Geological Contemporaniety and Persistent Types of Life | Page 5

critical examination than they have hitherto received, in order to
ascertain how far they rest on an irrefragable basis; or whether, after all,
it might not be well for paleontologists to learn a little more carefully
that scientific "ars artium," the art of saying "I don't know." And to this

end let us define somewhat more exactly the extent of these pretensions
of paleontology.
Every one is aware that Professor Bronn's 'Untersuchungen' and
Professor Pictet's 'Traite de Paleontologie' are works of standard
authority, familiarly consulted by every working paleontologist. It is
desirable to speak of these excellent books, and of their distinguished
authors, with the utmost respect, and in a tone as far as possible
removed from carping criticism; indeed, if they are specially cited in
this place, it is merely in justification of the assertion that the following
propositions, which may be found implicitly, or explicitly, in the works
in question, are regarded by the mass of paleontologists and geologists,
not only on the Continent but in this country, as expressing some of the
best-established results of paleontology. Thus:--
Animals and plants began their existence together, not long after the
commencement of the deposition of the sedimentary rocks; and then
succeeded one another, in such a manner, that totally distinct faunae
and florae occupied the whole surface of the earth, one after the other,
and during distinct epochs of time.
A geological formation is the sum of all the strata deposited over the
whole surface of the earth during one of these epochs: a geological
fauna or flora is the sum of all the species of animals or plants which
occupied the whole surface of the globe, during one of these epochs.
The population of the earth's surface was at first very similar in all parts,
and only from the middle of the Tertiary epoch onwards, began to show
a distinct distribution in zones.
The constitution of the original population, as well as the numerical
proportions of its members, indicates a warmer and, on the whole,
somewhat tropical climate, which remained tolerably equable
throughout the year. The subsequent distribution of living beings in
zones is the result of a gradual lowering of the general temperature,
which first began to be felt at the poles.
It is not now proposed to inquire whether these doctrines are true or
false; but to direct your attention to a much simpler though very
essential preliminary question--What is their logical basis? what are the
fundamental assumptions upon which they all logically depend? and
what is the evidence on which those fundamental propositions demand
our assent?

These assumptions are two: the first, that the commencement of the
geological record is coeval with the commencement of life on the globe;
the second, that geological contemporaneity is the same thing as
chronological synchrony. Without the first of these assumptions there
would of course be no ground for any statement respecting the
commencement of life; without the second, all the other statements
cited, every one of which implies a knowledge of the state of different
parts of the earth at one and the same time, will be no less devoid of
demonstration.
The first assumption obviously rests entirely on negative evidence. This
is, of course, the only evidence that ever can be available to prove the
commencement of any series of phenomena; but, at the same time, it
must be recollected that the value of negative evidence depends entirely
on the amount of positive corroboration it receives. If A B wishes to
prove an 'alibi', it is of no use for him to get a thousand witnesses
simply to swear that they did not see him in such and such a place,
unless the witnesses are prepared to prove that they must have seen him
had he been there. But the evidence that animal life commenced with
the Lingula-flags, 'e.g.', would seem to be exactly of this unsatisfactory
uncorroborated sort. The Cambrian witnesses simply swear they
"haven't seen anybody their way"; upon which the counsel for the other
side immediately puts in ten or twelve thousand feet of Devonian
sandstones to make oath they never saw a fish or a mollusk, though all
the world knows there were plenty in their time.
But then it is urged that, though the Devonian rocks in one part of the
world exhibit no fossils, in another they do, while the lower Cambrian
rocks nowhere exhibit fossils, and hence no living being could have
existed in their epoch.
To this there are two replies: the first, that the observational basis of the
assertion that the lowest rocks are nowhere fossiliferous is an
amazingly small one, seeing how very small an area, in comparison to
that of the whole world, has yet been fully searched; the second, that
the argument is good for nothing unless