GEOGRAPHICAL DISTRIBUTION. 663 



On inquiring into the origin of the North American 

 fauna, in the light of the geological history of the conti- 

 nent, we shall find, first, that immediately preceding the 

 glacial period, Arctic America was peopled by a flora and 

 fauna of which the larger proportion of the animals of the 

 continent north of latitude 30° are probably the descend- 

 ants ; and, second, that a number of species migrated north- 

 ward from the South American Continent. Now, when 

 the glacial period came in, the semi-tropical and warm tem- 

 perate animals of the northern two-thirds of the continent 

 were mostly swept out of existence ; a scanty arctic fauna 

 took their place ; as the ice melted and retreated to its pres- 

 ent limits, the present assemblage of temperate animals, 

 mostly modified descendants of those originally driven south, 

 migrated back again and colonized the region laid compara- 

 tively bare by the ice and cold of the glacial period. This 

 is an illustration of the sweeping extinctions, recolonizations, 

 and extended migrations of animals on our continent in 

 former times, by which the existing relations of faunae have 

 been brought about. Parallel events have occurred on the 

 Europeo-Asiatic Continent, and thus geological extinctions 

 and widespread migrations and recolonizations have taken 

 place ; and it is only in this way that the existing relations 

 in the geographical distribution of animals as well as plants 

 can be accounted for. 



It should also be observed that in the beginning of 

 things the continents were built up from north to south — 

 such has been at least the history of the North and South 

 American and the Europeo-Asiatic and African Conti- 

 nents ; and thus it would appear that north of the equator, 

 at least, animals slowly migrated southward, keeping pace, 

 as it were, with the growth and southward extension of the 

 grand land masses which appeared above the sea in the Pa- 

 leozoic Age. Hence, scanty as are the arctic and temperate 

 regions of the earth at the present time, in former ages these 

 regions were as prolific in life as the tropics now are, the 

 latter regions, now so vast, having all through the Tertiary 

 and Quaternary ages been undisturbed by great geological 

 revolutions, and meanwhile been colonized by emigrants 

 driven down bv the incoming cold of the glacial neriod. 



