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.