M II. S. WILLIAM Till: CUBOIDBS ZONE AM> ITS FAUNA. 



AJso, ihf Bame species may indicate likeness of conditions which, though 

 shifting geographically, may have continued through the time indicated by 

 a considerable oscillation of the conditions <»f deposition. Two species, or 

 two fauna- made up of entirely different species, may indicate only difference 

 of environment, although of precisely contempora >us period. 



Bui according to our present knowledge, it appears to be positively cer- 

 tain thai organisms have changed for the whole world more or less rapidly 

 and completely with the progress of geologic time This being the ease, if 

 we could ascertain the laws of change as expressed in the several orders and 

 genera of organisms we would he able to determine by them the geologic 

 period at which the deposits containing them were made. 



[f organisms remained constant under all conditions of environment, or if 

 their differences due to changed environment were of a different nature 

 from those coordinate with continued reproduction, we might use them to 

 determine actual contemporaneity of strata ; but this is not the fact. 



It is a fact that the characters which present a degree of constancy 

 among closely related forms of two widely separate areas are also in like 

 degree COnstanl for a relatively long time geologically. On the other hand, 

 characters which in series of closely allied species are constant for only 

 limited areas, their variation- constituting differences between the species of 

 separate regions, are also different for the species of each successive geologic 



.-t:i_ 



Hence, in the use of fossils for purposes of correlation, it happens that a 

 knowledge of the habits, history, laws of constancy ami of variation of each 

 species and of the genus to which it belongs are essential 'lenient- iii the 

 problem. 



The mere identity of some species in two compared formations, or even 

 identity of genera with closely allied Bpecies, is not alone evidence of con- 

 temporaneity. And in this respect, no doubt, the application of the term 

 hoTnotaxy to such similar formation-.;:- proposed by Huxley, is preferable 

 to conU mporaneity. 



Winn, however, we consider the fact that all groups of fossils, when 

 Btudied comparatively and with a view to ascertaining their historical muta- 

 tions, do presenl regular modifications of some of their characters coordinate 

 with geologic sequence, the question is raised whether fossils may not pre- 

 sent intrinsic evident fthe position they may occupy in the lite history of 



the '.''-nil- to which they belong. In the belief thai this i< possible, I have 

 mail'- :in exhaustive study of a fauna which, in Germany, Prance, Belgium, 

 England, Russia, and eastward, is found between typical middle and upper 

 Devonian fauuas, and I have compared with it a fauna occurring in New 

 York in what is called the 'fully lime-tone. In the following discussion I 

 -hall endeavor to point out the nature of the evidence by which it seeme pos- 

 sible to determine relative contemporaneity of strata by means of fossils. 



