GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 367 
Tropical America is alone in the possession of true ant-eaters (Myr- 
mecophagide), sloths (Bradypodide), marmosets (Hapalide), armadillos 
(Dasypodide), and agouties (Dasyproctide). 
Africa is the home of many groups not known elsewhere. Among 
them are the giraffe, hippopotamus, Orycteropus, elephant shrews 
(Macroscelidide), Potomogale, and Chrysochlorida. 
Besides this class of cases, in which particular groups are restricted to 
particular countries, there is another class, in which the living represen- 
tativesof single groups exist in isolated colonies in widely separated parts 
of the world. Illustrations of this kind are furnished by the tapirs, which 
inhabit tropical America and the Malay Peninsula, but do not exist in 
intermediate lands; by the family Camelida, represented in South 
America by the Hamas and in parts of Eurasia by the true camels; 
and by a group of insectivorous mammals in which all the genera but 
one are restricted to Madagascar, the one exception (Solenodon) living 
in Cuba and Haiti. Examples of this sort are known as eases of dis- 
continuous distribution, and indicate that the ancestors of the animals 
in question formerly inhabited a vast extent of country; that some 
sort of land connection, however indirect, existed between the colonies 
now so widely separated, and that the surviving descendants of these 
groups are probably approaching extinction. 
The examples thus far cited relate to the disconnected land areas in 
the neighborhood of the equator or in the southern hemisphere, and 
their explanation is to be sought in the history of the past. In the 
northern hemisphere animals and plants in general have a much more 
extended distribution than in the southern, the majority of the larger 
groups being common to North America, Europe, and Asia, and the 
limits of their distribution are encountered in traveling in a north and 
south direction and are evidently the result of causes now in opera- 
tion. It is to this class of cases as presented on the North American 
continent that your attention is invited this evening. 
In passing from the tropics to the Aretic pole on the eastern side of 
America a number of distinet zones are crossed, the most conspicuous 
features of which are well known. In the plant world the palms, man- 
groves, mahogany, mastic, Jamaica dogwood, and cassias of the tropical 
coast districts are succeeded by the magnolias, papaws, sweet gums, 
blackberries, and persimmons of the Southern States. These give place 
gradually to the oaks, chestnuts, and hickories of the Middle States, 
and the latter to the groves of aspen, maple, and beech which reach 
the southern edge of the great coniferous forest of the north,—a forest 
of spruces and firs that stretches completely across the continent from 
Labrabor to Alaska. Beyond this forest is a treeless expanse whose 
distant shores are bathed in the icy waters of the Arctic Ocean. 
Concurrently with these changes in vegetation from the south north- 
ward occur equally marked differences in the mammals, birds, reptiles, 
