Nos. 455-456.] 



MAMMALIA. 



for the development of this anomaly, and would explain {a) the 

 presence of but two phalanges on digit I, three on each of the 

 others ; {b) the epiphysis of digit I being proximal as in the pha- 

 langes, instead of distal as in the remaining metapodials ; {c) the 

 anomalous musculature of digit I on all mammals, the object of 

 which is clearly seen when the digit is opposable, but is quite 

 unexplained otherwise. 



It would be beyond the purpose of this article to give at all 

 fully the facts which support the above characterization of the 

 primitive mammalia, but enough has been said to show the gen- 

 eral nature and force of the evidence on which it is based. 

 The modern orders of mammals have departed to a varying 

 degree from this primitive type. The arboreal primates retam 

 most nearly the primitive character, except that the limbs are 

 elongated, the face is shortened and the brain greatly increased 

 in size and complexity. The rodents, largely perhaps on 

 account of their small size, have usually retained a great 

 deal of the primitive skeleton character, but the dentition is 

 much modified— least of all in the arboreal squirrels, which 

 are almost unaltered in dental characters from the first known 

 rodentia of the early Tertiary (Paramys, etc.). The Insectivora 

 likewise retain many primitive characters, but in most cases m 

 combination with one or another high specialization of an 

 unusual kind. The Carnivora retain the primitive character 

 more or less completely in the neck, trunk and legs, but show 

 much greater adaptive changes in^the feet and especially in the 

 teeth. (Several of the Creodonta have a more or less opposable 

 pollex, but this is lost in the later carnivora.) The Ungulata 

 are by far the most highly altered group among the land mam- 

 mals, almost every part of the body having gone through great 

 adaptive changes, although the evolution is most marked in 

 teeth and feet. We are able to trace the history of these 

 changes more fully in this than in any other order, as its geo- 

 logical record is more complete. The Edentata early developed 

 certain remarkable specializations which soon obscured their 

 primitive characters. Of the remaining orders of placental 

 mammals our geological record is very imperfect, and their 

 relationship to the central type mainly hypothetical. me 



