2 



EVOLUTION AND GENETICS 



past {fig. 1) whose bones and shells have been pre- 

 served in the rocks {fig. 2) . He thmks of these ani- 

 mals as having in the past given birth, through an 



Ecjuus 



'liohippus 



Nerychippus 



Mesohippus Orohippus Ppotopohippus 



Fig. 1. — Outline of horses of different geological periods, showing 

 their relative sizes and the decrease in the number of toes. {After Lull.) 



