No. 3.] MODE OF EVOLUTION IN THE MAMMALIA. 37 1 



be very direct, and subject to comparatively little fluctuation, ad- 

 vancing steadily in a definite direction, though with slight 

 deviations. Thus in the cameline series, the size and position of 

 the orbit, the shape and character of the posterior nares appear 

 to change first in one direction and then in another, so that in 

 some respects the skull of Pcebrotheriwn is more modernized 

 and ruminant-like than that of the camel. But these fluctuations 

 are slight and of no great importance. On the whole we are 

 impressed by the steady march of differentiation ; thus, in the 

 equine series the premolars one by one become molariform, the 

 molar pattern more complex, the face elongated, the digits are 

 continually reduced in number, the median digit becomes more 

 and more enlarged, and the carpal and tarsal bones adjusted to 

 the new character of the strains, the limbs become more and 

 more elongated, and the stature of the whole animal increased. 

 In many genera the cycle of variation appears to be a singularly 

 small one, and as Neumayr has observed in the Mollusca, the 

 same or a very similar cycle appears in successive genera. Thus 

 in the genera Palceosyops, Diplacodon, and Titanotherium, of the 

 Bridger, Uinta, and White River formations respectively, there 

 are curious similarities in the specific variations, which are re- 

 peated in each of the successive formations. The same thing is 

 true of the Oreodons of the White River, and Eporeodons of the 

 John Day, though perhaps these cases are in part to be explained 

 as belonging to genera of multiple origin, where several species 

 of the older genus have similarly and simultaneously taken on 

 the character of the new. Indeed, the limited plasticity of the 

 mammals, except along certain definite lines, is very marked. 

 In every formation the majority of species appear to die out 

 without leaving any successors behind them, and too early a 

 specialization would seem to be fatal to the perpetuation of a 

 group. With rare exceptions the progenitors of permanent lines 

 seem to be those forms which have not strayed too far in any 

 direction from the safe middle course ; every formation contains 

 numerous examples of what from one point of view may be called 

 premature differentiations, advantageous, no doubt, to their pos- 

 sessors, but not sufficiently plastic to adapt themselves readily to 

 new conditions. 



These facts are opposed to the assumption that in the evolu- 

 tion of a mammalian phylum frequent alternations of advance 



