RULES FOR USE OF FOSSILS IN STRATIGRAPHY 187 



(h) variation in the numerical proportion of abundance of the competent 

 species of the fauna. 



5. LAPPING OF TWO SUCCESSIVE HOMCEOTOPIC FA UNAS 



A third rule of importance, but of less practical value, is the lapping 

 over in time of two successive homoeotopic faunas. This has been 

 established to be a fact, and its practical bearing is that it must be 

 recognized as a possibility in classifying formations, though generally 

 a difficult fact to establish, since the lapping will be discovered only 

 where both the faunas are obscure and in a region where the limits of 

 neither fauna is clear or represented by its full quota of species. 



4. DETECTION OF STAGES OF MUTATION IN A FAUNA 



Having first ascertained by a thorough study of a continuous fauna, 

 in the region in which it is typically represented, what are its abundant 

 and rare species and the nature and extent of the plasticity of its species, 

 it then becomes not a difficult matter to detect slight deviations from its 

 normal characters ; and by observing the modifications, both in compo- 

 sition of the fauna and in variation of its species as they occur in long 

 continuous sections, their time values become evident. These stages in 

 the evolutional mutations of a continuous fauna become almost certain 

 evidences of horizons which are not repeated. Often a single specimen 

 will furnish indisputable evidence of mutational conditions which are 

 very limited in range. Examination of the composition of a single 

 faunule and of the varietal modifications presented by common species 

 or traces of species is therefore far more valuable for making chrono- 

 logical diagnosis than the presence of what are supposed to be character- 

 istic species. This precision with which purely evolutional stages of a 

 fauna may be applied is probably due to the fact that they are of shorter 

 duration than the life of any one of its species. The facts appear to 

 warrant the conclusion that some of these transition stages in the evo- 

 lution of a fauna are coextensive with the geographical distribution of 

 the fauna. The more delicate and satisfactory modifications seem to be 

 directly associated with epochs of shifting of the faunas. The first trace 

 of decided mutation in the form of a common species is often found to be 

 associated with other evidence that there has been a disturbance of the 

 equilibrium of the fauna, and where thoroughly investigated such dis-^ 

 turbances have been connected with change in the center of distribution. 



5. MUTATION ASSOCIATED WITH RAPID SHIFTING OF A FA UNA 



The study of the faunas in question leads to the h3^pothesis (though 

 more facts should be gathered before adopting the hypothesis as certain) 



