596 CENOZOIC ERA— AGE OF MAMMALS. 



so and were left stranded there. Thus is explained the remarkable fact 

 that Alpine plant-species in Europe are similar to and largely identical 

 with those in America ; and both with the present arctic species. This 

 indicates a former wide distribution of identical arctic species all over 

 Europe and America, and their subsequent retreat northward into 

 polar regions and upward into Alpine isolation. Grote has observed 

 a similar isolation of Labrador insect-species on Mount Washington, 

 and on the Colorado mountains.* 



There was probably a similar movement, to a less extent, of temper- 

 ate species. In the Taxodium of the Southern Atlantic and Gulf 

 swamps, and the Sequoias of California, we doubtless have examples of 

 species wide-spread in Miocene times, which have been destroyed by 

 these climatic changes, except in certain limited areas. 



But plants and lower species of animals are far less affected by 

 changes in physical conditions than are the higher species of animals. 

 This is shown by the wide range both in space and time of the former 

 as compared with the latter. Under these great changes and enforced 

 migrations, therefore, plants and invertebrate animal species maintained 

 their specific characters mostly unchanged, or but slightly changed. 

 But in the case of mammals destruction or change was inevitable. 

 Both took place — destruction of some and change of the remainder. 



In America some time during Quaternary, perhaps during the 

 period of northern subsidence, there was probably a broad land-con- 

 nection of North America with South America by the Caribbean Sea 

 region ; and certainly, as shown by the similarity of plants, with North- 

 ern Asia by the region between the Aleutian Isles and Bering Strait. 

 Thus migrations were not only enforced by climatic changes, but per- 

 mitted by geographical connections with adjacent continents. Also 

 the great Pliocene lake 1,000 miles long (p. 505) which separated West- 

 ern from Eastern North America was abolished, and migrations became 

 freer between the East and West. It is evident that from all these 

 causes mammalian faunas from widely-different regions were precipi- 

 tated upon each other, and struggled together for mastery. Large 

 numbers of species were destroyed, and the fittest only survived, and 

 these only under changed forms. It is quite possible that man came 

 to America with the Asiatic mammalian invasion. If so, his earliest 

 remains in America may be looked for on the Pacific coast. 



Of course, we use the word migrations in its widest sense, as change 

 of habitat of species as well as of individuals. In the case of plants 

 and many lower animals, the place of species only moved slowly from 

 generation to generation. In the case of mammals there was more de- 

 cided movement of individuals. 



* American Journal of Science, 1875, vol. x, p. 335. 



