74o PERISSODACTYLS xxx. 12 



the much fuller sequence in North America. It is probable that there 

 are many equally unjustified conclusions in our current beliefs about 

 evolution. 



The outstanding conclusion from a study of horse evolution is that 

 it is very difficult to describe the change as occurring in a single 

 direction, as supposed by believers in 'orthogenesis'. Apart from the 

 fact that many 'side-lines' can be detected beside the line that happens 

 to have survived, it is important to remember that not every line 

 evolves in the same direction. Thus in at least two genera of horses 

 size became progressively smaller (*Nannippus and *Archaeohippus). 

 However, it is certainly true that in some lines evolution may proceed 

 for long periods in one direction. We have no clear evidence why this 

 should be so, but it is reasonable to suppose that it is due to 'ortho- 

 selection', that is to say, the survival of animals that adopt a particular 

 method of life for which they are suited by a particular make-up. 

 The effect of this would be gradually to select all those genetic factors 

 that make for success in one environment (say, grazing on grassy 

 plains) and hence to produce evolutionary change in one direction. 



