264 



THE ORIGIN AND EVOLUTION OF LIFE 



separated regions in a closely similar manner and develop 

 closely similar characteristics in approximately a similar length 

 of time. The same is true of the widely separated lines of 



descendants from the mas- 

 todons, elephants, and rhi- 

 noceroses. This law of 

 uniform evolution and of 

 the development inde- 

 pendently in descendants 

 from the same ancestors of 

 closely similar characters 

 is confirmed in Osborn's 

 study of the evolution of 

 the titanotheres (Fig. 127). 

 In these animals, which 

 have been traced through 

 discoveries of their fossil 

 remains over a period of 

 time extending from the 

 beginning of the Lower 

 Eocene to the beginning 

 of the Middle Oligocene, 

 inclusive, is exhibited a 

 nearly continuous, ^ un- 

 broken transformation 

 from the diminutive Eoti- 

 tanops of the Lower Eocene 

 to the massive Brontothe- 

 rium of the Lower Oligocene, the latter form being so far as 

 known the most imposing product of mammalian evolution, 



1 The continuity is broken by the extinction of one branch and the survival of an- 

 other.^ It IS a continuity of character rather than of lines of descent. In some cases 

 there is a continuity both of characters and of branches. 



Fig. 128. Stages in the Evolution of the 

 Horn in the Titanotheres. 



This shows that these important weapons arise 

 as rectigradations, /. c, orthogenetically and 

 not as the result of the selection of chance or 

 fortuitous variations. Horns, large, 4, Bron- 

 totheriinn platyccras, Lower Oligocene; horns, 

 small, 3, Protitanothcrium emarginaliim, Upper 

 Eocene; horns, rudimentary, 2, Manteoccras 

 manteoceras, Middle Eocene; hornless stage, 

 I, Eotitanops horealis, Lower Eocene. 



Models in the American Museum of Natural 

 History, prepared for the author by Erwin S. 

 Christman. 



