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 palaeonto- 

 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 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 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 palaeontologists to describe them as dif- 

 ferent species. Thus, the same fossil, occurring in successive 

 groups of strata, and with the merely trivial and varietal difter- 

 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 gTOups 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, 



