_PRE-DETERMINATION IN NATURE._
The natural prejudice of persons not acquainted with geology is that
in the world all things continue as they were from the beginning.
But a little observation and experience dispels this delusion, and
perhaps replaces it with an opposite error. When our minds have been
familiarized with the continuous processes by which vaporous nebulæ
may be differentiated into distinct planets, and these may be slowly
cooled from an incandescent state till their surfaces become resolved
into areas of land and water; and still more, when we contemplate the
grand procession of forms of life from the earliest animals and plants
to man and his contemporaries, we become converts to the doctrine that
all things are in a perpetual flux, and that every succeeding day sees
them different from what they were the day before. In this state of
mind the scientific student is apt to overlook the fact that there are
many things which remain the same through all the ages, or which, once
settled, admit of no change. I do not here refer to those fundamental
properties of matter and forces and laws of nature which form the basis
of uniformitarianism in geology, but to determinations and arrangements
which might easily have been quite different from what they are, but
which, once settled, seem to remain for ever.
We have already considered the great fact that the nuclei and ribs of
the continental masses were laid down as foundations in the earliest
periods, and have been built upon by determinate additions, more
especially upon their edges and their hollows, so that while there
has been a constant process of removal of material from the higher
parts of the land, and deposition in the sea, and while there have
been periodical elevations and subsidences, the great areas of land
and water have remained substantially the same, and the main lines
of elevation and folding have conformed to the directions originally
fixed. Thus, in regard to the dry land itself, there has been fixity,
on the one hand, and mutation on the other, of a most paradoxical
aspect, till we understand something of the great law of constant
change united with perennial fixity in nature. From want of attention
to this, the permanence of continents is still a debated question,
and it is difficult for many to understand how the frequent dips
of the continental plateaus and margins under the sea, and their
re-elevation, often along with portions of the shallower sea bottom,
can be consistent with a general permanence of the position of the
continents and of the corresponding ocean abysses; yet, when this is
properly understood, it becomes plain that the union of fixity with
changes of level has been a main cause of the continuity and changes
of organic beings. Only the submergence of inland plateaus under
shallow and warm waters could have given scope for the introduction
of new marine faunas, and only re-elevation could have permitted
the greatest extension of plants and animals of the land. Thus, the
continuity of life with continual advance has depended on the permanent
existence of continental and oceanic areas; and the continents that
remain to us with all their diversity of elevation and outline, their
varied productions, both mineral and organic, and their life, which
is a select remainder of all that went before, have been produced and
furnished by a succession of changes, modified by the most conservative
retention of general arrangements and forms.
It is evident, however, that it is not merely permanence we have
to deal with here, but permanence of position along with change of
elevation; and this modified by the fact that there have always been
mountain ridges, internal plateaus, and marginal areas affected in
various ways by the vertical movement of the land. Further, the
elevation and subsidence of the land have not always been uniform,
but often differential, while every movement has tended to produce
modifications of ocean currents and of atmospheric conditions. The
whole subject, more especially in its relations to life, thus becomes
very complicated, and it is perhaps in consequence of partial and
imperfect views on these points that so much diversity of opinion has
arisen. For example, it is evident that we can gain nothing by adding
to the continents those submerged margins delineated by Murray in
the _Challenger_ reports, and which have in periods of continental
elevation themselves formed portions of the land. Nor do we establish a
case in favour of perished oceanic continents by the argument that they
are needed to furnish the materials of marginal mountains which are due
to the continuous sweeping of arctic material to the south by currents,
as we see in the coast of North America to-day. Nor do we invalidate
the permanence of the continents by the bridges of land, islands,
and shallow water at various times thrown across the Atlantic. The
distribution of Cambrian Trilobites, as illustrated by Matthew,[151]
seems to show a bridge of this kind in the north in very early times,
and similar evidence is furnished by the animals and plants of the
Devonian and Carboniferous, and by the sea animals and plants of the
later Tertiary and modern. Gardener has postulated a southern bridge
in the region of the West Indies for the migrations of plants, and
Gregory has adduced the evidence of those conservative and slow-moving
creatures, the sea urchins, in favour of similar connection in the West
Indian region at two distinct periods of time (the Lower Cretaceous and
the Miocene Tertiary). But bridges do not involve want of permanence
in their termini. Because an engineer has bridged the Firth of Forth,
it does not follow that the banks of this inlet did not exist before
the bridge was built; and if the bridge were to perish, the evidence
that trains had once passed that way would not justify the belief that
the bed of the Firth had been dry land, and the areas north and south
of it depressed. The more we consider this question the more we see
that the permanence, growth and sculpture of the continents are parts
of a great continuous and far-reaching plan. This view is strengthened
rather than otherwise, when we consider the probable manner in which
the enormous weight of the continents is sustained above the waters. We
may attribute this, on the one hand, to rigidity and lateral arching
and compression, or, on the other, to what may be termed flotation of
the lighter parts of the crust; and there seems to be little doubt
that both of these principles have been employed in constructing the
"pillars which support the earth." It is evident, however, that an arch
thrown over the internal abyss of the earth, or a portion of its crust
so lightened as to be pressed upward by its heavier surroundings, must,
when once established, have become a permanent feature of the earth's
foundations, not to be disturbed without calamitous consequences to its
inhabitants.
[151] Transactions Royal Society of Canada, 1892.
It is the part of the philosophical naturalist to bring together these
apparent contrarieties of mutation and permanence; both of which are
included, each in its proper place, in the great plan of nature. It is
therefore my purpose in the present chapter to direct attention to some
of the terminal points or fixed arrangements that we meet with in the
course of the geological history, and even in its earlier parts, and
more particularly in reference to the organic world. This, which is in
itself constantly changing, has been placed under necessity to adhere
to certain determinations fixed of old, and which regulate its forms
and possibilities down to our own time.
The argument, as we have seen in a previous chapter, for the animal
nature of Eozoon depends on our assuming certain parts of this fixity.
We suppose that then as now calcium carbonate had been selected as the
material for the skeletons of such creatures; that then, as now, minute
tubuli and large canals were necessary to enable the soft animal matter
to permeate and pass through the skeleton, and that the protoplasmic
animal matter of these far back geological periods had the same vital
properties of contraction and extension, digestion, etc., that it has
to-day. Could any one prove that these determinations of vital and
other forces had not been established, or that living protoplasmic
matter, with all its wonderful properties, had not been constructed in
the Laurentian period, the existence of this ancient animal would be
impossible. Yet how much is implied in all this, and though nothing
is more unstable chemically or vitally than protoplasm, if it were
introduced in the Laurentian, it has continued practically unchanged up
to the present time.
If we pass on to the undoubted and varied life of the Cambrian period,
we shall find that multitudes of things which might have been otherwise
were already settled in a way that has required no change.
In the oldest Trilobites the whole of the mechanical conditions of an
external articulated skeleton had been finally settled. The material
chitinous or partly calcareous, its microscopic structure, fitted to
combine lightness and strength with facility for rapid growth, the
subdivision of its several rings, so as to form a protective armour
and a mobile skeleton, the arrangement of its spines for defence
without interfering with locomotion, the contrivance of hinge joints
arranged in different planes in the limbs, all these were already in
full perfection, and just as they are found to-day in the skeleton of a
king-crab or any other Crustacean. They have, it is true, been modified
into a vast number of subordinate forms and uses, but the general
principles and main structures all stand. I was much struck with this
recently in studying a remarkable specimen now in the National Museum
at Washington. It is a large species of Asaphus; the same genus which
gave to the late Mr. Billings the limbs of a Trilobite, the first ever
described; but in the Washington specimen they are remarkably perfect.
Each limb presents a series of joints resembling those of the tarsus of
an insect, each joint being of conical form with the narrow proximal
end articulated to the enlarged distal end of the previous one, so
as to give great facility of movement and accommodation for delicate
muscular bands. This tells us of muscular fibre and tendon fitted for
flexing and extending these numerous joints, of motor nerves to work
that marvellous contractile power of the striated muscle, whose mode
of action is still an insoluble mystery, yet one practically solved in
the remote Cambrian age for the benefit of these humble inhabitants
of the sea. If we could imagine that the inventive power to perfect
such machinery was present in the brains of these old Crustaceans or
Arachnidans, we might wish that some of them had survived to instruct
us in matters which baffle our research.
It is long since the compound eyes of these Trilobites, as illustrated
by Burmeister, gave Buckland the opportunity to infer that the laws
of light and of vision were the same from the first as now. But what
does this imply? Not only that the light of the sun penetrating to the
depths of the Cambrian sea, was regulated by the same laws as to-day,
but that a series of cameras was perfected to receive the light as
reflected from objects, to picture the appearance of these objects on
a retinal screen as sensitive as the film of the photographer, and
thereby to produce true perceptions of vision in the sensorium of these
ancient animals. I have before me a fragment of the eye of a Trilobite
(_Phacops_), in which may be seen the little radiating tubes provided
for the several ocelli of the compound eye, just as we see in the
modern Limulus; and each of these ocelli must have been a perfect
photographic camera, and more than this, since absolutely automatic,
and probably having the power to represent colour as well as light
and shade. We know also, from the recent experiments of an Austrian
physiologist on the eyes of insects, that such compound eyes are so
constructed as to present a single picture, just as we can see the
whole landscape in looking through the many little panes of a cottage
window. In our own time the king-crab and lobster no doubt see just
as their predecessors did millions of years ago, and with precisely
similar instruments.
But the eyes of the modern Crustaceans have to compete with eyes of a
dissimilar type, constructed on the same general optical principles,
but quite different in detail. These are the simple or single eyes
of the cuttle-fishes and the true fishes. The same rivalry existed
in the oldest seas, when the competition of Crustaceans and cuttles
was just as keen as now. Though the eyes of the latter have not been
preserved, or at least have not yet been found, we have a right to
infer that the cuttles of the Cambrian and Silurian seas must have
been able to see as well as their Crustacean foes and competitors. If
so, the other type of eye must have been perfected for aquatic vision
as early as the compound type. In any case we know that a little
later, in the Carboniferous period, we have evidence that the eyes of
fishes conformed to those of their modern successors. I have myself
described[152] a carboniferous fish (_Palæoniscus_) from the bituminous
shales of Albert County, New Brunswick, in which the hard globular lens
of the eye had been sufficiently firm and durable to retain its form,
and to be replaced by calcite, showing even that like the lens of the
eye of a modern fish it had been constructed of concentric laminæ. In
the Carboniferous period also, both types of eye, the compound and
the single, experienced the further modifications necessary to fit
them for vision in air, the compound eye in insects, the simple eye in
Batrachians.[153] The original photographic cameras, strange though
this may appear to us, were intended for use under water; but at a very
early time they were adapted to work in air.
[152] _Canadian Naturalist._
[153] See _ante_, chapter on Air-breathers.
But we must bear in mind that this early solving of advanced problems
in mechanics, optics and physiology was in favour of Crustaceans
and cuttles, which were lords of creation in their time. There were
in those early days humbler creatures whose structures also present
wonderful contrivances.
I have already referred, in the chapter on imperfection of the
geological record, to the fossil sponges which have been found in so
great number and perfection in some of the oldest rocks of Canada, and
which have for the first time enabled us to appreciate the forms and
structures of the wonderful silicious sponges which preceded those
with which the dredgings of the _Challenger_ have made us familiar in
the modern seas. Humble sarcodous animals, without distinct muscular
or nervous system or external senses, the sponges have at least to
live and grow, and to that end they must already, in the dawn of life
on our planet,[154] have perfected those arrangements of ciliated
cells in chambers and canals which the microscope shows us driving
currents of water through the modern sponges, and thereby bringing to
them the materials of food and means of respiration. It is true we
know as little as the sponges themselves of the _modus operandi_ of
those perpetually waving threads which we call cilia or flagella, yet
they must have existed with all their powers even before the Cambrian
period.[155]
[154] I have found spicules of sponges in the chert nodules from the
Huronian limestones of Canada.
[155] Many species of hexaclinelled sponges have been described from
the upper Cambrian or lower Cambro-Silurian of Canada. See paper by the
author in the Transactions of the Royal Society of Canada, 1889.
[Illustration: A Giant Net-sponge.--_Palæosaccus Dawsoni_, Hinde. From
the Quebec group (Ordovician), Little Metis, Canada. Reduced to 3/7 the
diameter. (From the _Geological Magazine_, 1803.)]
The sponge, in order to support its delicate protoplasmic structures,
must have a skeleton. In modern times we find these creatures
depositing corneous or horny fibres, as in the common washing sponges,
or forming complex and beautiful structures of needles, or threads of
silica or calcite, and they seem from the first to have been able to
avail themselves of all these different materials. The oldest species
that we know had silicious or calcareous skeletons, though some of them
must also have had a certain amount, at least, of the ordinary spongy
or corneous fibres. But the most astonishing feature in what remains
of their skeletons, flattened out as they are on the surfaces of dark
slaty rock, is the manner in which they worked up so refractory a
material as silica into fibres like spun glass rods and crosses, and
built these up into beautiful basket-like forms, globular, cylindrical
or conical. It was necessary that they should fix themselves on the
soft muddy bottoms on which they grew, and to this end they produced
slender silicious fibres or anchoring rods, which, fine though they
were, had the form of hollow tubes. Sometimes a single rod sufficed,
but in this case it had a cross-like anchor affixed to its lower end, to
give it stability. Sometimes there were several simple rods, and then
they were skilfully braced by spreading them apart at the ends, and by
flattening their extremities into blades. Sometimes four rods joined in
a loop at the end gave the required support. Some larger species wound
together many threads like a wire rope, and even added to this flanges
like the thread of a screw, anticipating the principle of the modern
screw pile.
The body of the sponge must be hollow within, and must have a large
aperture or opening for the discharge of water, and smaller pores for
its admission. Various general forms were adopted for this. Some were
globular, or oval, or pear-shaped; others cylindrical, concave, or
mitre-shaped. To give form and strength to these shapes there were
sometimes vertical and transverse rods soldered together. In other
cases there were four-rayed or six-rayed needles of silica, with their
points attached so as to form a beautiful lattice-work, with its
meshes either square or lozenge-shaped. For protection sharp needles
were arranged like _chevaux de frize_ at the sides and apertures, and
these last were sometimes covered with a hood or grating of needles,
to exclude intruders from the interior cavity. Other species, however,
like some in the modern seas, seemed to despise these niceties, and
contented themselves with long straight needles placed in bundles, or
radiating from a centre, and thus supporting and protecting their soft
and sensitive protoplasm.
Curiously enough, these old sponges did not avail themselves of the
natural cystallization of silica, which, left to itself, would have
formed six-rayed stars, with the rays at angles of sixty degrees, or
six-sided plates, rods, or pyramids. They adopted another and peculiar
form of the mineral, known as colloidal silica, and being thus relieved
from any need to be guided by its crystalline form, treated it as we do
glass, and shaped it into cylindrical tubes, round needles and stars or
crosses, with the rays at right angles to each other.
The sponges whose skeletons are thus constructed, and which anticipated
so many mechanical contrivances long afterwards devised by man,
belonged to a group of silicious sponges (_Hexaclinellidæ_) which is
still extant, and represented by many rare and beautiful species of
the deep sea, which are the ornaments of our museums, and of which the
beautiful Eupleectella or Venus flower-basket, from the Philippine
Islands, and the glass-rope sponge (_Hyalonema_), from Japan, are
examples. But contemporary with these there was another group
(_Lithistidæ_), constructing skeletons of carbonate of lime, and which
preferred, instead of the regular mechanical structures of the others,
a kind of rustic work, made up of irregular fibres, very beautiful and
strong, but as a matter of pattern and taste standing quite by itself.
If there were any sponges with altogether soft and spongy skeletons in
these old times, their remains do not seem to have been preserved.
Here, it will be observed, are a great variety of vital and mechanical
contrivances devised in the very early history of the earth, settled
for all time, and handed down without improvement, and with little
change, to our later day. They are indeed vastly more wonderful than
the above general account can show; for to go into the details of
structure of any one of the species would develop a multitude of minor
complexities and niceties which no one not specially a student of these
animals could appreciate.
These are not solitary cases. The student of fossils meets with them
at every turn; and if he possesses the taste and imagination of a true
naturalist, cannot fail to be impressed with them.
To turn to a later but very ancient period, what can be more
astonishing than those first air-breathing vertebrates of the Coal
formation referred to in a previous chapter, with all their special
arrangements for an aërial habitat? I have mentioned their footprints,
and when we see the quarrymen split open a slab of sandstone and expose
a series of great plantigrade tracks, not unlike those of a human foot,
with the five toes well-developed, we are almost as much astonished as
Crusoe was when he saw the footprints on the sand. Crusoe inferred the
presence of another man in his island; we infer the earliest appearance
of an air-breathing vertebrate and the pre-human determination of the
form and number of parts of the human foot and hand, to appear in the
world long ages afterward. We see also that already that decimal system
of notation which we have founded on the counting of our ten fingers
was settled in the framework of most unmathematical Batrachians. It has
approved itself ever since as the typical and most perfect number of
parts for such organs.
If sceptically inclined, we may ask, Why five rather than four or six?
In the case of man we see that individuals who have lost one finger
have the use of the hand impaired, while the few who happen to have
six do not seem to be the better. How it was with the old Batrachians
we do not know; but it is certain that if we could have amputated the
claw-bearing little toe of _Sauropus unguifer_, or the reflexed little
toe of _Cheirotherium_, we should have much injured their locomotive
power.
The vegetable kingdom is full of similar examples of the early
settlement of great questions. Perhaps nothing is more marvellous
than the power of the green cells of the leaf as workers of those
complex and inimitable chemical changes whereby out of the water,
carbon dioxide and ammonia of the soil and the atmosphere, the living
vegetable cell, with the aid of solar energy, elaborates all the varied
organic compounds produced by the vegetable kingdom. Yet this seems all
to have been settled and perfected in the old Silurian period, long
before any kind of plant now living was on the earth. Perhaps in some
form it existed even in the Laurentian age, and was instrumental in
laying up its great beds of carbon. So all that is essential in plant
reproduction, whether in that simpler form in which a one-celled spore
is the reproductive organ, or in that more complex form in which an
embryo plant is formed in the seed, with a store of nourishment laid up
for its sustenance.
These arrangements were obviously as perfect in the great club mosses
and pines of the Devonian and Carboniferous as they have ever been
since, and we have specimens so preserved as to show their minute parts
just as well as in recent plants. The microscope also shows us that
the contrivances for thickening and strengthening the woody fibres and
trunk of the stem by bars or interrupted linings of ligneous matter, so
as to give strength and at the same time permit transudation of sap,
were all perfected, down to their minutest details, in the oldest land
plants. It is true that flowers with gay petals and some of the more
complicated kinds of fruit are later inventions, but the additions
in these consist mainly of accessories. The essentials of vegetable
reproduction were as well provided for from the first.
The same principle applies to many of the leading forms and types of
life, considered as genera or species. While some of these are of
recent introduction, others have continued almost unchanged from the
remotest ages. Such creatures as the Lingulæ, some of the Crustaceans
and of the Mollusks, the Polyzoa and some Corals have remained with
scarcely any change throughout geological time, while others have
disappeared, and have been replaced by new types.
We began this chapter with a consideration of the permanence of
continental areas, and may close with a reference to the same great
fact in connection with the continuity of life. Whether with some we
attach more importance to the support of the continents by lateral
pressure and rigidity, or with others to what may be termed flotation,
by virtue of their less density, as compared with that of the lower
parts of the earth; there can be little doubt that both principles have
been applied, and that both admit of some vertical movement. Thus the
stability of the continents is one of position rather than height, and
their internal plateaus as well as their partially submerged marginal
slopes have undergone great and unequal elevations and depressions,
causing most important geographical changes. Among these are the
formation of connecting bridges of shoals, islands, or low land,
connecting the continental masses at different periods, and permitting
migrations of shallow-water animals and even of denizens of the land.
The facts adduced in previous pages are sufficient to show connections
across the north of the Atlantic at intervals reaching from the
Cambrian to the Modern.
The conclusion of the whole matter is that there is a fixity and
unchangeableness in determinations and arrangements of force just
as much as in natural laws; and that while God has made everything
beautiful in its time He has also made everything beautiful and useful
in some sense for all time. With all this, while the great principles
and modes of operation remain unchanged, there is ample scope for
development, modification and adaptation to new ends, without deviation
from essential properties and characters. It is a wise and thoughtful
philosophy which can distinguish what is fixed and unchangeable from
that which is fluctuating and capable of development. Until this
distinction is fully understood, we may expect one-sided views and
faulty generalizations in our attempts to understand nature.
References:--"The Chain of Life in Geological Times." London. New
Species of Fossil Sponges from the Quebec Group at Little Metis.
Trans. Royal Society of Canada, 1889. Fossil Fishes from the Lower
Carboniferous of New Brunswick. _Canadian Naturalist_, "Acadian
Geology," 1855, and later editions to 1892. London and Montreal.
"The Story of the Earth," 1872 and later editions to 1891. London.
_THE GREAT ICE AGE._
DEDICATED TO THE MEMORY OF
MY LATE FRIEND
DAVID MILNE HOME, LL.D., F.R.S.E., ETC.,
An eminent and judicious Advocate of sound and
moderate Views respecting the Glacial Age.
Exaggerated Ideas--The St. Lawrence Valley--Modern Ice Action in the
St. Lawrence--Coast Ice--The Icebergs of Belle-Isle--Mt. Blanc and its
Glaciers--Effects of Glaciers--Possible Extension of Glaciers--Facts
of Glaciation in Canada--Cordilleran Glacier, Laurentide Glacier,
Appalachian Glacier--Submerged Valleys and Plains--Double Submergence
and Intermediate Partial Elevation--Interglacial Periods--Questions as
to Alternate Glaciation of Northern and Southern Hemispheres
[Illustration: Modern Boulder Beach.--Little Metis, St. Lawrence
Estuary. (From a Photograph.) Showing the manner in which travelled
boulders are piled up against the beach by the floating ice of the
Modern time (p. 346).]