70 EVOLUTION AND DISEASE. 
Teeth furnish much that is interesting in connection 
with vestigial structures. ‘The enamel which constitutes 
a covering to the crown of teeth in many mammals may 
be looked upon as a vestige: no tissue resembling it 
occurs in any other part of the body of a mammal. 
Teeth are essentially calcified cutaneous papilla ; at one 
time in the history of our planet, her seas were peopled 
with numerous ichthyic forms furnished with an armour 
of enamel; some of the best specimens being the mailed 
ganoids. Under pathological conditions, however, teeth 
may spring up in such extraordinary situations as in 
cysts of the ovary. 
Turning to particular cases, we may study an 
instructive example in the horse. This admirably 
specialized animal possesses three incisors and one 
canine on each side; then an interval follows until 
we reach the pre-molars: a study of closely allied 
fossil forms indicates that this gap, or diastema, was 
occupied by well-formed teeth in the ancestors of the 
horse, and this view finds support from the circumstance 
that the first pre-molar is vestigial and presents itself as 
a tiny socketless tooth. This functionless pre-molar is, 
as a rule, shed early ; when persistent it is frequently a 
source of considerable annoyance to the animal, as every 
animal developed antlers weighing from twenty-seven to thirty kilo- 
grams. Thatsuchrapid growth as this must tax the vital powers of 
an animal is clearly shown by the circumstance that during the 
growth of the antlers the Wapiti required, and was supplied daily 
with, nearly twice the quantity of food consumed by it at the time 
when the antlers were fully grown. The bearing of this fact from 
an evolutionary point of view is too.obvious to need any pointing 
out. wt o> 
