l6 SMITHSONIAN* MISCELLANEOUS COLLECTIONS VOL. 59 



but Dr. C. H. Merriam has shown that the controlling- factor in the 

 distribution of recent animals and plants in North America is tem- 

 perature (Proc. Biol. Soc., Washington, Vol. 7, 1892, pp. 1-64, pi. 

 i.; Nat. Geog. Mag., Vol. 6, 1894, pp. 229-238, pis. 12-14). It 

 would seem, too, that if there was ever a time since the beginning 

 of the Pleistocene epoch when differences in amount of moisture 

 would determine the distribution of life in our country, that time 

 is at present. 



It cannot, of course, be denied that humidity and drought have 

 their influence on both animals and plants, and that at all times 

 during the Pleistocene the distribution of species of mammals was 

 modified thereby, although this was subordinate to more powerful 

 factors. 



It might, therefore, be expected that the Pleistocene faunas rec- 

 ognized here would be a northern and a southern, with, perhaps, 

 an intermediate one and, certainly, we must recognize the presence 

 of arctic, temperate, and subtropical elements in our lists of species 

 of Pleistocene mammals. However, since, through the influence of 

 four or five glacial ice-sheets, the arctic and cool-temperate animals 

 have again and again been driven southward upon the warmth- 

 loving species, and the latter have as many times swarmed far north, 

 and the remains of all that existed at each stage are often mingled 

 in practically the same deposits, it is impossible to base faunal di- 

 visions on predilections for a warm or a cold climate. 



A factor far more potent in determining modifications of faunas 

 than either moisture or heat is time. It never ceases to act and its 

 influence is inexorable. A great assemblage of animals is swept 

 away and another is put in the place of the old, and this is doomed 

 itself to disappear in its turn. The classification of the mammalian 

 faunas of the Pleistocene here proposed, is, therefore, based on the 

 changes supposed to have been wrought by this potent agency. 



Had one, however, lived during any of the interglacial stages, 

 one would doubtless, as now, have recognized a number of faunas 

 that occupied zones determined by temperature. 



