No. 2.] THE HARD PARTS OF THE MAMMALTA. 223 
close together, and of slender form. The canines form part of 
the series, and are not distinguishable by external characters 
from the incisors. The reason of the peculiar characters dis- 
played by these teeth is to be found in their present habits. 
These organs are used as combs for dressing the fur, and for 
removing parasites from the skin, and the apparatus is a most 
effective one. The gradual evolution of this character and its 
accompanying function may be traced from the Adapidz of the 
Eocene period. In some of these forms (as Tomitherium) the 
incisors are quite oblique, and in Adapis the canines are inci- 
siform. The mechanical effect of combing the hair with inci- 
sors of ordinary structure, would be to first wear off any lateral 
expansions of the crowns, and then to narrow their apices. The 
strains would also gradually cause a successively procumbent, 
and then decumbent direction of the crowns. Lack of use, 
owing to lack of opposition of the inferior incisors, would be 
followed in the superior incisors by the reduction in size which 
characterizes them. I have not seen Galeopithecus in life, but . 
I suspect that its very peculiar comb-like inferior incisors are 
the product of a similar habit. In this genus, however, the 
incisors have transversely expanded crowns, and they are divided 
from the edge to near the base by parallel fissures. Thus is 
produced a comb, which is as effective functionally as is that 
of the Lemurs. I suspect that this result was brought about 
by use, on the part of the ancestors of this genus, of incisor 
teeth already so expanded as to suffer division rather than con- 
traction by the long-continued friction of hair. 
The atrophy of incisors is a fact for which mechanical reasons 
are not readily found. It has occurred in the higher Ambly- 
poda (Fig. 66) and Artiodactyla, and in some of the Perissodac- 
tyla (Menodontide, Fig. 81), as well as in the Edentata. In the 
three groups first named, this loss has been accompanied by 
the development of horny processes on the skull, and this 
modification, occurring on three so diverse groups of Ungulata, 
excites the suspicion that there is some necessary connection 
between the two phenomena. I have suggested that the atrophy 
of the incisors in these cases was due to the abstraction of 
growth energy and material, from the premaxillary region, for 
use in horn-building. But there is no demonstration as yet 
attainable that such is a dynamic law. The loss of the incisors 
