EVOLUTION OF ANIMALS 



523 



have gone on developing until they became a handicap and perhaps were 

 ultimately a factor in its extinction. Another animal often given as an 

 example of overspecialization is the Irish elk the antlers of which were 

 greatly developed; at first this was to the advantage of the animal but 

 later they reached such a size as to impede its progress in the forests and 

 place it at a disadvantage in escaping from enemies. To explain such 

 cases as this it has been suggested that the development of a character is 

 due to a hereditary tendency accompanying a progressive change in the 

 genes, which causes the animal to develop constantly in a certain direc- 

 tion. If the result is to make the animal more effective, natural selection 

 tends to perpetuate the type, but when overdevelopment and disadvan- 

 tage follow, the hereditary tendencies cannot be reversed and the result 

 is extinction. 





Fig. 319. — Saber-toothed tiger; restoration. {Redrawn from Scott, "History of Land 

 Mammals in the Western Hemisphere," by the courtesy of The Macmillan Company.) 



589. Evolutionary Series. — Several evolutionary series exist the best 

 known of which are those of the elephants, horses, and camels. 



The earliest known elephant, known as Moeritherium, is found in the 

 upper Eocene deposits in northeastern Africa. This animal somewhat 

 resembled a hog in form, with a projecting snout, and was of only moder- 

 ate size, being between 3 and 4 feet in length. From Eocene time to the 

 present many types have appeared (Fig. 320) 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 hp, is at a maximum in the modern 

 types. It is a powerful but delicate 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 

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



