34 INTRODUCTION. 



that the laws of the distribution of fossil organisms are not 

 always the same in different formations, and that they are 

 liable to vary under conditions which are only partially or 

 not at all understood. To both these points our attention 

 may be directed for a few moments. 



As regards imperfections in the methods of palaeontological 

 research, by far the most important arises from the fact that 

 far too much weight has been attached by observers, espe- 

 cially in the earlier periods of the science, to the age of the 

 rocks in which any given fossil occurred. So long as the 

 opinion was current that fossils occurring in different for- 

 mations must be different, it followed of necessity that the 

 smallest and most trivial varietal or even individual pecu- 

 liarities of form or structure were considered as sufficient to 

 establish specific distinction. At present, however, palaeon- 

 tologists are tolerably agreed that the mere fact of a differ- 

 ence of physical position, and consequently of age, ought 

 never to be taken into account at all in considering the true 

 affinities and systematic position of a fossil. At the same 

 time it is, for many reasons, most important that palaeontolo- 

 gists should have a general personal acquaintance with the 

 rocks in which occur any fossils that they may have to 

 examine and describe ; and many errors have arisen from 

 the neglect of this sound rule. 



Again, palaeontologists are not agreed as to the relative 

 value of different classes of fossils in determining the age and 

 stratigraphical position of the rocks in which they occur. 

 If all the fossils point towards the same conclusion, there 

 is, of course, no difficulty in the matter ; but it sometimes 

 happens that the vegetable fossils of a given formation would 

 lead one to conclude that it was of a given age, whereas the 

 Invertebrate or Vertebrate fossils would induce us to place 

 it at some different horizon in the stratified series. There 

 has thus arisen a controversy as to the relative value of 

 plants and animals as tests of the age of a given series of 

 rocks ; and in at least two instances this controversy has 

 affected questions of considerable general importance. In 

 one of the cases referred to, we find plants which are ad- 

 mitted to be the same as those of the Coal-measures (Car- 



