INTRODUCTION TO MAMMALIAN PALEONTOLOGY 



17 



are rapid and conspicuous. They are in part hereditary 

 (germinal) diflferences, which pass down for generations 

 unmodified by habit or environment. 



For example, the American genus Peromyscus (the 

 white-footed mouse), as studied by Osgood (1909.1), 



(dolichocephalic). Peromyscus may have been widely 

 distributed from some common center during the last 

 40,000 years, and during this long period there may 

 have been both geographic or space evolution and 

 geologic or time evolution, the evolution in time being 



Figure 15.- 



-The family tree of the titanotheres, showing the relation between the branches (phyla), sub- 

 families, and genera, as known to science in 1919 



The shaded areas show connections that f 



3 well established; the dotted lines show gaps that remain to be filled by future discovery, especially 

 in the Uinta formation of Utah. 



presents a continuous series of transition changes in 

 color and form in species having a geographic range 

 from Tehuantepec to Alaska. In the northern re- 

 gions Peromyscus is larger and has relatively longer 

 teeth and a skull that may be somewhat elongate 



comparable to that which we observe in the geologic 

 phyla of the titanotheres. 



The existing genus Cervus affords another example, 

 significant because its geographic range is similar to 

 that of certain Oligocene titanotheres. 



