include some aberrant and rudimentary Molluscoids, is now best
known to us by its medium types, the univalve and bivalve Shell-fishes;
the higher group of the Cuttle-fishes and Nautili, though not uncommon,
being much less numerous, and one at least of the lower groups, the
Lamp-shells or Brachiopods, being represented in the modern world
by but few forms. The extension of the Mollusks backwards into the
Cambrian is remarkable as being on the whole meagre in comparison with
that of the Crustaceans, and as presenting only in small numbers the
types most common in later times. One or two shells, and perhaps some
tracks, represent the highest group: some forms resembling the floating
species of Sea-snails, and a very few ordinary bivalves represent
the types best known in the modern seas; while the Brachiopods, and
probably some still simpler forms, are in great comparative excess.
The individual specimens are also of small size, as if these creatures
were but insinuating themselves on the arena of life in insignificant
and humble forms. So far as yet known, the lowest groups supposed
to be allied to the Mollusks, the Ascidians or Sea-squirts, and the
Sea-mosses (Polyzoa), do not appear; but they may have been represented
by species which possessed no hard parts capable of preservation.
This leads us to the consideration that while all the Crustacea
necessarily possess some kind of crust or external skeleton, the
Mollusks are very different in this respect. While some of them have
ponderous shells, others even of the highest forms are quite destitute
of such protective parts. This again leads to a curious question
respecting the armature of the Trilobites. Some of these, even of the
larger species, have strong and formidable spines, like those of the
King-crabs and other modern Crustaceans. Now in the modern species we
know these organs to be intended to defend their possessors against the
attacks of fishes more swift and powerful than themselves. But what
enemies of this kind had the Trilobites to dread? Yet species a foot or
more in length presented great bayonet-like spines.
[Illustration: Fig. 4.--_Ctenichnites ingens_, Matthew.
A slab with markings of aquatic animals. From specimen in Peter Redpath
Museum.]
All that we know on this subject is that on the surfaces of the Lower
Cambrian rocks there are in some places complicated and mysterious
tracks or scratches, which seem to have been produced when the rock
was in the state of soft mud, by large and swiftly swimming animals
possessing some sort of arms or similar appendages (Fig. 4). Matthew
has ingeniously suggested that they may have been large Mollusks allied
to the modern gigantic Squids which still abound in the ocean, that
they may have been sufficiently powerful to prey on the Trilobites,
and, being swift swimmers, would have found them a helpless prey but
for their defensive spines. Yet such large Mollusks might have perished
without leaving any remains recognisable in the rocks, except what
may be termed their hand-writing on clay. A few small examples of the
shell-bearing species of these highest Mollusks, however, appear in
the Cambrian, and in the succeeding ages they become very abundant and
attain to large dimensions, again dwindling toward modern times. It
would thus seem that for some unknown reason the highest and lowest
Mollusks may have been locally plentiful, but the intermediate types
were rare.
The much lower group of Echinoderms, or Sea-urchins and Sea-stars,
curiously enough puts in but a small appearance in the Early Cambrian,
being represented, as far as yet known, by only one embryonic group,
the Cystideans. A little later, however, Feather-stars became greatly
abundant, and a little later still the true Star-fishes and Urchins.
The aberrant group of the Sea-slugs seems, so far as known, to be of
more modern origin; but most of these animals are soft-bodied, and
little likely to have been preserved.
The great group of the coral animals, so marked a feature of later
ages, is scarcely known in the oldest Cambrian, except by some
highly generalized forms[5] (Fig. 5). There are, however, small
Zoophytes referable to the lower type of Hydroids, and markings which
are supposed to be casts of stranded Jelly-fishes. If, with some
naturalists, we regard the Sponges as very humble members of the
coral group (Cœlenterata), then we have a right to add them to its
representatives in the lowest Cambrian; but perhaps they had better be
ranked with the next and lowest group of all--the Protozoa.
[Footnote 5: Dr. G. J. Hinde has carefully studied these forms, and
also similar species occurring in Lower Cambrian beds in different
parts of North America, Spain, Sardinia, and elsewhere. See note in the
Appendix, and _Journal Geol. Society of London_, vol. xlv. p. 125.]
[Illustration: Fig. 5.--_Archæocyathus profundus_, Billings.
Possibly a Coral of generalized type from the Lower Cambrian of L'Anse
à Loup, Labrador. A small specimen.]
[Illustration: Fig. 6.--_Structures of A. profundus (magnified)._
From specimens in Peter Redpath Museum.
(_a_) Lower acervuline portion. (_b_) Upper part, with three of the
radiating laminæ and section of pores, (_c_) Portion of lamina, with
pores, the calcareous skeleton unshaded.]
These are the humblest of all the inhabitants of the sea, presenting
very simple, jelly-like bodies with few organs, but sometimes producing
complex and beautiful calcareous and siliceous coverings or tests.
Animals of this type have been found in the Lower Cambrian, though not
in such vast multitudes as in some later formations. There are also in
the Cambrian some large, laminated, calcareous bodies (Cryptozoon of
Hall), to be noticed more fully below, and which have recently been
traced in still lower deposits even below the lowest Cambrian (Figs.
7, 8). These have some resemblance to the layer-corals or stromatoporæ
of the Silurian and Ordovician, which are by many regarded as the
skeletons of coral animals of a low type; but the microscopic structure
of Cryptozoon rather allies it with some of the larger forms of
Protozoa found higher up in the series of formations. We shall have to
discuss this later in connection with still older fossils.
[Illustration: Fig. 7.--_Cryptozoon proliferum_, Hall.
Portion of slab reduced in size. (After Hall.) See also Fig. 59, p.
237.]
[Illustration: Fig. 7_a_.--_Portion of thin section of Cryptozoon
proliferum (magnified × 50)._]
(_a_) Corneous layers, (_a¹_) One of these dividing, (_b_) Intermediate
stroma with granules of calcite, dolomite and quartz, traversed by
canals.
_From a Micro-photograph by_ Prof. Penhallow.
[_To face p. 39._
If now in imagination we cast our tow-net or dredge into the sea of the
Lower Cambrian, we may hope to take specimens illustrative of all our
six groups of invertebrate animals, and under several of them examples
of more than one subordinate group. Of the Crustaceans we might have
representatives of four or five ordinal groups, and of the Mollusca
as many. These are the two highest and most complicated. In the four
lower groups we would naturally have less variety, though it would seem
strange, were it not for so many examples in later periods, that the
dominant and highest groups should be most developed in regard to the
number of their modifications.
[Illustration: Fig. 8.--_Diagrammatic section of two Laminæ of
Cryptozoon, showing the Canals of the intermediate space, or Stroma
(magnified)._
Specimen in Peter Redpath Museum.]
Of the whole we might perhaps have been able to secure at least 200
species even in one locality. The likelihood is that if there had been
a collecting expedition like that of the _Challenger_ in Early Cambrian
times, it could have secured thousands of specific forms representing
all the above types, more especially as we probably know very little of
the softer and shell-less animals of these old seas, and there is some
reason to believe that these may have been in greater proportion than
in the present ocean.
In illustration of the richness of some parts of the lowest Cambrian
sea, I may refer here to the large and beautifully illustrated Memoir
of Walcott on the Lower Cambrian, containing fifty folio plates of
species collected in a few districts of North America; and, as a minor
example, to the contents of a loose boulder of limestone of that age,
found at Little Metis on the Lower St. Lawrence, under the following
circumstances (Fig. 9):--
[Illustration: Fig. 9.--_Lower Cambrian Fossils found in a few cubic
inches of limestone in a conglomerate at Little Metis; viz., Trilobites
of genera Olenellus, Ptychoparia, Solenopleura, Protypus; Brachiopod
of genus Iphidea; Pteropod of genus Hyolithes; Gastropod, genus
Stenotheca; Sponge, undetermined._]
Along what is now the valley of the Lower St. Lawrence and the gulf
of the same name, there seem to have been deposited in the oldest
Cambrian or Olenellus period beds of limestone rich in shells of marine
animals and fragments of these. These can be seen in place in some
parts of Newfoundland, and here and there on the hills bounding the
St Lawrence River; but for the most part they have been swept away
by the sea when these districts were being elevated to form parts
of the American land. Their ruins appear as boulders and pebbles in
thick beds of conglomerate or pudding-stone, constituting portions of
the Upper Cambrian and Lower Ordovician series, which now occupy the
south coast of the Lower St. Lawrence. In one of these boulders, less
than a foot in diameter, removed from its hard matrix and carefully
broken up, I found fragments representing eleven different species,
of which no less than eight were trilobites, one a gastropod, one a
brachiopod, and one probably a sponge--and this forms an interesting
illustration of the number of species sometimes to be found in a
limited space, and also of the great prevalence of the Trilobites in
these beds. The statistics of these groups for North America, as given
by Walcott, show 165 species belonging to all the groups enumerated
above, and of these the Trilobita constitute one-third of the whole;
so that the Olenellus Zone, as it has been called from one genus of
these Crustaceans, might well be named the reign of Trilobites, unless,
indeed, as the indications already referred to seem to show, giant
cuttle-fishes, destitute of shells, were then the tyrants of the sea,
but are represented only by the markings of their long and muscular
arms on the soft sea mud while dashing after their Crustacean prey.
What I desire, however, chiefly to emphasize is, that in the lowest
beds of the Cambrian we have evidence of sea-bottoms swarming with
representatives of all the leading types of marine invertebrate life,
and therefore seem to be still far from the beginning of living things,
if that was a slow and gradual process, rather than a sudden or rapid
series of events.
_PRE-CAMBRIAN LIFE_
III
_PRE-CAMBRIAN LIFE_
Having traced the chain of life through the long geological ages, from
the present day back to the Cambrian Period, we may now take our stand
on the fauna of the lowest Cambrian or Olenellus Zone, as a platform
whence we may dive into still deeper abysses of past time. Here,
however, we seem to have arrived at a limit beyond which few remains
of living things have yet been discovered, though there still remain
pre-Cambrian deposits of vast thickness and occupying large areas of
our continents. These pre-Cambrian formations are as yet among those
least known to geologists. The absence of fossils, the disturbances and
alterations which the rocks themselves have undergone, and which make
their relative ages and arrangement difficult to unravel, have acted
as deterrents to amateur geologists, and have to some extent baffled
the efforts of official explorers. In addition to this, workers in
different regions have adopted different methods of arrangement and
nomenclature; and in a very recent address, the Director-General of the
Geological Survey of Great Britain expresses his inability to satisfy
himself of the equivalency of the different pre-Cambrian groups on the
opposite sides of the Atlantic, and in consequence prefers to retain
for those of Britain merely local names.
On the other hand, those who hold the modern theories of gradual
evolution repudiate the idea that the Lower Cambrian fauna can be
primitive, and demand a vast series of changes in previous time to
prepare the way for it. In any case this comparatively unexplored
portion of geological time holds out the inducement of mystery and the
possibility of great discoveries to the hardy adventurers who may enter
into it. It must now be our effort to explore this dim and mysterious
dawn of life, and to ascertain what forms, if any, are visible amid its
fogs and mists.
The Kewenian or Etcheminian.
In certain basal Cambrian or infra-Cambrian beds, found by Matthew in
Southern New Brunswick, by Walcott in Colorado, and by Scandinavian
and English geologists in their respective countries, we find a
few remains referred to Algæ, or seaweeds; small tests or shells of
Protozoa; burrows and trails similar to those of modern sea-worms;
a few bivalve shells allied to modern Lingulæ, but presenting some
remarkable generalized characters; some bivalve and shrimp-like
Crustaceans, spicules of sponges, and large laminated forms
(Cryptozoon) similar to those already referred to as occurring in the
Upper Cambrian; also certain mysterious markings that are supposed to
have been produced by the arms or tentacles of free-swimming animals
of various kinds. In these lower beds the Trilobites have nearly or
quite disappeared, being represented only by doubtful fragments. The
beds of rock, originally sandy or muddy sediments, contain fossils very
sparingly, and only in certain layers separated by great thicknesses
of barren material, as if earthy matters were being deposited very
rapidly, or as if animal life was rare on the sea-bottom except at
intervals. It has, however, been suggested as possible[6] that much
of the marine population in those early times consisted of pelagic or
swimming animals destitute of any hard parts that could be preserved.
In addition to biological arguments in favour of this view, there is
the fact that some of the beds are stained with carbonaceous or coaly
matter, as if the sediment had been mixed with decomposed remains of
plants or animals retaining no determinate forms. Future discoveries
may increase our knowledge of the life of this period preceding the
Cambrian, but it is evident that so far as these rocks have been
examined, they indicate a great step downward in regard to the variety
and complexity of marine life.
[Footnote 6: By Prof. Brookes, of Johns Hopkins University.]