THE BEGINNING OF LIFE ON THE EARTH.
The day must have been when the first living being appeared for the
first time on our planet. Was it plant or animal? or a generalised
organism uniting in some mysterious way the properties and powers of two
kingdoms of nature, now so distinct, and even contrary to each other in
their manifestations? Did it appear suddenly, or was it slowly evolved
from dead matter by some process in which the albuminous or protoplasmic
matter, which we know forms the basal substance of living beings, was
first produced and then endowed with life? Did the first living being
appear in a mature state, or was it merely a germ from which the mature
individual could be produced? These are questions which science in its
present state has no means of answering. We do not know any process by
which the ingredients of protoplasm can be combined so as to produce
that substance without a previous living being. We do not know what
molecular differences may exist between dead albumen and that which we
see growing and moving and instinct with life; still less do we know how
to set up or establish these differences. We do not know the precise
nature or relation to other forces of the energy which actuates living
organisms. In our experience the simplest creatures that have life
spring from previous germs, themselves the products of previous
generations of living beings. Thus we are in the presence of great
mysteries which it might be impossible for us to solve, even if we were
permitted to visit some new planet on which the dawn of life was
breaking.
Some things, however, we can infer as to the conditions of the
introduction of life.
First, there is every reason to believe that the earth we inhabit was
once a glowing, incandescent mass, condensing from a vaporous condition,
and quite unfit for the abode of living beings, and which, even if in
some previous state its materials had constituted the mass of an
inhabited world, must have lost every trace of any living germ in the
fervent heat to which it had been subjected. There must, therefore, have
been in some way an absolute creation or origination of life and
organisation.
Secondly, we may infer that in the earlier stages of the earth, when it
was perhaps wholly or almost entirely covered with the waters, when it
was still uniformly warmed with its own internal heat, when it was
surrounded with a pall of dense vapours preventing radiation, and
nursing its heat within itself, though in a condition entirely unsuited
to the higher forms of life, it may have presented circumstances more
favourable to the origination and multiplication of living beings of low
organisation than at any subsequent time. This incubation of creative
power in the vaporous mantle over the primA|val ocean was a favourite
imagination of old thinkers, and is not obscurely hinted at in the Book
of Genesis. It has been revived and much insisted on by evolutionists in
our own time, though it has no certain foundation in scientific
observation or experiment.
Thirdly, from the fact that plant-life alone has the power of subsisting
on inorganic matter, and that plants furnish all the nourishment of
animals, we may fairly infer that the life of the plant preceded that of
the animal. It has, indeed, been suggested that some of the humbler
forms of life may combine in a rude and simple way enough of the powers
of the plant and the animal to enable them to bridge over the double
gap between the animal and the plant, and the animal and the mineral, or
that such creatures may in their early stages carry on vegetable
functions, and in their later those of the animal. It is theoretically
possible that life may have begun with such creatures, which some of the
results of microscopical research would lead us to believe still exist.
It is, however, on the whole more probable that simple plants first
existed, and furnished pabulum to animals of low grade introduced almost
contemporaneously.
Fourthly, all our knowledge of the succession of life leads us to
believe that it was not the higher plants and animals that first sprang
into existence from the teeming earth, but creatures of low and humble
organisation, suited to the then immature and unfinished condition of
the planet. It is also in accordance with the amazing fecundity of the
seas in all geological periods in these lower forms of life, to suppose
that the earliest living things originated in the waters, and that the
plants and animals of the land are of later date.
Do we know anything from actual observation of this earliest population
of the world? Such knowledge we can hope to acquire only by studying the
oldest formations known to us; and these, it must be confessed, exist in
a state so highly crystalline, and so much affected by internal heat, by
mechanical pressure, and by movement, as to render it little likely that
organic remains should be preserved in them in a state fit for
recognition.
In many parts of the world, and notably in Canada and Scandinavia, as
well as in Wales, Scotland, and Bavaria, the older PalA|ozoic rocks, the
lowest containing plants in great abundance, rest on still older
crystalline beds, which have become hard and crystalline in
pre-PalA|ozoic times, and have contributed sand and pebbles to the
succeeding very ancient deposits. These old rocksaEuro"the Eozoic series of
our tableaEuro"may be grouped in two great systems, the Laurentian and
Huronian (Fig. 14). The former may be conveniently divided into three
members: First, the Bojian, or Ottawa gneiss, consisting of stratified
granite rocks, usually of a red colour, and of very great thickness.
This contains, so far as known, no limestone, and has afforded as yet no
trace of fossils. Secondly, the Middle Laurentian, the greater part of
which consists of gneiss, but containing important beds of other rocks,
as quartzite, iron ore, and limestone. It is in this series that we have
the first evidence of life, and it is here also that we find the
greatest abundance of carbon, in the form of graphite or plumbago, and
also large quantities of calcium phosphate, or bone earth. Thirdly, the
Upper Laurentian or Norian series. This consists in great part of
Labadorite, or lime feldspar, but has also beds of ordinary gneiss,
limestone, and iron ore.
[Illustration: FIG. 14.aEuro"Ideal section, showing the relations of the
Laurentian and Huronian.
_a_, Lower Laurentian.
_b_, Middle Laurentian.
_c_, Upper Laurentian.
_d_, Huronian.
_e_, Cambrian and Silurian.]
The latter, the Huronian, is much less crystalline, and is divisible
into two seriesaEuro"the Lower Huronian, which includes many beds of
volcanic origin, and the Upper Huronian, which has afforded some obscure
fossils. The Huronian was first recognised by Sir W. E. Logan in Canada,
but corresponding rocks exist in Europe. The Pebidian series of Hicks in
Wales is probably of this age.
It is likely that much of the present appearance and condition of the
most ancient rocks may be attributed to metamorphism, that is, to the
slow baking under the influence of heat, heated water, and pressure, to
which they have been subjected in the lower parts of the earthaEuro(TM)s crust,
when buried deeply under newer deposits. It is also true, however, as
Dr. Sterry Hunt has pointed out in detail, that they present mineral
characters which show a mode of deposition different from that which has
prevailed subsequently, and probably indicating great ejections of
heated mineral matter into the primitive ocean, and comparatively little
of that deposit therein of mere sand and clay which has prevailed in
subsequent geological periods. In short, these rocks have an
unmistakably primitive aspect, distinguishing them from those of later
times, and conveying the impression that they approach at least to the
records of that time when a heated ocean first rested on the thin and
recently solidified crust of our planet. If this is really the case,
then our Lower LaurentianaEuro"hard, compact, destitute of limestone, and
composed of material which may be little else than the _dA(C)bris_ of
products of internal heat merely spread out into bedded forms by
wateraEuro"may represent a time when no living thing as yet tenanted the
waters; and the dawn of life may have appeared in that period when the
Middle Laurentian beds were laid down. Here at least we find two kinds
of evidence pointing to the existence of certain forms of life in the
waters.
The first depends on the mineral character of the beds themselves. This
formation holds several very thick beds of limestone. Now although this
kind of rock may, under certain circumstances, be deposited directly
from solution in water, it is not ordinarily so deposited, but more
usually through the agency of living beings inhabiting the waters, and
forming their skeletons or hard parts of limestone derived from the
water, usually through the medium of humble forms of plant life. In this
way are formed reefs of coral and beds of shells and of chalky ooze, all
composed of material once constituting the skeletons of animals. The
study of limestones of all geological ages shows that this has been the
usual mode of their formation. If the Laurentian limestones had a
similar origin, the seas of that period must have swarmed with animals
having calcareous coverings; and the study of more modern limestones
which have become highly crystalline shows that it is quite possible
that the forms and structures of these organisms may have been
obliterated.
Again, the Middle Laurentian abounds in carbon or coaly matter. True,
this is in the form of graphite or plumbago, but this condition may be a
result of metamorphism; and we know that the carbon of coal-beds and
bituminous shales of much more modern times has been altered into
graphite. Further, the graphite occurs in the way in which we should
expect it to occur if of organic origin. It is found disseminated in the
limestone, just as bituminous matter is found in unaltered rocks of this
kind. It is found interlaminated with gneiss, as carbonaceous and
bituminous matters are found in the shales of the ordinary fossiliferous
rocks, where these substances are known to be of organic origin. The
graphite also occurs in a very pure form in irregular veins, just as in
some bituminous formations the rock oil, oozing into fissures, has been
hardened into asphalt or coaly matter.[3]
To these facts may be added the presence of thick beds and veins of iron
ore and of apatite or calcium phosphate (bone earth). Both of these
substances occur in a disseminated state in nearly all rocks, but they
are concentrated into definite deposits by the action of life. Iron is
usually dissolved out and redeposited by acids produced in the decay of
vegetable matter, as we see in the clay ironstones of the coal formation
and in bog-iron ores. Calcic phosphate is taken up by many animals, and
forms their shells or skeletons, and on their death is deposited in beds
on the sea-bottom, sometimes to a very considerable extent.
The concurrence of all these phenomena in the Middle Laurentian may be
held to afford a strong presumption that, could we discover these rocks
in an unaltered state, we should find the limestones filled with marine
fossils and the graphite showing the forms or structure of plants. The
only startling feature in this conclusion is, that if we admit it, we
must also admit that life was developed in the Laurentian time in an
exuberance not surpassed, if equalled, in any subsequent period. Still,
there is nothing incredible in this, for if the forms of life were few
and low, their increase may have been rapid, because unchecked; and they
no doubt found in the ancient seas a surplusage of material on which to
feed and with which to construct their skeletons. Dr. Hunt has estimated
that the amount of carbon now sealed up as coaly matter would, if
diffused in the atmosphere as carbon dioxide, afford 600 times the
quantity of that gas at present floating in the air. A still more vast
amount is sealed up in the limestone of the several geological
formations. The same chemist has shown that the quantity of lime held in
solution in the ocean must have been much greater in Laurentian times
than at present. These facts at least allow us to suppose that in the
Eozoic times there were great supplies of carbon and of lime available
to such creatures of low organisation as were capable of profiting by
them; and we have no reason to doubt that there may have been plants and
animals so constituted as to flourish in conditions of this kind, in
which perhaps scarcely any modern species could exist.
These probabilities have caused geologists anxiously to search for any
traces of fossil organic remains in the old Laurentian rocks; and they
have been rewarded by the discovery of one species, _Eozoon Canadense_,
still often referred to as only a problematical fossil; but this arises
to a large extent from the prevalent want of knowledge sufficient to
appreciate the evidence for its organic character. This being once
admitted, we have in the existence of _Eozoon_ alone a sufficient cause
for the accumulation of much of the Laurentian limestone, though there
is reason to believe that it was not the only inhabitant of those
ancient seas.
[Illustration: FIG. 15 (Nos. 1 to 4).aEuro"Small weathered specimen of
_Eozoon_. From Petite Nation.
1, Natural size; showing general form, and acervuline portion above and
laminated portion below. 2, Enlarged casts of cells from upper part. 3,
Enlarged casts of cells from the lower part of the acervuline portion.
4, Enlarged casts of sarcode layers from the laminated part.]
The best specimens of _Eozoon_ occur as rounded, flattened, or more or
less irregular lumps or masses in certain layers of the Laurentian
limestone. When weathered on the surface of the rock, these lumps show a
regular concentric lamination, caused by thin fibres of limestone,
alternating with other mineral substances, filling up the spaces between
them. When these intervening layers are composed of such minerals as
Serpentine, Loganite, Pyroxene, or Dolomite, which are more resisting
than the limestone, they project when weathered, or when the limestone
is etched by an acid, so as to show the lamination very distinctly. At
the lower surface of the masses the layers are seen to be thicker than
they are above, and in perfect specimens they are seen toward the
surface to break up into small rounded vesicles of calcite, like little
bubbles, which constitute the so-called acervuline condition of _Eozoon_
(Fig. 15, No. 2). Slices of the fossil etched with an acid show these
appearances very perfectly, and can even be printed from, so as to
present perfect nature-prints of the structure (Fig. 16).
FIG. 16.aEuro"Nature-printed specimen of _Eozoon_ slightly etched with acid.
It shows the lamination, and at one side fragmental _Eozoon_ (_LifeaEuro(TM)s Dawn
on Earth_).
On etching a small fragment or slice with very dilute acid, so as to
dissolve away the calcite slowly, if the specimen be well preserved, we
find that the calcite layers have a very curious structure. This is
indicated by the appearance of little white or transparent threads of
Serpentine, Dolomite, or Pyroxene, which ramify throughout the substance
of the limestone layers, and are left intact when they have been
dissolved. These little processes must originally have been pores in the
limestone layers, which have been filled with the substance which
constitutes the alternate laminA|. In addition to this, if we use a
somewhat high microscopic power, and especially if we study the
structures as seen in thin transparent slices, we can perceive a still
finer tubulation along the sides of the calcite layers, represented by
extremely minute parallel rods of mineral matter (Figs. 17, 18).
Now if we regard these structures as those of an infiltrated fossil, as
described in last chapter, their interpretation will not be difficult.
The original organism was composed of calcareous matter in thin
concentric laminA|, connected with each other by pillars and plates of
similar material. Between these laminA| was lodged the soft, jelly-like
substance of a marine animal, growing by the addition of successive
layers, each protected by a thin calcareous crust. The layers were
originally traversed by very numerous parallel tubuli, permitting the
soft protoplasm to penetrate them; and when, in the progress of growth,
it was necessary to strengthen these layers, they were thickened by a
supplemental deposit traversed by larger and ramifying canals. When the
animal was dead, and its soft parts removed by decay, the chambers
between the laminA|, as well as the minute canals and tubuli, became
infiltrated with mineral matter, in the manner described in the last
chapter, and when so preserved became absolutely imperishable under any
circumstances short of absolute fusion.
[Illustration: FIG. 17.aEuro"Magnified group of canals in supplemental
skeleton of _Eozoon_.
Taken from the specimen in which they were first recognised (_LifeaEuro(TM)s
Dawn on Earth_).]
[Illustration: FIG. 18.aEuro"Portion of _Eozoon_ magnified 100 diameters,
showing the original cell-wall with tubulation, and the supplemental
skeleton with canals.aEuro"After Carpenter.
_a_, Original tubulated wall or aEurooeNummuline layer.aEuroŁ More magnified in
Fig. A. _b_, _c_, Intermediate skeleton, with canals.]
This interpretation leads to the conclusion, at which I arrived from the
study of the first well-preserved specimen ever submitted to microscopic
examination, that the animal which produced the calcareous skeleton of
_Eozoon_ was a member of that lowest grade of Protozoa known as
Foraminifera; and which, after living through the whole of geological
time, still abound in the sea. The main differences are, that _Eozoon_
presents a somewhat generalised structure, intermediate between two
modern types, and that it attained to a gigantic size compared with most
of these organisms in later periods. How near it approaches in structure
to some modern forms may be seen by comparison of the recent species
represented in Fig. 19, in which the parts corresponding to the
chambers, laminA|, tubuli, and canals of _Eozoon_ can be readily
distinguished.
[Illustration: FIG. 19.aEuro"Magnified portion of shell of _Calcarina_.aEuro"After
Carpenter.
_a_, Cells.
_b_, Original cell-wall with tubuli.
_c_, Supplementary skeleton with canals.]
The modern animals of this group are wholly composed of soft gelatinous
protoplasm or sarcode, the outer layer of which is usually somewhat
denser than the inner portion; but both are structureless, except that
the inner layer may present a more or less distinct granular
appearance. Many of them show a distinct spot or cell, called the
nucleus, and some have minute transparent vesicles, which contract and
expand alternately, and appear to be of the nature of circulatory or
excretory organs. They have no proper alimentary canal, but receive
their food into the general mass and digest it in temporary cavities.
Their means of locomotion and prehension are soft thread-like or
finger-like processes, extended at will from the surface of any part of
the body, and known as false feet (pseudopodia). From these processes
the whole group has obtained the name of Rhizopods, or rootfooted
animals. They may be regarded as constituting the simplest and humblest
form of animal life certainly known to us.
The very numerous species of these creatures existing in the waters of
the modern world may be arranged under three principal groups. The first
and highest includes those which have lobate or finger-like pseudopods,
and a well-developed nucleus and pulsating vesicle (Fig. 20, _a_). They
are mostly inhabitants of fresh water, and destitute of a hard crust or
shell. A second group, including many inhabitants of the sea as well as
of fresh waters, has thread-like radiating pseudopodia[4] (Fig. 20 _b_).
Some of these form beautiful silicious skeletons. A third group,
essentially marine, consists of those with reticulated pseudopodia, and
usually destitute of distinct nucleus and pulsating vesicle (Fig. 21).
They produce beautiful calcareous skeletons, often very complex, or
sometimes are content to cover themselves with a crust of agglutinated
grains of sand. It is to this last group that _Eozoon_ belongs, and to the
highest division of itaEuro"that which has the shell perforated with minute
pores, often of two kinds. It is curious that just as we have the
chambers and pores of _Eozoon_ filled with serpentine, so in all
geological formations and in the modern seas it is not uncommon to find
Foraminifera having their cavities filled with glauconite and other
hydrous silicates allied to serpentine.
[Illustration: FIG. 20.aEuro"_a_, _AmA"ba_, a fresh-water naked Rhizopod; and
_b_, _Actinophrys_, a fresh-water Protozoon of the group Radiolaria,
with thread-like pseudopodia.]
[Illustration: FIG. 21.aEuro"_Nonionina_, a modern marine Foraminifer.
Showing its chambered shell and netted pseudopodia.aEuro"After Carpenter.]
If we attempt to trace the Rhizopods onward from the Middle Laurentian,
we are met with a great hiatus in the Upper Laurentian. The species
_Eozoon Bavaricum_ has, however, been found in rocks apparently of
Huronian age; but this is the last known appearance of _Eozoon_, properly
so-called. In the Cambrian or Siluro-Cambrian, however, we meet with
many gigantic Protozoa, more especially those known as _Stromatopora_,
_ArchA|ocyathus_, _Receptaculites_, and _Cryptozoon_.
[Illustration: FIG. 22.aEuro"_Stromatopora concentrica._aEuro"After Hall.
_a_, Section of the same, magnified.
_b_, Small portion highly magnified, showing laminA| and pillars.]
The typical StromatoporA|, or Layer-corals, consist, like _Eozoon_, of
concentric layers, connected by numerous pillars, which are often,
though not always, more definite and regular than in the Laurentian
fossil. The laminA| are perforated, but more coarsely than in _Eozoon_, and
they are often thickened with supplemental deposit which, in some of the
forms, presents canals radiating from vertical tubes or bundles of tubes
penetrating the mass (Figs. 22, 23). The mode of growth of _Stromatopora_
must have closely resembled that of _Eozoon_, and the forms produced are
so similar that it is often quite impossible to distinguish them by the
naked eye. Like _Eozoon_, they form the substance of important limestones,
and single masses are sometimes found as much as three feet in diameter.
The StromatoporA| extend from the Upper Cambrian to the Devonian
inclusive. In the Carboniferous they are continued by smaller and more
regular organisms of the genus _Loftusia_,[5] and this genus seems to
extend without marked change up to the Eocene Tertiary. Recent students
of the StromatoporA| seem disposed to promote them from the province of
Protozoa to that of the Hydroids.[6] The reasons for this seem cogent in
the case of some of the forms, but in my judgment fail in others, more
especially in the older forms. It may ultimately be found that the group
as now held includes very different types of structure. In modern times
I know of no nearer representative than the animal whose skeleton often
adheres in red encrusting patches to our specimens of corals, and which
is known as _Polytrema_. In general structure it is not very far from
being a very degenerate kind of _Stromatopora_.
[Illustration: FIG. 23.aEuro"_Caunopora planulata._ Showing the radiating
canals on a weathered surface. Devonian.aEuro"After Hall.]
It is curious that in the line of succession above stated, the beautiful
tubulated cell-wall of _Eozoon_ disappears; and this structure seems,
after the Laurentian, to be for ever divorced from the great laminated
Protozoans. It reappears in the Carboniferous, in certain smaller
organisms of the type of the _Nummulites_, or Money-stone Foraminifers,
and is continued in this group of smaller and free animals down to the
present time. In the Cretaceous and early Tertiary periods, the
Foraminifera of different types have been nearly as great rock-builders
as they were in the Laurentian. Some of these later rock-builders,
however, have belonged to the lower or imperforate group; others to the
higher or Rotaline and Nummuline groups; and, as a whole, they have been
individually small, making up in numbers what they lacked in size.
Probably the conditions for enabling animals of this type rapidly, and
on a large scale, to collect calcareous matter, were more favourable in
the Laurentian than they have ever been since.
[Illustration: FIG. 24.aEuro"_ArchA|ocyathus minganensis._ A Primordial
Protozoon.aEuro"After Billings.
_a_, Pores of the inner wall.]
In the Siluro-Cambrian age two other forms of gigantic Foraminiferal
Protozoans were introduced, widely different from _Eozoon_, and destined
apparently not to survive the period in which they appeared. These were
_ArchA|ocyathus_, the ancient Cup-corals, and Receptaculites, which may
perhaps be called the Sack-corals. Both are quite remote from _Eozoon_ in
structure, wanting its complexity in the matter of minute tubules, and
having greater regularity and complication on the large scale.
_ArchA|ocyathus_ had the form of a hollow inverted cone with double
perforated walls, connected by radiating irregular plates, also
perforated (Fig. 24). It has been regarded as a sponge, and some species
are certainly accompanied with spicules; but these I have ascertained to
be merely accidental, and will be referred to in the next chapter. The
true structure of _ArchA|ocyathus_ consists of radiating calcareous plates
enclosing chambers connected by pores. ArchA|ocyathus came in with the
Later Cambrian, and seems to have died out in the Siluro-Cambrian. The
only more modern things which at all resemble it are the Foraminifera
called _Dactylopora_, which belong to the Tertiary period.
[Illustration: FIG. 25.aEuro"Receptaculites. Restored.aEuro"After Billings.
_a_, Aperture.
_b_, Inner wall.
_c_, Outer wall.
_n_, Nucleus, or primary chamber.
_v_, Internal cavity.]
Receptaculites is a still more complex organism. It has a sack-like
form, often attaining a large size, and the double walls are composed of
square or rhombic plates, connected with each other by hollow tubes from
which proceed canals perforating the plates (Fig. 25). This curious
structure is confined to the Siluro-Cambrian, and is so dissimilar from
modern forms that its affinities have been subject to grave doubts.
[Illustration: FIG. 26.aEuro"Section of _Loftusia Persica_. An Eocene
Foraminifer. Magnified five diameters.aEuro"After Carpenter and Brady.]
We thus have presented to us the remarkable fact that in the PalA|ozoic
age we have no precise representative of _Eozoon_, but instead three
divergent types, differing from it and from each other, all apparently
specialised to particular uses, all temporary in their duration; while
in later times nature seems to have returned nearer to the type of
_Eozoon_, though on a smaller scale, and separating some characters
conjoined in it. Some portion of this curious result may be due to our
ignorance; and it would be interesting to know, what we may know some
day, how this type of life was represented in the long interval between
the Huronian and the Upper Cambrian, when perhaps there may have been
forms that would at least enable us to connect _Eozoon_ and _Stromatopora_.
Another link in the chain of being remains to be noticed here. In the
Laurentian limestones we meet with numerous minute spherical bodies and
groups of spheres with calcareous tubulated tests.[7] These may either
be small ForaminiferA|, distinct from _Eozoon_, or may be germs or detached
cells from its surface. Similar bodies are found in the lower part of
the Siluro-Cambrian, in the Quebec group at Point Levis; and there they
are filled with a species of glauconite constituting a sort of greensand
rock. Still higher, in the Carboniferous, there are very numerous
species of Foraminifera, presenting forms very similar to those in the
modern seas, so that in the smaller shells of this group we seem to have
evidence of a continuous series all the way from the Laurentian to the
present time. The greater laminated forms co-exist with these up to the
Eocene Tertiary. Throughout the whole of geological timeaEuro"from the
formation of the Laurentian limestones to that of the chalky ooze
accumulating in the modern oceanaEuro"these humble creatures have been among
the chief instruments in seizing on the calcareous matter of the waters
and depositing it in the form of limestone.
[Illustration: FIG. 27.aEuro"Foraminiferal Rock Builders, in the Cretaceous
and Eocene.
_a_, _Nummulites lA|vigata_aEuro"Eocene. _b_, The same, showing chambered
interior. _c_, Milioline limestone, magnifiedaEuro"Eocene, Paris. _d_, Hard
Chalk, section magnifiedaEuro"Cretaceous.]
I have said nothing of the development of higher forms of animal life
from _Eozoon_, simply because I know nothing of it. We shall see in the
next chapter that these are introduced seemingly in an independent
manner. We may be content to trace foraminiferal life along its own line
of development, waxing and waning, but ever confined within the same
general boundaries, from the Laurentian to the present time. It is
likely that if, in any of the ages constituting this vast lapse of time,
a dredge had been dropped into the depths of ocean, it would have
brought up Foraminifera not essentially different in form and structure.
If any one asks to what extent the successive species constituting this
almost endless chain may be descendants one of the other, we have no
absolutely certain information to give. On the one hand, it is not
inconceivable that such forms as _Stromatopora_ or _Nummulina_ may have
descended from _Eozoon_. On the other hand, it is equally conceivable that
the same power which produced _Eozoon_ at first, whether from dead matter
or from some unknown lower form of life, may have repeated the process
in later times with modifications. In any case it is probable that the
Foraminifera have experienced alternations of expansion and shrinkage,
of elevation and decadence, in the lapse of geological time. There were
times in which many new forms swarmed into existence, and times in which
old forms were becoming extinct without being replaced by others. In so
far as the areas of the continents and the adjacent waters are
concerned, those periods when the land was subsiding under the ocean
must have been their times of prosperity, those in which the crust of
the earth shrunk and raised up large areas of land must have been their
times of decay. Still this lowest form of animal life has never
perished, but has always found abundant place for itself, however
pressed by physical change and by the introduction of higher beings.
[Illustration: _Paradoxides Regina_ (Matthews). Lower Cambrian of New
Brunswick.
1/6th Nat. Size.]