discussion of the materials which compose them, as 

 well as the origin of these materials, in order to con- 

 vey a real idea of their true natures. However, it might 

 be well to direct attention to a few of the more general 

 features. 



The teeth of horses, cattle, beavers, etc., continue 

 to grow in length for a long time and do not form 

 roots until late in life. This is because they feed on 

 abrasive substances and the growth compensates for 

 the wear upon them. The chisel-like incisors of ro- 

 dents grow during the entire life of the animal and are 

 kept at a more or less constant length by use. Should a 

 wood-chuck, for instance, lose one of its incisors, there- 

 by leaving the opposing one without a surface to grind 

 against, the existing tooth would continue to grow in 

 length until it would eventually lock the jaws and 

 prevent feeding. 



It might be mentioned that the hard enamel is 

 present only on the front face of the incisors in rodents 

 and, as the animals feed, the softer portion (dentine) 

 behind wears away much more rapidly, thus maintain- 

 ing a sharp, beveled, cutting surface. This is quite 

 noticeable in Beavers. 



The Skeleton 



To a person unfamiliar with the Phylogenetic 

 development of animals, and especially to anyone who 

 is not acquainted with the details of vertebrate con- 

 struction, the skeletons of mammals would seem to be 

 of diverse character. However, a comparative study 

 of the corresponding parts shows the skeletons of mam- 

 mals in general, to be remarkably similar in character, 

 in fact, they are, bone for bone, almost identical. The 



