INTRODUCTION 25 



It is obvious that a mammal may hunt for its herbivorous diet in sev- 

 eral (Hff(U"(^nt habitats, in the water, on kind, on trees, beneath the surface 

 of the earth, and since the Hmbs are the means of locomotion in these 

 hal)itats a mammal may be herbivorous and natatorial, like the manatee 

 {Manatus) and dugong {Halicore), herbivorous and arboreal, like the tree 

 sloths (Bradypodidae), herbivorous and cursorial, like the horses. This 

 makes it perfectly clear why there is no fixed correlation between the 

 structure of the teeth and that of the limits, and is another proof of our 

 inability to predict the form of one part of an animal from our knowledge 

 of another part. 



The prolonged operation of the adaptive radiation of mammals from 

 primitive and generalized forms into specialized and adaptive forms has in 

 the vast periods of geologic time (see p. 63) evolved or created the existing 

 Orders of Mammals. 



The Orders of Mammals^ 



A brief review of the chief kinds of mammals living and extinct is 

 needed before we can take a survey of their history; otherwise the reader 

 will be lost in details without being able to comprehend general kinds and 

 relations. A full conspectus of the classification of the principal types 

 of mammals living and extinct will be found in the Appendix of this volume, 

 to which the student or reader may make rapid reference through the 

 index. 



With reference to adaptive radiation, each order of mammals should 

 be thought of as having a typical mode of life from which its various mem- 

 bers diverge in various degrees, sometimes so far as to take up an entirely 

 different mode of life. The typical life is usually the original, ancestral or 

 primitive life which characterized the order when it first diverged from 

 other orders; as a rule it is the typical mode of life which gives or has 

 given the dominant or profound anatomical characters to the teeth and 

 skeleton. For example, the rodents were originally herbivorous, gnawing 

 animals, and this is still typical of most rodents, but certain rodents have 

 departed so far from their ancestral habits as to become not only aquatic 

 but fish-eating. Thus there is a clear distinction between the primary, 

 typical, original, fundamental adaptation of an order, and the secondary 

 or acquired adaptations which many of its members may enter upon and 

 thus imitate the typical adaptation of another order. 



The grand divisions and subdivisions of the Class Mammalia of Linnaeus 

 are as follows: 



' For a review of the evolutionary relationships of the principal groups of mammals, see 

 Gregory, W. K., The Orders of Mammals. Bull. Amcr. Mus. Nat. Hist., Vol. XXVII, 1909. 



