432 Williams — Fossil Faunas and their use, etc, 



The relative value of the constituent species is expressed in 

 terms of the abundance or rarity of the individuals of each 

 faunnle sample, viz : bionic value. 



The purity or integrity of the fauna may be recognized by 

 the list of its dominant species. 



The geographical distribution of the fauna may be recog- 

 nized by the presence of the dominant species and their 

 holding their standard dominance in the list of species with 

 which they are associated in the faunule. 



The region, over which the bionic equilibrium of the fauna 

 is expressed by occurrence of the same dominant species, is 

 the metropolis of the fauna. 



The geological range of a fauna is recognized by the 

 persistence of the bionic equilibrium of the species. 



Two faunas may coexist in time in distinct geographical 

 areas ; but in the same area, the two faunas can appear in their 

 integrity only by displacement by which the bionic equilibrium 

 will be disturbed. Hence two faunas in their purity will 

 always appear in succession in any single section. 



As the geological changes are, in general, in one direction 

 for any particular region, the shifting of faunas is likely to be 

 in the same direction for long periods of time, and thus the 

 recurrence of two distinct faunas is rare. Occasionally oscilla- 

 tion of two faunas can be recognized in a single section ; this 

 fact may be interpreted as migration back and forth over the 

 same region. The occurrence of two faunas each occupying a 

 distinct metropolis will thus rarely ever show itself in lapping 

 of the faunas ; but occasionally evidence of the coexistence of 

 the faunas will be seen in the intercalation of a colony of one 

 of the faunas in the midst of the other. The lapping of 

 faunas, stratigraphically, is the necessary interpretation of the 

 coexistence of two faunas at the same period of time. 



Yale University, New Haven, Conn. 



