EVOLUTION OF ANIMALS 



553 



present many types have appeared (Fig. 381) which show, in general, 

 an increase in size, the production of a trunk, or proboscis, the develop- 

 ment of tusks, and changes in the molar teeth. Present-day elephants 

 are not the largest in the series, which culminated in a type 14 feet high; 

 the largest exact measurement recorded for a modern elephant is 11 feet, 

 though 13 feet has been reported. The trunk, or proboscis, representing 

 an elongation of the nose and upper lip, is at a maximum in the modern 

 types. It is a powerful but dehcate prehensile organ used in gathering 

 food and in taking up water, which is then passed into the mouth. In 

 the evolution of the elephants tusks were developed from both the lower 

 and the upper jaws, but in the more recent types those in the lower jaw 



Fig. 382. — The evolution of the horse during many millions of years showing an increase 

 in its size and a decrease in the number of its toes, from four to one. A, four-toed Eohippus 

 of the Eocene epoch. B, three-toed Mesohipptis of the Oligocene epoch. C, Merychippus 

 of the Miocene epoch with its large central toe and smaller side toes. D, Equus of the 

 Recent epoch with only one visible toe. {Redrawn from Mavor, "General Biology.") 



have been suppressed while those in the upper have been retained and 

 developed. The length of the tusks possessed by the modern elephants 

 is far exceeded, however, by the tusks of some of the extinct types. This 

 is perhaps an example of excessive specialization. The skull has become 

 very high, a fact which makes it easier for the animal to carry the weight 

 of the large tusks. The molar teeth have become greatly developed and 

 reduced in number, not more than eight being functional at the same time. 

 These teeth, as in all herbivorous animals, are fitted for grinding the food. 

 The most ancient horses, that from Europe known as Hyracotherium 

 and that from western North America as Eohippus, also belong to the 

 Eocene period. Eohippus (Fig. 382^) was an animal about the size of a 

 small dog, standing only about 12 inches high. The head was elongated, 



