80 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
Elephas.—In this genus are included a number of extinct forms 
(the mammoths) from three or four continents, and the living ele- 
phants. The extinct forms, though called mammoths, were not large 
animals, being no larger than the Indian elephant of today, and not 
so large as the living African species. Some of the features of the 
elephants, their size, the short neck, the long proboscis, and the heavy 
tusks are matters of common observation. The skull is very high 
and short (Fig. 5, A’). The height is due chiefly to the development 
of cancellate bone, not to the enlargement of the brain, which is still 
quite small. As stated above, the high skull affords the necessary 
leverage for the muscles that support the weight of the tusks. The 
molar teeth are distinctly grinding teeth (Fig. 5, A). Each tooth 
bears a number of transverse ridges, about ten in the African elephant 
and two dozen or more in the Indian species. These ridges are worn 
down by the chewing of harsh food, so that the upper surface displays 
a number of flattened tubular plates of enamel inclosing dentine and 
bound together by cement. A tooth is completely worn out by use, 
and is replaced by another. The method of replacement, however, 
is peculiar. While the tusks (incisors) are of two sets, one following 
the other like milk and permanent teeth of other mammals, the 
grinders succeed one another in continuous fashion. There are never 
more than two visible grinders on each side of each jaw. As they 
wear out they move forward in the jaw, and are replaced by new teeth 
appearing behind. New molars thus enter at intervals of two to four 
years in young elephants, and at intervals of 15 to 30 years in later 
life. If an elephant lives long enough (60 years or more) it develops 
a total of 28 teeth, including tusks, but has not more than ten (often 
less) at any one time. 
Correlated with the nature of the teeth of the elephants are their 
food and chewing habits. Whereas the ancestral forms whose molars 
bore prominent elevations lived on twigs and tender herbage which 
they crushed in mastication, the mammoths with their flattened tooth 
surfaces devoured grasses, sedges, and other harsh vegetation which 
they ground with lateral motion of the teeth upon one another. In 
this respect modern elephants are like the mammoths. 
In the changes described above is found one of the most beautiful 
and best established evolutionary series with which the palaeontolo- 
gist is acquainted. Only a few others equal or approach it in clearness 
and completeness. 
