422 Williams — Fossil Faunas and their use in 



When a fauna is geologically considered, it is of first import- 

 ance to note these characteristics, for by them we discover the 

 relative dominance of the species in the faunal aggregate. 

 And the dominant species, by the fact of being dominant, 

 show that they have conquered the difficulties presented to 

 their continuing life, and in the struggle for existence have 

 thereby proven themselves to be the fittest to survive in the 

 particular conditions they occupy. To express this character- 

 istic of evolutional strength of an organism, the term bionic has 

 been proposed. The term was defined in the following words: 



" Ine bionic quality of an organism may, then, be defined 

 as its quality of continuing, and repeating in successive gener- 

 ations, the same morphologic characters"* 



This quality of continuing and repeating the same morpho- 

 logic characters is expressed in successive strata by the occur- 

 rence of the same fossil species ; it is expressed geographically 

 by the same species occurring in different geographic areas. 



The relative bionic values of different species are expressed 

 by the relative number of individuals in a faunal aggregate, 

 and hence the equilibrium between the several species of a 

 fauna is expressed in terms of the numbers of individuals of 

 each species living together under a common set of conditions. 

 Looking then at faunas from this bionic standpoint, we have a 

 delicate test of the time relations of the faunas which is of 

 extreme value in establishing correlations. For the integrity 

 of a fauna expressed in terms of its dominant species can be 

 conceived of as continuing only so long as the relative bionic 

 values of each of the species remains constant. Any slight 

 change of conditions, or incursion by migration of other species, 

 may upset the bionic equilibrium, as will be expressed in terms 

 of relative frequency of the constituent species of each faunule 

 (that is, the local sample of the general fauna under considera- 

 tion). 



Having established this method of recording and measuring 

 the time significance of a fossil fauna, its use in making corre- 

 lations becomes evident. We have here a means of distin- 

 guishing the effects of change of geographic conditions (viz : 

 geographical distribution) from change coincident with passage 

 of time (viz : geologic range). 



In the same section we may discover, by noting the bionic 

 values of the successive faunules, how long the integrity of 

 the fauna is preserved, and whether any evolution of species 

 takes place during that life period of the fauna. So long as 

 the same species continue to be the dominant species in the 

 successive faunules, the faunal integrity is preserved. When 



* " The discrimination of time-values in geology." Journ. GeoL, vol. ix, 

 p. 578. 



