PRINCIPLES OF PALEONTOLOGIC CORRELATION 679 



migrations, which can occur only at times of readjustment of 

 faunal provinces. There can be none in the intermediate periods 

 of stability and quiescence when the fauna is endemic. 



The writer proposes to retain the term " zone," in the sense 

 intended by' Oppel, as a chronologic term for a limited horizon, 

 or time division, characterized by an interregional fauna. Use 

 of the term in this significance would recognize not only biologic 

 development, but also geologic events, for an interregional fauna 

 can appear only in times of readjustment of biologic regions, of 

 trangressions of the sea on the land, or of opening up connections 

 between regions that before were separated. These are nature's 

 periodic trial balances, during which the geologic columns in 

 various regions, for a while divergent in biologic development 

 and thus in stratigraphic classification, are brought into harmony. 



A zone, in this sense, means a comparatively short time in 

 which a certain characteristic, limited group of animals or plants 

 lived — too short for any great faunal change, but long enough 

 for this group to diffuse itself over a great area. To illustrate 

 this let us take a well known example. It must have taken a 

 long time for Productus semireticitlatus to be dispersed through 

 the seas of Australia, Eurasia, and America, for it is found in 

 all those regions. But no stratigrapher would choose this 

 species as a zone fossil, since it ranges from the Mountain Lime- 

 stone into the Permian ; often characteristic of a certain province 

 during a given time, but of no one horizon everywhere. And 

 during this long time the greater part of the accompanying 

 faunas underwent enormous changes, until most of the genera, 

 even, were new. During all these migrations Productus semireti- 

 culatus itself underwent modifications until it might be divided 

 into a number of geographic species or varieties, and each of 

 these into mutations or varieties in an upward-ranging genetic 

 series. But accompanying Productus semiretiadatus there are 

 many species and genera that were short-lived and widely dis- 

 tributed, in some one region appearing as a link in a genetic 

 series, but in some other region appearing sporadically or 

 unheralded by local ancestors, and brought in by immigration 



