22 



ing problem underlying the whole subject. There is no 

 more fascinating or profitable work for the student of the 

 present mammalian fauna of North America than the trac- 

 ing out of the habitats, and determining the intergradation 

 or non-intergradation of such forms as compose many of 

 our leading genera of mammals. We have as yet scarcely 

 reached the point where this can be done with entire cer- 

 tainty for any group, but here and there boundaries have 

 been established, and we can begin to foresee in some 

 instances what will be the final results. Should the activity 

 of the last five years be continued for the next decade, it 

 will doubtless be possible at the end of that time to map 

 the distribution of most of our mammals with considerable 

 accuracy ; to know what forms intergrade, and over what 

 areas and under what conditions the intergradation occurs ; 

 also what are sharply isolated and localized, though 

 closely related to others ; and in many instances to deter- 

 mine the lines of evolution and of closest genetic relation- 

 ship among the congeneric forms of several of the leading 

 families. 



A few statistics will throw into strong light some at 

 least of the results of work during the last ten years. In 

 1884 Mr. Frederick W. True, Curator of Mammals in the 

 United States National Museum, published "A Provisional 

 List of the Mammals of North and Central America, and 

 the West Indies," 1 which fairly reflected the status of the 

 subject as then understood. Mr. True's List contained 378 

 species and 45 subspecies — a total of 423 recognized forms. 

 At the present time the number, as nearly as can con- 

 veniently be determined, is 732 species and 131 subspecies, 

 or a total of 863 — an increase exceeding 100 per cent., 

 mainly within the last five years. These additions include 

 8 new genera and about 12 new subgenera, while 10 groups 

 rated as subgenera in 1884 have been raised to the rank of 

 genera. On the other hand, however, several genera have 



1 Proc. U. S. Nat. Mus., VII., 1884, pp. 587-611. 



