172 THE AGE OF MAMMALS 



vertebrae differ from those of any other known animal. The ancestral 

 zeuglodonts are known in the Eocene of Africa. It appears possible that 

 these great American forms are migrants from the Mediterranean seas of 

 the Old World ( see p. 73). 



Causes of Extinction of the Archaic Orders of Eocene Mammalia ' 



Extinction is not on the same scale nor tlue to the same causes through- 

 out the successive geologic epochs. The great law of mammalian improve- 

 ment through the elimination of the least fitted becomes less sweeping in 

 its effects as time goes on and the Mammalia become perfected. Eocene 

 extinction is chiefly that of whole orders of archaic mammals. Late Eocene 

 and Oligocene extinction is preeminently that of inadaptive families. 

 Miocene times complete the elimination of families and are characterized 

 by the extinction of inadaptive genera. This is also true of Pliocene times. 

 The especial feature of Pleistocene times is the ruthless and world-wide 

 extinction of highly adaptive kinds of mammals in certain parts of the 

 world, both of genera and of species. 



Competition of lower and higher types. — It is a very striking fact that 

 not only the archaic but a very large proportion of what we may term 

 the prophetic, modernized, Eocene mammals became extinct at or before 

 the close of this period. The causes of extinction were probably not the 

 same in the two groups. The archaic mammals are very generally dis- 

 tinguished by extremely small brains. This is certainly true of many of 

 the creodonts, of Phenacodus, Coryphodon, and the Dinocerata. This 

 limited brain power placed these quadrupeds at a disadvantage in com- 

 petition with the higher placentals. Under contemporary or prevailing 

 conditions of life, intelligence and instinct are matters of first importance 

 in relation to quickness, alertness, adaptability to new conditions. Modern 

 quadrupeds differ widely in this regard; on the western plains of North 

 America, for example, the horses by their resourcefulness save their lives 

 where cattle perish. The cursorial Phenacodontidse measured their psychic 

 powers with the cursorial Equidae; the tooth structure in the two families 

 was substantially the same, but the phenacodonts were handicapped by a 

 lower brain organization and by an inferior foot mechanism. The long 

 survival and steady increase in size of the clumsy Amblypoda is one of the 

 most astonishing phenomena of Eocene mammal life. The extinction of 

 these mammals may be attributed to two causes: the low brain power, 

 which may have inhibited the proper defense and care of the young, and 

 the arrested evolution of the grinding teeth, which are actually no larger 

 and little more effective for the comminution of food in the giant Uinta- 

 therium than in the smaller Coryphodon. It is noteworthy that where the 



■ Osborn, H. F., The Causes of Extinction of Mammalia. Amer. Xatural., Vol. XL, 

 no. 479, Nov. 1906, no. 480, Dec. 1906. See especially pp. 856-857, 850-854, 842. 



