70 E VOL UTION 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 papillae ; 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. That such rapid 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. 



