xxi. 9 CONSERVATIVE EUTHERIANS 577 



fossil horse brains that as the body size increases the brain-body 

 ratio decreases. 



8. Conservative eutherians 



Much of this change was the result of adopting a fully terrestrial 

 and often herbivorous mode of life, in place of the earlier arboreal and 

 insectivorous or carnivorous one. Changes in these four directions 

 have taken place in many separate mammalian lines, but the evidence 

 contradicts the thesis that they are the result of some force of ortho- 

 genesis, driving the animals infallibly along. In nearly every group 

 there are examples of some animals that have remained nearly un- 

 changed for long periods, e.g. opossums and shrews since the Creta- 

 ceous (80 million years), lemurs and tarsiers since the Eocene (50 

 million years), pigs and tapirs since the Oligocene (35 million years), 

 and deer since the Miocene (20 million years), to name only a few. 



It is important to study such animals in which there has been little 

 change; they form the 'controls', and may enable us to recognize the 

 factors inducing change when it does occur. Moreover, in many 

 further lines there has been change in some but not all of the above 

 directions, for instance, many of the most successful mammals have 

 remained small. There may even be changes in the directions opposite 

 to those listed, for instance, some edentates and whales have more 

 than the original number of teeth. Mammals do not commonly de- 

 crease in size during evolution (but they may do so), and they prob- 

 ably never reacquire lost digits, though in a few claws have reappeared 

 after they had been lost. 



9. Divisions and classification of Eutheria 



Infraclass 3. Eutheria 

 Cohort 1. Unguiculata 

 Order 1. Insectivora 

 Order 2. Chiroptera 

 Order 3. Dermoptera 

 *Order 4. Taeniodonta 

 *Order 5. Tillodontia 

 Order 6. Edentata 

 Order 7. Pholidota 

 Order 8. Primates 

 Cohort 2. Glires 



Order 1. Rodentia 

 Order 2. Lagomorpha 



