18G H. S. WILLIAMS — SHIFTING OF FAUXAS 



abrupt succession in a continuous section, have no genetic relationship 

 to each other, while, on the other hand, two successive faunas belonging 

 to the same environment (that is, homoeotopic faunas) will owe whatever 

 differences they present to one of two causes : (1) evolutional change 

 through direct descent, or (2) loss by destruction and gain b}^ immigra- 

 tion of new species. 



The horizon marked in a section by transition from one fauna to an- 

 other whose species are heterotopic is of entirel}^ different value from 

 that marked by the separation between two homaeotopic faunas, for, as 

 has been shown, heterotopic faunas may be contemporaneous and yet 

 b}' shifting appear one above the other in the sections of a region ex- 

 tending over hundreds of miles ; yet it is horizons of the first kind that 

 are more frequently selected for the boundaries of formations and faunas, 

 and chiefly because they are more readily recognized by virtue of the 

 stronger contrasts between the faunas below and above. 



In conclusion, let me state some of the practical rules regarding the 

 use of fossils as time-markers suggested by these investigations. 



Practical Rules for Use of Fossils in Stratigraphy 



1. abrupt tea xsi tions 



Abrupt transitions in a continuous section from one fauna to another, 

 made of entirely different species and genera, which we are accustomed 

 to use in defining the boundar}^ of formations, are of decidedlj^ local and 

 not wide importance in making correlation ; and I think I have pre- 

 sented the correct reason for this fact. It is the effect of a mere local 

 shifting of events, for less than a hundred miles it may be, and the two 

 faunas which in the section are brought into sharp contrast, one of 

 them definitely successor to the other, are actually contemporaneous 

 faunas, adjusted to separate conditions of environment — that is, they 

 are heterotopic. 



2. SLIGHT MUTA TIOXS OF A CONTINUOUS FA UNA 



The second rule is perhaps little more than the converse of the first, 

 namely, the most satisfactor}^ and reliable evidence of definite epochs 

 of time is to be found by the study of the slight mutations which take 

 place in the history of a continuous fauna — that is, a fauna, or series of 

 faunules, whose species are homoeotopic — that is, adjusted to like con- 

 ditions of environment. These mutations are of two kinds : (a) Varia- 

 tion in the form of successive descendants of a common ancestr}^ and 



