The History and Affinities 

 of the Recent Land Mammals 

 of Western North America 



William H. Burt 



Museum of Zoology, University of Michigan^ 

 Ann Arbor 



When we speak of the origin of a fauna or 

 of some component of a fauna we imply that a more or less definite 

 place and time is involved. If our present concept of evolutionary 

 process is correct, faunas or parts thereof do not arise de novo. 

 Instead, they constitute a continuum. How, then, without being 

 strictly arbitrary, can we reasonably designate a place and time as 

 a starting point in this continuum — a place and time for the origin — 

 when the fauna or taxon is merely changing from one complex to 

 another. Can we really talk about the origin of a fauna or a taxon 

 without going all the way back to the origin of life itself, which may 

 also have been a continuum? Would it not be more nearly accurate 

 to speak of a phase in the history of a fauna or a taxon? In mammals, 

 for example, we might speak of the reptilian-mammalian phase, 

 which must have persisted for some millions of years, then the 

 mammalian phase, which has continued to the present. Geolog- 

 ically speaking, we might designate a pre-Tertiary phase and a 

 Tertiary phase in mammalian history. In this way we would indi- 

 cate a gradual change, not an abrupt one. Part of our thinking, 

 perhaps, has been influenced by the great discontinuities in the 

 geologic record. These break the continuum in the fossil record and 

 give to the uninitiated the impression of great steps in evolution. 

 They make for easy categorizing, but the animals and plants were 

 living and evolving during these great intervals of time when, 

 in the history of land mammals, no terrestrial deposits were being 

 formed, and no record was left. We are apt to pass over these 

 unknown intervals of time and say, for example, that the mammals 



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