38 THE AGE OF MAMMALS 



used for purposes of convenience, that they express the truths of geographic 

 distribution in their grander outhnes but not in their details. 



In the shifting geographic and physiographic scenes of the enormously 

 protracted Age of Mammals the lines of division are now sharpened by 

 continental depression and separation, by isolation and insulation, and 

 again blended by continental elevation or ])y the formation or renewal 

 of migration routes, and by the opportunities, of which the mammals are 

 prone to take immediate advantage, to extend their geographic range by 

 migration. 



Thus we shall see in past times the Nearctic and Palaearctic now blend 

 into the Holarctic, and again separate more widely than at present. Again, 

 we shall see the Ethiopian, Holarctic, and Neotropical blending far more 

 closely than they do at present. In truth, there can he no iinijorm system 

 of past and present distribution, as the present writer once fondly hoped. 

 Every geologic Era, every Epoch, and even every Period has its own laws 

 of distribution. 



Migration Routes and Barriers 



Checking the tendencies of mammals to migrate are inconspicuous 

 barriers of many kinds in the living world and in animals themselves, as 

 well as the more obvious and conspicuous physiographic and climatic 

 barriers. Mountain chains, broad rivers, stretches of sea, of desert, or of 

 forest, which freely facilitate the migration of some kinds of mammals and 

 form absolute checks to the migration of other kinds, present barriers no 

 more formidable than those indirectly brought about by certain degrees 

 of moisture and of heat or cold. Thus ''temperature zones" form barriers 

 wherever they control the periods of reproduction.^ Moisture may infest 

 a country with insect pests, such as flies, ticks, and mosquitoes, which form 

 as absolute a boundary to migration as a broad mass of water. Animals 

 which are so specialized as to be restricted or confined to certain habitats 

 may be said to impose their own barriers upon themselves. 



Most barriers are ultimately traceable to changes in the greater and 

 lesser land masses and their connections, caused by the elevation or sub- 

 sidence of various parts of the earth. These changes both ■ make and 

 destroy land routes, and cause a series of changes in all physical conditions 

 of climate, moisture or desiccation, of temperature, or heat and cold. The 

 succession of faunas during the Miocene and Pliocene periods in Euro^^e is 

 positive proof of a succession of environments. 



In this connection it is highly interesting to compare the stable con- 

 tinents of North America and Africa, which show relatively slight fluctua- 

 tions of land and sea level during the Age of Mammals, with the highly 

 unstable continent of Europe. During a considerable part of the Age of 



' Merriam, C. H., The Geographical Distribution of Life in North America with Special 

 Reference to the Mammalia. Proc. Biol. Soc. Washington, Vol. VII, April 13, 1892. 



