396 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
ORIGIN OF 1'YPES AND FAUNAS—GEOLOGIC EVIDENCE. 
In speaking of the Boreal and Sonoran origin of species and groups 
in the present paper, the term “ origin” is used exclusively in a sense 
intended to indicate present centers of distribution—not real or ancient 
centers of origin; for it must be borne in mind that the history of the 
inhabitants of the earth is not only a history of the successive appear- 
ance and disappearance of types now extinct, but a history of great 
movements—of vast migrations to and fro over the surface of the globe, 
and little is known of the real points of origin of our Boreal and Tropi- 
cal faunas and floras. The geologic evidence demonstrates that in the 
past large land areas have been many times joined together and many 
times rent asunder. The establishment of land continuity between 
areas previously disconnected has made it possible for new forms of 
animals and plants to obtain a footing and spread over regions pre- 
viously uninhabited by them,—often doubtless at the expense of the 
indigenous fauna and flora. Even great continents, as North and 
South America, have been more than once united and separated; and 
the last union of these continents is so recent we can distinctly trace at 
the present day the course and distribution of the intrusive forms. 
On the other hand, in comparatively recent times, multitudes of 
species and genera, and even families and higher groups, have sud- 
denly disappeared from large areas where they were formerly abundant, 
and some of them from the face of the earth, so that the fauna of the 
recent past compared with that of to-day presents some strange con- 
trasts. North Ameriea in Pleistocene times was inhabited by associa- 
tions of mammals not now living on this continent, but found in as far 
distant parts of the earth as Asia and South America; for horses, 
camels, and elephants then lived here with Mamas, tapirs, and capy- 
baras. With them were others now altogether extinct, as huge tigers, 
wolves, cave bears, the great Mastodon, the Megatherium, Megalonyx, 
Mylodon, and other gigantic sloths. 
GLACIAL EPOCH. 
The cause of this sudden extermination of dominant types is believed 
to have been the Glacial epoch, which is known to have driven species 
of animals and plants from the poles to the tropies, and which explains 
several of the otherwise inexplicable problems presented in the study 
of the past and present distribution of life. 
The snows at the beginning of the Glacial epoch fell upon a conti- 
nent of great forests—forests that gave shelter to multitudes of mam- 
mals and birds and other forms of life, a large proportion of which no 
longer inhabit America, and many of which do not exist in any part of 
the globe. 
During the period of maximum development the great glacier is be- 
lieved to have been not less than 8,000 feet in thickness in northern 
