CHRONOLOGICAL SUCCESSION. 39 
our present observation goes, none of the undoubted Grapto- 
lites have ever been discovered in rocks later than those known 
upon other grounds to be Silurian ; but it is possible that they 
might at any time be detected in younger deposits. Similarly, 
the species and genera which we now regard as characteristic 
of the Lower Silurian, may at some future time be found to 
have survived into the Upper Silurian period. We should not 
forget, therefore, in determining the age of strata by palzeonto- 
logical evidence, that we are always reasoning upon generalisa- 
tions which are the result of experience alone, and which are 
liable to be vitiated by further and additional discoveries. 
When the paleontological 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 palzeontological 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 paleontological 
evidence extant or available as to the age of a given group 
of strata. In the second place, palzontological observers in 
different parts of the world are lable to give different names 
to the same fossil, and in all parts of the world they are occa- 
sionally 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 paleontologists to describe them as dif- 
ferent species. Thus, the same fossil, occurring in successive 
groups of strata, and with the merely trivial and varietal differ- 
ences 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 forma- 
tions. 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 some- 
times liable to operate as a source of error. When once, 
