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 faunz 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- 
-netts; 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 temporate 
regions of the earth at the present time, in former ages these 
regicns 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 by the incoming cold of the glacial period. 
