52 GEOLOGICAL BIOLOGY. 



fauna-floras, or sets of organic species, which can be recog- 

 nized wherever on the face of the globe they are found. 



The time-duration of each of these fauna-floras may be 

 called a period, and the successive periods thus distinguished 

 constitute the divisions of the eras which at present are recog- 

 nizable in each of the continents with greater or less fulness. 



Locally greater precision in classification has been attained, 

 but differences arising from adjustment of the organisms to 

 conditions of environment, and in living species expressed in 

 geographical distribution, make it doubtful if we are able to 

 correlate fossil faunas or floras, the world around, with greater 

 precision than to recognize the marks of the same period in 

 each district. 



Period a Recognized Division of an Era. For the present, 

 also, it seems more likely to conduce to real progress of knowl- 

 edge to consider the periods to be divisions of the standard 

 era, rather than absolute units of time-duration, dependent 

 on their own criteria alone for definition. 



In naming them, therefore, the subdivision of the era into 

 early, middle, and later divisions is preferable to the adoption 

 of separate distinctive names, and each continent or geologi- 

 cal province will then be free to adopt its own interpretation of 

 the local limits and marks of the period in its series of strata. 



Standard Periods and their Names. There are already de- 

 fined such divisions in the several eras, as follows : in the Cam- 

 brian era, an early or Eocainbrian period, a middle or Meso- 

 cambrian period, and a later or Neocambrian period. Walcott 

 has called the faunas of these periods the Olenellus, the Para- 

 doxides, and the Dicellocephalus faunas. 



In the same way the Ordovician era is made up of the 

 Eoordovician, or period of the Calciferous formation of 

 American geology, and a Neoordovician period, or the period 

 of the Trenton group of North America. 



The Silurian era is composed of the Eosilurian period 

 (Oneida, Medina, Clinton, Niagara), and a Neosilurian period 

 (Salina and Lower Helderberg). 



In the Devonian era are the Eodevonian period (Oriskany, 

 Corniferous), Mesodevonian period (Hamilton) and Neodevo- 

 nian (Chemung). In the Carboniferous era are the Eocarbon- 



