GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 399 
mination of this period was not complete, and our continent has never 
regained its former warmth. Hence the expelled species were not per- 
mitted to advance more than a short distance into the region formerly 
occupied by them, and the tropical species have been held back and at 
the present day are not found except along the extreme southern con- 
fines of our territory. For example, pecearies in early Pleistocene times 
ranged northward over a large part of western America, while at pres- 
ent they are restricted to parts of Texas and Louisiana below the Red 
River of the South; and the capybaras, tapirs, and other tropical forms 
whose fossil remains have been found in many parts of the United 
States have not been able to return. The same is true of plants; for 
the palms, tree ferns, and numerous other tropical types that formerly 
ranged over much of our country, are now either altogether extinet or 
exist only in the tropics. 
Thellama—and many plants—now inhabiting the Andes may belooked 
upon as representing a class of cases in which Boreal forms were driven 
so far south that they actually reached the great mountain system of 
South America, and spread southward over its elevated plateaus and 
declivities to the extreme end of the continent in Patagonia and Terra 
del Fuego. This fact has been long recognized by botanists. 
The paleontologic history of the earth shows that many groups now 
unknown came into existence from preceding groups, gradually attained 
a maximum development, and as gradually passed away; but there are 
few records of breaks in the geologic series, or of disturbances of any 
kind from the earliest appearance of life to the present time, that have 
resulted in the destruction of so many types as the cold of the Glacial 
epoch. 
CAUSES CONTROLLING DISTRIBUTION. 
It is now pretty generally conceded that temperature and humidity 
are the chief factors governing the distribution of life, and that tem- 
perature is more potent than humidity. Illustrations of this law have 
been already given in contrasting the humid and arid elements of the 
several zones with the zone elements as limited by temperature, and it 
has been found in the case of mammals and birds that the effects of tem- 
perature, estimated numerically, are more than three times greater than 
the effects of humidity upon genera, and many times greater upon the 
higher groups. 
Authors differ as to the exact period during which temperature exerts 
the greatest influence, but there can be little doubt that for both ani- 
mals and plants it is the season of reproductive activity, and hence 
varies inversely with latitude and altitude. In high arctic latitudes 
this period is very brief, while in the humid tropics it seems to extend 
over nearly if not quite the whole year.* 
* This was nainied out by the author in North Am. Fauna, No. 3, September, 1890, 
pp. 26, 27 
