328 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Elephants, Mastodons, Bhinoceroses, and Horses, which, though extinct 

 in America, have living representatives in the tropies of the so-called 

 "Old World", to say nothing of the evidence afforded by the remains of 

 still earlier types of arctopolitan range. The succeeding epochs of cold 

 caused extensive migrations of some groups and'the extinction of others; 

 with the diverse climatic conditions subsequently characterizing high 

 and low latitudes came the more pronounced differentiation of faunae, 

 and the development, doubtless, of many new types adapted to the 

 changed conditions of life the development of boreal types from a warm- 

 temperate or semi-tropical stock. The accepted theories respecting the 

 modification of type with change in conditions of environment changes 

 necessarily due mainly to climatic influences render it certain that 

 if animals are so far under the control of circumstances dependent upon 

 climate, and emphatically upon temperature, as to be either exterminated 

 or greatly modified by them, the same influences must govern their geo- 

 graphical distribution. 



Recent discoveries respecting the mammalia inhabiting North Amer- 

 ica during the Tertiary period have shown that many of the leading 

 types of mammals including not only those above named, but also 

 many others now found only in the eastern hemisphere, originated in 

 North America, and migrated thence to Asia, Europe, and even Africa, 

 either as somewhat generalized types, or after they had nearly reached 

 their present degree of differentiation 5 in short, so far as mammalian 

 life is concerned, that America is the "Old World" from which the 

 so-called "Old World" has been mainly peopled. The present genetic 

 convergence of life about the northern pole seems to show that not only 

 has there been here a comparatively free intercommunication, but that 

 the mammalian life now existing there has lived there for a long period 

 under similar conditions of environment ; and that these conditions are 

 unfavorable, in consequence of a comparatively low temperature, to rapid 

 change of form or structure. 



This is shown not only by the great diversity of life met with in the 

 intertropical regions, as compared with the uniformity met with in the 

 semi-frigid regions (equal areas being, of course, compared), but by the 

 coincident occurrence of a simple, homogeneous arctic marine fauna, 

 with the low temperature over the sea-floor far to the southward of where 

 such forms occur in the warmer surface and shore-waters. The intimate 

 relation between temperature and the distribution of life is most forci- 

 bly shown by the existence under the same parallel of latitude of diverse 

 faunae not only at different elevations above the sea on mountain-slopes, 

 but at different depths beneath the surface of the ocean, where the 

 several faunae are characterized not only by the presence of different 

 species, but by the prevalence of different genera, and even families. In 

 fact, it is to me a matter of surprise that, with our present knowledge 

 of the subject, any naturalist of note should assume that temperature 

 has nothing to do with the circumscription of faunae, or that any law 



