632 



THE AMERICAN NATURALIST 



[Vol. LI 



Osborn in 1902 pointed out the evolutionary progress observable in 

 the species of Hyopsodus from successive stages of the Lower and 

 Middle Eocene; this is in general confirmed and extended by the far 

 larger collections [comprising more than a thousand specimens] now 

 available and the somewhat wider geologic range of the genus; but it is 

 evident that not one but three or four phyla are present in each horizon ; 

 the relations of the Lower Eocene species to those of the Middle Eocene 

 are not wholly clear, and the geological overlap of stages of each struc- 

 tural phylum suggests rather progressive displacement of older by 

 newer stages coming in from some other region, than gradual evolution 

 in loco. It might equally well be interpreted as the displacement of 

 older by newer " mutants," in the DeVriesian sense of the term. 



However this may be, the Lower Eocene species are distinguished 

 from those of the Middle Eocene by the less molariform premolars, and 

 this is most noticeable in H. simplex from the lowest horizon, while 

 the Lost Cabin species [from the upper part of the Lower Eocene] 

 approach nearest to those of the Bridger [Middle Eocene] . 



Examples of this kind miglit be multiplied, tending to 

 show that the evolution of Tertiary mammals has often 

 been more or less continuous, or by small successive 

 changes, at least during the relatively brief geological 

 periods that are represented by a large series of speci- 

 mens from closely sequent levels of an uninterrupted 

 stratigraphic series. And although mutations may well 

 be a paleontological reality, there is little danger that ver- 

 tebrate paleontologists are likely to draw false inferences 

 regarding the history of structures and of races through 

 mistaking independent contemporaneous mutants for suc- 

 cessive stages, for the simple reason that their observa- 

 tions are based on long series of specimens which are ar- 

 ranged in their true chronological sequence, from ascend- 

 ing geological horizons covering the whole Tertiary 

 Period. 



In this connection I submit an accurate diagram (Fig. 

 1) by Mr. Granger, which is fairly representative of the 

 kind of evolution demonstrated among many, but not all, 

 known races of mammals during the Tertiary and Quater- 

 nary Periods, a period of time conservatively estimated at 

 4,000,000 years. 



The character of the evidence tending to show that the 

 paleontologist is dealing with truly successive stages and 



