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THE AGE OF MAMMALS 



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question of time, and it is 6ne of our chief objects in this review of the his- 

 tory of the fossil mammals of the Old and New Worlds to use this history 

 as a means of closely establishing similar divisions of past time in these 

 two widely separated geographic regions. 



Employing the suggestive terms of Louis Agassiz, the wliole life history 

 of the earth may be divided into Ages of Invertebrates, of Fishes, of Rep- 

 tiles, of Mammals; the latter Age comes as the last episode before the 

 final Age of Man. The Age of Mammals is technically known as the 

 Ccenozoic Era (Age of Recent Life), a term which is partly equivalent to 

 but has generally replaced the older term Tertiary, which signifies the 

 third period in the history of life. The Caenozoic is subdivided into two 

 Periods and six Epochs, as follows: 



Epochs 



'HoLOCENE, (from oAcs, entire, Kaivos, recent), or recent time, characterized 



by the world-wide destruction and elimination of mammals through 



the agency of man. 

 Pleistocene (from TrAeto-ros, most, Kaivo's, recent), a life period in which 



the majority of the recent forms of mammals appear and in which 



there occurs the last glacial period and a great natural extinction of 



earlier forms in all parts of the world. 



Pliocene (from TrAetwv, more, Kairds, recent), a vast modernization of 

 the mammals in which all the existing orders and families are known, 

 as well as many of the existing genera, but few or no existing species. 



Miocene (from fxeLwv, less, Katvos, recent), an earlier stage of moderniza- 

 tion, in which lived many mammals closely similar to existing forms. 



Oligocene (from oXtyos, little, /catvo's, recent), characterized by the ap- 

 pearance of many existing types of mammals and the gradual dis- 

 appearance of many of the older types. 



Eocene (from •^ws, dawn, Kaiv6s, recent), characterized by the first ap- 

 pearance of many of the ancestors of the modernized mammals and 

 the gradual disappearance of many of the archaic types characteristic 

 of the Age of Reptiles. 



These grand time divisions of the Csenozoic are the work of the nine- 

 teenth century, and the incessant trend of discovery is to multiply time 

 divisions and make them more minute. The work of the twentieth cen- 

 tury is precise correlation. The ardent studies of the great French natural- 

 ists Lamarck, Alexandre Brongniart (1770-1847), Cuvier, and Deshayes 

 in the early part of the nineteenth century, the golden age of palaeontol- 

 ogy in France, were accompanied by a growing realization of the vast 

 stretches of geologic time as witnessed in the vast changes which have 

 taken place in the animal life of the globe and in the enormous thickness of 

 some of the sedimentary rocks which had been deposited even during this 

 later or Tertiary Period. It became absolutely necessary to make divi- 

 sions of the Tertiary; the threefold division was in the first instance due 



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