When the palaeontological evidence as to the age of any given set of strata is corroborated by the physical evidence, our conclusions may be regarded as almost certain; but there are certain limitations and fallacies in the palaeontological method of inquiry which deserve a passing mention.  In the first place, fossils are not always present in the stratified rocks; many aqueous rocks are unfossiliferous, through a thickness of hundreds or even thousands of feet of little-altered sediments; and even amongst beds which do contain fossils, we often meet with strata of many feet or yards in thickness which are wholly destitute of any traces of fossils.  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.  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, the observer has to rely wholly upon the fossils which he may meet with.