EVOLUTION OF ANIMALS 



551 



may be developed constantly as a result of hybridization, not in the sense 

 of crossing two species but in the sense of crossing genetic characters. 



Still another method is that known as orthogenesis. Paleontologists 

 have been very active in urging the importance of this as a method and 

 the evidence to support it comes largely from fossil types. According to 

 the theory of natural selection, w^hen a characteristic becomes harmful 

 to an animal it should disappear, but there is paleontological evidence 

 to the effect that in the past many types that have speciaHzed in certain 

 directions have gone on developing in that direction, even when over- 

 specialization has resulted in harm and has ultimately led to the extinc- 

 tion of the animal. An example of such a type is the saber-toothed tiger 

 (Fig. 380) ; its upper canines developed until they became exceedingly 

 effective both in the securing of prey and in defense, but they seem to 





Fig. 380. — Saber-toothed tiger; restoration. {Redrawn from Scott, "History of Land 

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



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, w^hich 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. 



591. 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 



