56 GENERAL EVOLUTION. 



and mandibular tusks permanent in some males ; in the third, 

 four cross crests and the mandibular tusks yariable ; in the fourth, 

 five cross crests on the third molar ; tusks unknown. In Stego- 

 don the mandibular tusks cease to appear, the crests of the third 

 molar become more numerous, and embrace between them, in the 

 bottoms of the valleys, a strong deposit of cementum. In Loxo- 

 don the crests have the whole interspaces filled with cementum, 

 while the same thing holds in Elephas, with a greatly increased 

 number of cross crests, which become vertical laminas. The lami- 

 nar character has become apparent from its rudimental condition 

 in Stegodon. 



Now, these are stages of development, though not in a con- 

 tinuous, single line. The shedding of the inferior tusks takes 

 place earlier and earlier in the genera from Dinotherium, till 

 they never appear in Stegodon. The molar teeth, it is well 

 known, present, as they succeed each other from back to front, 

 a regularly increasing number of transverse crests in the same 

 species. Thus, in Trilopliodoii ohioticus the first molar presents 

 but two, while the last presents six. The last molars of other 

 genera present a very much increased number. What is it, 

 then, but that the increased number of crests in the third molar, 

 definitive of these genera, is an acceleration of growth ; the 

 fourth in Trilophodon is structurally third in Mastodon, and the 

 fourth of Mastodon being third in Pentalophodon ; the fourth 

 of Pentalophodon becoming third in Stegodon, and so to the 

 end ? This is confirmed from the proved fact of the disap- 

 pearance of the premolars. They are fewer in Trilophodon 

 than in Dinotherium, and are soon shed ; they are also early 

 shed in Mastodon and Stegodon {insignis Falc. Caut.), and are 

 not known to exist in the succeeding types ; the acceleration of 

 succession of teeth has caused them to be entirely omitted. The 

 young tooth of Elephas, moreover, is represented by a series of 

 independent parallel laminae at first, which, when they unite, 

 form a series of crests similar to the type of the genus ]\Iastodon 

 and others of the beginning of the series. The deposit of cemen- 

 tum takes place later, till the valleys are entirely filled up. Thus 

 the relations of this part of the tooth-structure in the series are 

 also those of the successional growth of those of Elephas, the ex- 

 treme of the series. 



It would be only necessary to show that two distinct condi- 

 tions, in any of these respects, occurred among the different indi- 



