INTRODUCTION 39 



IVIammals Europe is like a peninsula budding off from the western side of 

 Asia or at times almost like an archipelago, so largely does the sea trans- 

 gress its northern and southern borders. Its varying coastlines, its insular 

 conditions, its archipelagic surfaces are to be followed in imagination in 

 connection with the evolution of its mammalian fauna. Nevertheless the 

 main trend of evolution and extinction in unstable Europe coincides with 

 that in relatively stable North America. 



IV. Geologic or Time Distribution of Mammals 



Time divisions. — A host of questions turn upon the geologic, or time 

 distribution of mammals, which is to be studied hand in hand with their 



* . . . 



geographic, or space distribution, as above described. The precise solu- 

 tion of all problems of origin and dispersal, or travel and migration of the 

 different kinds of mammals, concluding finally with the most absorbing 

 question of the center of origin and dispersal of the human race, turns upon 

 the question of geologic time. 



At present, from astronomical reckoning, we may all ascertain the time 

 and readily fit all that is occurring in different parts of the world into the 

 days, weeks, and months. In the past, however, in the geologic time 

 divisions ^ which are kno\vn as Stages (Etages), Periods, and Epochs, 

 we directly invert our present order of procedure, because we must first 

 discover what is occurring in the different parts of the world, and from 

 these occurrences we must deduce, estimate, and establish geologic time. 



If the question is asked when did the Age of Reptiles close and the Age 

 of Mammals begin, in France, in the Rocky Mountains, or in South Amer- 

 ica, the answer is sought not through the rocks, but through the fossils 

 which the}' contain, or through a i^rocess of observation and reasoning 

 which is kno^\^l technically as palceontological correlation. When we com- 

 pare all the fossil mammals which are kno^^^l in the dawn of the Eocene in 

 Europe, in North America, and South America, we are able to establish a 

 homotaxis or general similitude in the life of these widely separated regions, 

 and a synchronism, or general similitude in the time of these different re- 

 gions. An exact synchronism is practically impossilile of attainment, but 

 approximate synchronism, or time correlation, is by no means beyond our 

 reach, although often a vastly long and difficult undertaking. 



The very title of this volume, ''The Age of Mammals," implies the 



' Comptps Rendus do la VIII': Session, en France, Congr^s Geologique International, 

 Paris, I'JOO (1901). This International Geological Congress ruled the following use of terms: 



1. £'ms = Palaeozoic, Mesozoic, Csenozoic. 

 (Cajnozoic = Tertiary + Quaternary.) 



2. Periods as Cambrian, Silurian, Cretaceous, etc. 



3. Epochs = 'Eo . . ., Meso . . ., Nco . . ., as Eodevonian, Mesodcvonian, Neode- 

 vonian, etc. 



4. Ages == Etages =(Ag.cs or Stages), as Astian, Bartonian, etc. 



5. Phases=Life Zones, such as "zone a Cardiola,'' etc. 



