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THE PAST CONDITION OF ORGANIC NATURE
THE PAST CONDITION OF ORGANIC NATURE THOMAS H. HUXLEY IN the lecture which I delivered last Monday evening I endeavoured to
sketch in a very brief manner but as well as the time at my disposal
would permit the present condition of organic nature meaning by that
large title simply an indication of the great broad and general
principles which are to be discovered by those who look attentively at
the phenomena of organic nature as at present displayed. The general
result of our investigations might be summed up thus: we found that the
multiplicity of the forms of animal life great as that may be may be
reduced to a comparatively few primitive plans or types of construction;
that a further study of the development of those different forms
revealed to us that they were again reducible until we at last brought
the infinite diversity of animal and even vegetable life down to the
primordial form of a single cell.
We found that our analysis of the organic world whether animals or
plants showed in the long run that they might both be reduced into
and were in fact composed of the same constituents. And we saw that
the plant obtained the materials constituting its substance by a
peculiar combination of matters belonging entirely to the inorganic
world; that then the animal was constantly appropriating the
nitrogenous matters of the plant to its own nourishment and returning
them back to the inorganic world in what we spoke of as its waste; and
that finally when the animal ceased to exist the constituents of its
body were dissolved and transmitted to that inorganic world whence they
had been at first abstracted. Thus we saw in both the blade of grass
and the horse but the same elements differently combined and arranged.
We discovered a continual circulation going on--the plant drawing in
the elements of inorganic nature and combining them into food for the
animal creation; the animal borrowing from the plant the matter for its
own support giving off during its life products which returned
immediately to the inorganic world; and that eventually the
constituent materials of the whole structure of both animals and plants
were thus returned to their original source: there was a constant
passage from one state of existence to another and a returning back
again.
Lastly when we endeavoured to form some notion of the nature of the
forces exercised by living beings we discovered that they--if not
capable of being subjected to the same minute analysis as the
constituents of those beings themselves--that they were correlative
with--that they were the equivalents of the forces of inorganic
nature--that they were in the sense in which the term is now used
convertible with them. That was our general result.
And now leaving the Present I must endeavour in the same manner to put
before you the facts that are to be discovered in the Past history of
the living world in the past conditions of organic nature. We have
to-night to deal with the facts of that history--a history involving
periods of time before which our mere human records sink into utter
insignificance--a history the variety and physical magnitude of whose
events cannot even be foreshadowed by the history of human life and
human phenomena--a history of the most varied and complex character.
We must deal with the history then in the first place as we should
deal with all other histories. The historical student knows that his
first business should be to inquire into the validity of his evidence
and the nature of the record in which the evidence is contained that
he may be able to form a proper estimate of the correctness of the
conclusions which have been drawn from that evidence. So here we
must pass in the first place to the consideration of a matter which
may seem foreign to the question under discussion. We must dwell upon
the nature of the records and the credibility of the evidence they
contain; we must look to the completeness or incompleteness of those
records themselves before we turn to that which they contain and
reveal. The question of the credibility of the history happily for us
will not require much consideration for in this history unlike those
of human origin there can be no cavilling no differences as to the
reality and truth of the facts of which it is made up; the facts state
themselves and are laid out clearly before us.
But although one of the greatest difficulties of the historical student
is cleared out of our path there are other difficulties--difficulties
in rightly interpreting the facts as they are presented to us--which
may be compared with the greatest difficulties of any other kinds of
historical study.
What is this record of the past history of the globe and what are the
questions which are involved in an inquiry into its completeness or
incompleteness? That record is composed of mud; and the question which
we have to investigate this evening resolves itself into a question of
the formation of mud. You may think perhaps that this is a vast
step--of almost from the sublime to the ridiculous--from the
contemplation of the history of the past ages of the world's existence
to the consideration of the history of the formation of mud! But in
nature there is nothing mean and unworthy of attention; there is
nothing ridiculous or contemptible in any of her works; and this
inquiry you will soon see I hope takes us to the very root and
foundations of our subject.
How then is mud formed? Always with some trifling exception which I
need not consider now--always as the result of the action of water
wearing down and disintegrating the surface of the earth and rocks with
which it comes in contact--pounding and grinding it down and carrying
the particles away to places where they cease to be disturbed by this
mechanical action and where they can subside and rest. For the ocean
urged by winds washes as we know a long extent of coast and every
wave loaded as it is with particles of sand and gravel as it breaks
upon the shore does something towards the disintegrating process. And
thus slowly but surely the hardest rocks are gradually ground down to
a powdery substance; and the mud thus formed coarser or finer as the
case may be is carried by the rush of the tides or currents till it
reaches the comparatively deeper parts of the ocean in which it can
sink to the bottom that is to parts where there is a depth of about
fourteen or fifteen fathoms a depth at which the water is usually
nearly motionless and in which of course the finer particles of this
detritus or mud as we call it sinks to the bottom.
Or again if you take a river rushing down from its mountain sources
brawling over the stones and rocks that intersect its path loosening
removing and carrying with it in its downward course the pebbles and
lighter matters from its banks it crushes and pounds down the rocks
and earths in precisely the same way as the wearing action of the sea
waves. The matters forming the deposit are torn from the mountain-side
and whirled impetuously into the valley more slowly over the plain
thence into the estuary and from the estuary they are swept into the
sea. The coarser and heavier fragments are obviously deposited first
that is as soon as the current begins to lose its force by becoming
amalgamated with the stiller depths of the ocean but the finer and
lighter particles are carried further on and eventually deposited in a
deeper and stiller portion of the ocean.
It clearly follows from this that mud gives us a chronology; for it is
evident that supposing this which I now sketch to be the sea bottom
and supposing this to be a coast-line; from the washing action of the
sea upon the rock wearing and grinding it down into a sediment of mud
the mud will be carried down and at length deposited in the deeper
parts of this sea bottom where it will form a layer; and then while
that first layer is hardening other mud which is coming from the same
source will of course be carried to the same place; and as it is
quite impossible for it to get beneath the layer already there it
deposits itself above it and forms another layer and in that way you
gradually have layers of mud constantly forming and hardening one above
the other and conveying a record of time.
It is a necessary result of the operation of the law of gravitation that
the uppermost layer shall be the youngest and the lowest the oldest
and that the different beds shall be older at any particular point or
spot in exactly the ratio of their depth from the surface. So that if
they were upheaved afterwards and you had a series of these different
layers of mud converted into sandstone or limestone as the case
might be you might be sure that the bottom layer was deposited first
and that the upper layers were formed afterwards. Here you see is the
first step in the history--these layers of mud give us an idea of time.
The whole surface of the earth--I speak broadly and leave out minor
qualifications--is made up of such layers of mud so hard the
majority of them that we call them rock whether limestone or
sandstone or other varieties of rock. And seeing that every part of
the crust of the earth is made up in this way you might think that the
determination of the chronology the fixing of the time which it has
taken to form this crust is a comparatively simple matter. Take a
broad average ascertain how fast the mud is deposited upon the bottom
of the sea or in the estuary of rivers; take it to be an inch or two
or three inches a year or whatever you may roughly estimate it at;
then take the total thickness of the whole series of stratified rocks
which geologists estimate at twelve or thirteen miles or about seventy
thousand feet make a sum in short division divide the total thickness
by that of the quantity deposited in one year and the result will of
course give you the number of years which the crust has taken to form.
Truly that looks a very simple process! It would be so except for
certain difficulties the very first of which is that of finding how
rapidly sediments are deposited; but the main difficulty--a difficulty
which renders any certain calculations of such a matter out of the
question--is this the sea-bottom on which the deposit takes place is
continually shifting.
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