CRITICAL PERIOD S'lN THE HISTORY OF THE EARTH. 527 



Southern Atlantic and Gulf coasts and in California were abundant in Green- 

 land. Evidently there could have been no Polar ice-cap at that time, and 

 •consequently no arctic species unless on mountain tops. During the latter 

 part of the Pliocene the temperature did not differ much from the present; 

 the Polar ice-cap had. therefore commenced to form, with its accompani- 

 ment of arctic species. With the coming on of the Glacial epoch, the polar 

 ice and arctic conditions crept slowly southward, pushing arctic species to 

 Middle Europe and Middle United States, and sub-arctic species to the shores 

 ■of the Mediterranean and the Gulf. With the return of more genial cli- 

 mate, arctic conditions went slowly northward again, and with them went 

 .arctic species slowly migrating, generation after generation, to their present 

 .arctic home. 



Similarly, molluscous shells migrated siowl}' southward and again north- 

 ward to their present position. But plants and some terrestrial inverte- 

 brates, such as insects, had an alternative which shells had not, namely, 

 that of seeking arctic conditions also upward on the tops of mountains. 

 Many did so, and were left stranded there\intil now. It is in this way that 

 we account for the otherwise inexplicable fact that Alpine species in Mid- 

 dle Europe are similar or even largely identical with those in the United 

 States, and also with those now living in arctic regions. These species were 

 wide-spread all over Europe and the United States in Glacial times ; and 

 while some of them afterward went northward to their present home, some 

 in each country sought arctic conditions in Alpine isolation. This explana- 

 tion, which has been long recognized for plants, has been recently applied 

 by Mr. Grote to arctic insects found on the top of Mt. Washington and 

 the mountains of Colorado.* 



Undoubtedly changes of climate during this time enforced similar mi- 

 grations among mammals also. But it is evident that while plants and in- 

 vertebrates might endure such modifications of climate and such enforced 

 migracions with little alteration of form, the more highly organized and 

 sensitive mammalian species must be either destroyed or else must undergo 

 more profound changes. Moreover, the opening of land connections between 

 regions previously isolated by barriers would be far more quickly taken 

 advantage of by mammals than by invertebrates and plants. The migra- 

 tions of plants are of necessity very slow, that is, from generation to gen- 

 eration. The migrations of mammals, too, so far as they are enforced by 

 changing climate, are of a similar kind; but the voluntary migrations of 

 mammals, permitted by removal of barriers, may take place much more rap- 

 idly, even in a few generations. This introduces another element of very 

 rapid local change, namely, the invasion of one fauna by another equally 

 well adapted to the environment, and the struggle for life between the in- 

 vaders and the autochthones. 



*This application, with reference to Mt. Washington and other arctic insects in Ameri- 

 ca, was previously made by Prof. A. S. Pa^'kard, Jr , in the Memoirs of the Boston Soc. Nat. 

 Hist., i. p. 256. 18C7. — Ed. American Naturalist. 



