life is dawn on the earth 11

life is dawn on the earth 11

Then the growth was arrested, and possibly these upper layers gave off reproductive germs, fitted

to float or swim away and to establish new colonies. We may have

such reproductive germs in certain curious globular bodies, like

loose cells, found in connection with irregular Eozoon in one of

the Laurentian limestones at Long Lake and elsewhere. These curious

organisms I observed some years ago, but no description of them was

published at the time, as I hoped to obtain better examples. I now

figure some of them, and give their description in a note. (Fig. 18).

I have recently obtained numerous additional examples from the beds

holding Eozoon at St. Pierre, on the Ottawa. They occur at this place

on the surface of layers of the limestone in vast numbers, as if they

had been growing separately on the bottom, or had been drifted over

it by currents. These we shall further discuss hereafter. Such was

the general mode of growth of Eozoon, and we may now consider more in

detail some questions as to its gigantic size, its precise mode of

nutrition, the arrangement of its parts, its relations to more modern

forms, and the effects of its growth in the Laurentian seas. In the

meantime a study of our illustration, Plate IV., which is intended as a

magnified restoration of the animal, will enable the reader distinctly

to understand its structure and probable mode of growth, and to avail

himself intelligently of the partial representations of its fossilized

remains in the other plates and woodcuts.

 

[Illustration: Fig. 18. _Minute Foraminiferal forms from the Laurentian

of Long Lake._

 

Highly magnified. (_a._) Single cell, showing tubulated wall. (_b, c._)

Portions of same more highly magnified. (_d._) Serpentine cast of a

similar chamber, decalcified, and showing casts of tubuli.]

 

With respect to its size, we shall find in a subsequent chapter that

this was rivalled by some succeeding animals of the same humble type

in the Silurian age; and that, as a whole, foraminiferal animals have

been diminishing in size in the lapse of geological time. It is indeed

a fact of so frequent occurrence that it may almost be regarded as

a law of the introduction of new forms of life, that they assume in

their early history gigantic dimensions, and are afterwards continued

by less magnificent species. The relations of this to external

conditions, in the case of higher animals, are often complex and

difficult to understand; but in organisms so low as Eozoon and its

allies, they lie more on the surface. Such creatures may be regarded

as the simplest and most ready media for the conversion of vegetable

matter into animal tissues, and their functions are almost entirely

limited to those of nutrition. Hence it is likely that they will be

able to appear in the most gigantic forms under such conditions as

afford them the greatest amount of pabulum for the nourishment of

their soft parts and for their skeletons. There is reason to believe,

for example, that the occurrence, both in the chalk and the deep-sea

mud, of immense quantities of the minute bodies known as Coccoliths

along with Foraminifera, is not accidental. The Coccoliths appear to

be grains of calcareous matter formed in minute plants adapted to a

deep-sea habitat; and these, along with the vegetable and animal debris

constantly being derived from the death of the living things at the

surface, afford the material both of sarcode and shell. Now if the

Laurentian graphite represents an exuberance of vegetable growth in

those old seas proportionate to the great supplies of carbonic acid

in the atmosphere and in the waters, and if the Eozoic ocean was even

better supplied with carbonate of lime than those Silurian seas whose

vast limestones bear testimony to their richness in such material, we

can easily imagine that the conditions may have been more favourable

to a creature like Eozoon than those of any other period of geological

time.

 

Growing, as Eozoon did, on the floor of the ocean, and covering wide

patches with more or less irregular masses, it must have thrown up from

its whole surface its pseudopods to seize whatever floating particles

of food the waters carried over it. There is also reason to believe,

from the outline of certain specimens, that it often grew upward in

cylindrical or club-shaped forms, and that the broader patches were

penetrated by large pits or oscula, admitting the sea-water deeply into

the substance of the masses. In this way its growth might be rapid and

continuous; but it does not seem to have possessed the power of growing

indefinitely by new and living layers covering those that had died,

in the manner of some corals. Its life seems to have had a definite

termination, and when that was reached an entirely new colony had to

be commenced. In this it had more affinity with the Foraminifera, as

we now know them, than with the corals, though practically it had the

same power with the coral polyps of accumulating limestone in the sea

bottom, a power indeed still possessed by its foraminiferal successors.

In the case of coral limestones, we know that a large proportion of

these consist not of continuous reefs but of fragments of coral mixed

with other calcareous organisms, spread usually by waves and currents

in continuous beds over the sea bottom. In like manner we find in

the limestones containing Eozoon, layers of fragmental matter which

shows in places the characteristic structures, and which evidently

represents the debris swept from the Eozoic masses and reefs by the

action of the waves. It is with this fragmental matter that the small

rounded organisms already referred to most frequently occur; and while

they may be distinct animals, they may also be the fry of Eozoon, or

small portions of its acervuline upper surface floated off in a living

state, and possibly capable of living independently and of founding new

colonies.

 

It is only by a somewhat wild poetical licence that Eozoon has been

represented as a "kind of enormous composite animal stretching from the

shores of Labrador to Lake Superior, and thence northward and southward

to an unknown distance, and forming masses 1500 feet in depth." We may

discuss by-and-by the question of the composite nature of masses of

Eozoon, and we see in the corals evidence of the great size to which

composite animals of a higher grade can attain. In the case of Eozoon

we must imagine an ocean floor more uniform and level than that now

existing. On this the organism would establish itself in spots and

patches. These might finally become confluent over large areas, just

as massive corals do. As individual masses attained maturity and died,

their pores would be filled up with limestone or silicious deposits,

and thus could form a solid basis for new generations, and in this way

limestone to an indefinite extent might be produced. Further, wherever

such masses were high enough to be attacked by the breakers, or where

portions of the sea bottom were elevated, the more fragile parts of the

surface would be broken up and scattered widely in beds of fragments

over the bottom of the sea, while here and there beds of mud or sand

or of volcanic debris would be deposited over the living or dead

organic mass, and would form the layers of gneiss and other schistose

rocks interstratified with the Laurentian limestone. In this way, in

short, Eozoon would perform a function combining that which corals and

Foraminifera perform in the modern seas; forming both reef limestones

and extensive chalky beds, and probably living both in the shallow and

the deeper parts of the ocean. If in connection with this we consider

the rapidity with which the soft, simple, and almost structureless

sarcode of these Protozoa can be built up, and the probability that

they were more abundantly supplied with food, both for nourishing their

soft parts and skeletons, than any similar creatures in later times, we

can readily understand the great volume and extent of the Laurentian

limestones which they aided in producing. I say aided in producing,

because I would not desire to commit myself to the doctrine that the

Laurentian limestones are wholly of this origin. There may have been

other animal limestone-builders than Eozoon, and there may have been

limestones formed by plants like the modern Nullipores or by merely

mineral deposition.

 

[Illustration: Fig. 19. _Section of a Nummulite, from Eocene Limestone

of Syria._

 

Showing chambers, tubuli, and canals. Compare this and fig. 20 with

figs. 10 and 11.]

 

[Illustration: Fig. 20. _Portion of shell of Calcarina._

 

Magnified, after Carpenter. (_a._) Cells. (_b._) Original cell-wall

with tubuli. (_c._) Supplementary skeleton with canals.]

 

Its relations to modern animals of its type have been very clearly

defined by Dr. Carpenter. In the structure of its proper wall and its

fine parallel perforations, it resembles the _Nummulites_ and their

allies; and the organism may therefore be regarded as an aberrant

member of the Nummuline group, which affords some of the largest and

most widely distributed of the fossil Foraminifera. This resemblance

may be seen in fig. 19. To the Nummulites it also conforms in its

tendency to form a supplemental or intermediate skeleton with canals,

though the canals themselves in their arrangement more nearly resemble

Calcarina, which is represented in fig. 20. In its superposition of

many layers, and in its tendency to a heaped up or acervuline irregular

growth it resembles _Polytrema_ and _Tinoporus_, forms of a different

group in so far as shell-structure is concerned. It may thus be

regarded as a composite type, combining peculiarities now observed in

two groups, or it may be regarded as a representative in the Nummuline

series of Polytrema and Tinoporus in the Rotaline series. At the time

when Dr. Carpenter stated these affinities, it might be objected that

Foraminifera of these families are in the main found in the Modern and

Tertiary periods. Dr. Carpenter has since shown that the curious oval

Foraminifer called _Fusulina_, found in the coal formation, is in like

manner allied to both Nummulites and Rotalines; and still more recently

Mr. Brady has discovered a true Nummulite in the Lower Carboniferous of

Belgium. This group being now fairly brought down to the Palæozoic, we

may hope finally to trace it back to the Primordial, and thus to bring

it still nearer to Eozoon in time.

 

[Illustration: Fig. 21. _Foraminiferal Rock Builders._

 

(_a._) Nummulites lævigata--Eocene. (_b._) The same, showing chambered

interior. (_c._) Milioline limestone, magnified--Eocene, Paris. (_d._)

Hard Chalk, section magnified--Cretaceous.]

 

Though Eozoon was probably not the only animal of the Laurentian seas,

yet it was in all likelihood the most conspicuous and important as

a collector of calcareous matter, filling the same place afterwards

occupied by the reef-building corals. Though probably less efficient

than these as a constructor of solid limestones, from its less

permanent and continuous growth, it formed wide floors and patches

on the sea-bottom, and when these were broken up vast quantities of

limestone were formed from their debris. It must also be borne in mind

that Eozoon was not everywhere infiltrated with serpentine or other

silicious minerals; quantities of its substance were merely filled

with carbonate of lime, resembling the chamber-wall so closely that