630 THE AMERICAN NATURALIST [Vol. LI 



a given formation the successive species show a regular 

 increase in size and a progressive molarization of the 

 pattern of one or more of the premolar teeth. A typical 

 case of this kind is recorded by Matthew^ in the genus 

 Cynodoyitomys, a small insectivorous mammal of the 

 Lower Eocene which is represented by three successive 

 species which do not overlap in time, but are separated 

 by small progressive differences in the premolars and 

 molars. Each species is represented by series of from 

 ten to twenty specimens, from successive horizons of the 

 Bighorn and Wind Eiver Basins in Wyoming. Another 

 instance of practically continuous evolution is furnished 

 by the Middle Eocene titanothere PalcBOsyops. Professor 

 Osborn and the present writer have observed that in this 

 genus the species named paludosus, major, leidyi and 

 robustiis form a regular and nearly continuous series ex- 

 tending from the lower to the higher levels of the Bridger 

 Basin, in which the lower and upper premolars gradually 

 evolve toward the molar pattern. A fifth species, P. copei, 

 from the uppermost fossiliferous levels of the Bridger 

 Basin is considerably more advanced than any of its 

 predecessors, and is connected with them by intermediate 

 specimens from the nearby Washakie Basin of the same 

 age. 



In other cases the material indicates that while some 

 phyla evolve at a nearly uniform rate others lag behind 

 at varying rates, the extreme cases furnishing the relicts 

 or ''living fossils" which give so many useful hints as to 

 the primitive characters of a race. 



Such an instance is furnished by the history of the 

 Eocene primates PeUjcodMS and Notharctus (Table I). 

 The oldest species, Pelycodus ralstoni, is of small size and 

 very primitive character. The latest species, Notharctus 

 crassns, is about twice as large and of very advanced 

 character. Many intermediate stages are known. Of 

 these P. relictus is an extremely conservative form which 

 has acquired only a few of the progressive characters 

 seen in its contemporaries. 



5 Bun. Amer. Mus. Nat. Hist., Vol. XXXIV, 19, p. 470. 



