breathing water-dissolved oxygen means that the fish are 

 unlikely ever to evolve any organs of intelligence which 

 would require a more efficient means of energy release 

 than the fish possess. 



The horses alive today form what is perhaps the most 

 completely studied animal group as regards their evolution. 

 The ancestors of the horses first appeared in the Eocene 

 period, some 50 million years ago. The diminutive Eohip- 

 pus, about as large as a spaniel dog, possessed four toes on 

 each front foot, and three on each hind one, and lived in the 

 newly appearing grasslands bordering the forests of this 

 warm climate. As the climate cooled in some regions in the 

 following Oligocene period, causing an increase in grass- 

 lands and dwindling forests, the larger Mesohippus and 

 Merychippus (Miocene) species evolved, the number of 

 toes diminishing to three on all feet, and the structure of 

 the biting surfaces of the teeth increasing in complexity to 

 give better grass-chewing ability. With their lengthened 

 legs and gregarious habit, these horses were enabled to 

 elude their feline predators, and as the new Alps (Oligo- 

 cene)) and Himalayas (Miocene) rose and divided Europe 

 and Asia into varied climatic zones the horses migra- 

 ted into new regions. The new great grass plains of North 

 America were invaded by horses crossing the Bering Strait 

 during one of its periods of elevation as a land bridge 

 between Asia and America. In the Pliocene still larger 

 horses (Pliohippus) with only one toe on each foot had 

 evolved, while later in the Pleistocene horses (Equus) as 

 we presently know them appeared. These had only the 

 single hoof which provides such excellent strength to the 

 very long legs of the horse — permitting this animal the 

 very high and sustained running speed which it has to- 

 day. The jaw was now greatly deepened and lengthened 



10 



