940 EXOLLTIONAL MODIFICATIONS 



complete the adaptive modifications of the primates, they were by no means 

 the only forms undergoing an active process of differentiation. From some 

 period in the Mesozoic a host of primitive mammals began to pass into the 

 great habitat zones along many different lines of adaptive radiation. This 

 process of mammalian differentiation led up to the era of geological time 

 properly called the age of mammals, more particularly, perhaps, the age of 

 placental mammals. Even at the beginning of the Tertiary period there had 

 appeared such diversified forms as the carnivores (creodonts), bats, insecti- 

 vores, hoofed mammals (protoungulates), rodents, lemuroids and tupaioids. 

 All of these orders developed a profusion of families, genera and species, 

 linked together, however, by one mutual bond of heritage in their placental 

 mode of development. The duration of this great division of time is variously 

 estimated between ten million^ and one hundred and forty million years. - 



' American Museum of Natural History estimate. 

 - Bv radium emanation method. 



