THE LIVING WORLD. 
485 
beneficent work, even though its execution involved new responsibilities, and 
demanded changes in the organization of the creature itself. The rodent is 
able to live everywhere and anywhere; to adjust himself to all climates, and 
to any regimen in the matter of food; its smallness of size and disproportion¬ 
ate courage, persistence and strength fit it to take care of itself under condi¬ 
tions which would prove fatal to the largest mammals, and its duty is limited 
to no one locality, no one geological, geographical or historical period. Denti¬ 
tion, Or the structure of the teeth, is, as has been said, the accepted scientific 
standard of classification, and the rodents are evidently named, solely with refer¬ 
ence to their dental peculiarities. The rodents have no canine teeth, for these 
would be useless to them. Hence, between the incisors and the canine teeth 
there is an unfilled gap. No living rodent has more than two developed inci- 
AARD-VARK, OR EARTH HOG. 
sors in each jaw. The typical number of molar teeth is six for each jaw, 
subject to increase in certain animals. 
Furthermore, all the teeth are solidly rooted, and yet grow without limit, as 
the tooth-making supplies are perennial. The arrangement of the dentine (or 
soft material of the tooth), and of the enamel is such that, when we add 
the way in which the teeth come together, we have every condition for the 
constant presence of teeth always in good condition, and constantly, sharpened 
by contact with each other. The molars, however, grow less readily, and in 
form are specially suited to the work which they are to perform. The rodents 
develop the cerebellum at the expense of the cerebrum, whose convolutions 
and complexities are few. It follows, therefore, that, while their intelligence is 
