The Past Condition of Organic Nature | Page 7

I repeat, it is absolutely
necessary, from the nature of things, that that record should be of the
most fragmentary and imperfect character. Unfortunately this
circumstance has been constantly forgotten. Men of science, like young
colts in a fresh pasture, are apt to be exhilarated on being turned into a
new field of inquiry, to go off at a hand-gallop, in total disregard of
hedges and ditches, losing sight of the real limitation of their inquiries,
and to forget the extreme imperfection of what is really known.
Geologists have imagined that they could tell us what was going on at
all parts of the earth's surface during a given epoch; they have talked of
this deposit being contemporaneous with that deposit, until, from our
little local histories of the changes at limited spots of the earth's surface,
they have constructed a universal history of the globe as full of
wonders and portents as any other story of antiquity.
But what does this attempt to construct a universal history of the globe
imply? It implies that we shall not only have a precise knowledge of
the events which have occurred at any particular point, but that we shall
be able to say what events, at any one spot, took place at the same time
with those at other spots.
Let us see how far that is in the nature of things practicable. Suppose
that here I make a section of the Lake of Killarney, and here the section
of another lake--that of Loch Lomond in Scotland for instance. The
rivers that flow into them are constantly carrying down deposits of mud,
and beds, or strata, are being as constantly formed, one above the other,
at the bottom of those lakes. Now, there is not a shadow of doubt that
in these two lakes the lower beds are all older than the upper--there is
no doubt about that; but what does 'this' tell us about the age of any
given bed in Loch Lomond, as compared with that of any given bed in
the Lake of Killarney? It is, indeed, obvious that if any two sets of
deposits are separated and discontinuous, there is absolutely no means

whatever given you by the nature of the deposit of saying whether one
is much younger or older than the other; but you may say, as many
have said and think, that the case is very much altered if the beds which
we are comparing are continuous. Suppose two beds of mud hardened
into rock,--A and B-are seen in section. (Fig. 5.)
[Fig. 5.]
Well, you say, it is admitted that the lowermost bed is always the older.
Very well; B, therefore, is older than A. No doubt, 'as a whole', it is so;
or if any parts of the two beds which are in the same vertical line are
compared, it is so. But suppose you take what seems a very natural step
further, and say that the part 'a' of the bed A is younger than the part 'b'
of the bed B. Is this sound reasoning? If you find any record of changes
taking place at 'b', did they occur before any events which took place
while 'a' was being deposited? It looks all very plain sailing, indeed, to
say that they did; and yet there is no proof of anything of the kind. As
the former Director of this Institution, Sir H. De la Beche, long ago
showed, this reasoning may involve an entire fallacy. It is extremely
possible that 'a' may have been deposited ages before 'b'. It is very easy
to understand how that can be. To return to Fig. 4; when A and B were
deposited, they were 'substantially' contemporaneous; A being simply
the finer deposit, and B the coarser of the same detritus or waste of land.
Now suppose that that sea-bottom goes down (as shown in Fig. 4), so
that the first deposit is carried no farther than 'a', forming the bed Al,
and the coarse no farther than 'b', forming the bed B1, the result will be
the formation of two continuous beds, one of fine sediment (A A1)
over-lapping another of coarse sediment (B Bl). Now suppose the
whole sea-bottom is raised up, and a section exposed about the point Al;
no doubt, 'at this spot', the upper bed is younger than the lower. But we
should obviously greatly err if we concluded that the mass of the upper
bed at A was younger than the lower bed at B; for we have just seen
that they are contemporaneous deposits. Still more should we be in
error if we supposed the upper bed at A to be younger than the
continuation of the lower bed at Bl; for A was deposited long before B1.
In