There are, therefore, to begin with,
many cases in which there is no palaeontological evidence extant
or available as to the age of a given group of strata. In the
second place, palaeontological observers in different parts of
the world are liable to give different names to the same fossil,
and in all parts of the world they are occasionally liable to
group together different fossils under the same title. Both these
sources of fallacy require to be guarded against in reasoning as
to the age of strata from their fossil remains. Thirdly, the mere
fact of fossils being found in beds which are known by physical
evidence to be of different ages, has commonly led palaeontologists
to describe them as different species. Thus, the same fossil,
occurring in successive groups of strata, and with the merely
trivial and varietal differences due to the gradual change in its
environment, has been repeatedly described as a distinct species,
with a distinct name, in every bed in which it was found. We know,
however, that many fossils range vertically through many groups
of strata, and there are some which even pass through several
formations. The mere fact of a difference of physical position
ought never to be taken into account at all in considering and
determining the true affinities of a fossil. Fourthly, the results
of experience, instead of being an assistance, are sometimes
liable to operate as a source of error.
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