CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



811 



has its distinctive velocity of the horn biocharacter; 

 and the same is true of the new cusps appearing on the 

 teeth. 



Therefore when we compare similar biocharacters of 

 every kind in two distinct phyla of the titanotheres we 

 find that not only the horns but every biocharacter has 



Figure 730. — The limbs of four embryos, showing the 

 ontogenetic velocity of the bones of the fore limb in 

 horses 



A, 20 millimeters, to D, 350 millimeters. After Ewart, 1894 (PI. 11)^ 

 rearranged. Observe the acceleration of proportions in the forearm 

 and manus. 



its distinctive phyletic velocity. For example, close 

 examination by Gregory of the actual percentage incre- 

 ments of proportions in the evolving Palaeosyops skull 

 demonstrates three principles of motion, namely, (1) 

 that each cranial, dental, and skeletal biocharacter is 

 evolving at a different rate, some faster, some slower; 

 (2) that a biocharacter which early in geologic time 

 evolves rapidly may afterward evolve more slowly, 

 while the velocity of other continuous proportion bio- 

 characters may be increasing; (3) that the movement 

 of a biocharacter may be reversed, so that after a long 

 period of phyletic velocity it may take an opposite 

 direction. 



The following five principles of biocharacter velocity 

 are well established in this monograph : 



1. All biocharacters are moving, each having its 

 separate velocity; the movement is either progressive 

 or retrogressive. 



2. The phyla of titanotheres are distinguished by 

 relative rates of velocity of similar biocharacters in 

 geologic time, also by the acceleration or retardation 

 of similar biocharacters in ontogeny. 



3. This biocharacter velocity may be cumulative 

 and run to an extreme — that is, to overdevelopment of 

 certain organs and structures. 



4. Juxtaposed biocharacters may show entirely dis- 

 tinct velocities. 



5. The retrogression and subsequent recession of a 

 biocharacter may reverse the order of its appearance. 



Having discussed the interpretation of the various 

 phenomena which are observed in the evolution of bio- 

 characters in titanotheres we are ready to interpret 

 the evolution of other series of American quadrupeds, 

 such as horses, rhinoceroses, elephants, which are now 

 under observation by Osborn. 



MODES OF BIOCHARACTER EVOIUTION ACTUAIIY 

 OBSERVED 



MAMMALIAN PHYLA DISTINGUISHED BY DIFFERENT 

 ONTOGENETIC AND PHYLOGENETIC VELOCITIES IN 

 THE SAME BIOCHARACTERS 



Members of different phyla may possess exactly the 

 same biocharacters, but each of these biocharacters 

 may exhibit a different rate of phylogenetic progress. 

 It is by these contrasts in the rate of evolution of a 

 large number of similar biocharacters that the phyla 

 of titanotheres and of other mammals may be clearly 

 distinguished. In 1902 the fact was established that 

 throughout the whole geologic period when the 

 Titanotherium-he&Tmg beds (Chadron formation) were 

 being deposited as many as eight different phyla of 



Figure 731. — Relative velocities (acceleration, 

 balance, or retardation) of a series of biochar- 

 acters (A- J) 



Some are progressive, developing rapidly; others, though 

 adjacent, are balanced or are retrogressive and degenerate. 



titanotheres were independently evolving in the same 

 general region. These eight phyla correspond more 

 or less closely to the several "genera" originally pro- 

 posed by Cope and Marsh. 



Study of these separate phyla by the author with 

 the cooperation of Dr. W. K. Gregory was directed to 

 show what each of these phyla was like at the begin- 

 ning of the deposition of the Tiiano^Aermm-bearing 



