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. When once, namely, a
generalisation has been established that certain fossils occur
in strata of a certain age, palaeontologists are apt to infer
that _all_ beds containing similar fossils must be of the same
age. There is a presumption, of course, that this inference would
be correct; but it is not a conclusion resting upon absolute
necessity, and there might be physical evidence to disprove it.
Fifthly, the physical geologist may lead the palaeontologist astray
by asserting that the physical evidence as to the age and position
of a given group of beds is clear and unequivocal, when such
evidence may be, in reality, very slight and doubtful. In this
way, the observer may be readily led into wrong conclusions as
to the nature of the organic remains--often obscure and
fragmentary--which it is his business to examine, or he may be
led erroneously to think that previous generalisations as to
the age of certain kinds of fossils are premature and incorrect.
Lastly, there are cases in which, owing to the limited exposure
of the beds, to their being merely of local development, or to
other causes, the physical evidence as to the age of a given
group of strata may be entirely uncertain and unreliable, and
in which, therefore, t