SELECTION 153 



The Eadiolarians are amongst the lowest of or- 

 ganisms. Now let us go to the highest class for an 

 illustration. The different species of elephants, both 

 living and extinct, are distinguished, among other 

 characters, by the different ways in which the 

 enamel and dentine of the molar teeth are folded 

 into transverse ridges. This folding of the hard 

 enamel, and the filling of the interstices with a 

 softer cement, is a very useful structure ; for, owing 

 to the different hardness of the different parts, an 

 uneven surface is constantly maintained, which 

 enables the tooth to fulfil its function of grinding 

 down leaves and shoots of trees, on which both the 

 species of living elephants feed. In the African 

 elephant the transverse ridges are thicker in the 

 middle than at the ends ; while in the Indian ele- 

 phant they are equally thick all through, and the 

 number of ridges in each tooth is nearly double the 

 number in those of the African elephant. In the 

 Mammoth this type of tooth was still more 

 specialised than in the Indian elephant. Elephants 

 of both types, now extinct, lived together in 

 India during the pliocene period. In the pleisto- 

 cene, the African elephant lived in South Europe, 

 with other species, some of which belonged to the 

 Indian type ; and at the close of the pleistocene, the 

 Mammoth, which had the most specialised teeth 

 of any, became extinct. As these two forms were 

 competing for so long a time without either of them 

 gaining any advantage, we cannot suppose that the 

 two living species of elephant owe their preserva- 

 tion to the superior pattern of their molar teeth. 



