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.