352 Louis Agassiz on the Natural 
association and combination of a variety of animals and plants within 
definite regions, forming peculiar fauna and flora. 
Considering the whole range of the temperate zone from east to 
west, we may divide it, in accordance with the prevailing physical 
features, into—1st, An Asiatic realm, embracing Mantchuria, Japan, 
China, Mongolia, and passing through Turkestan—into, 2d, The 
European realm, which includes Iran as well as Asia Minor, Meso- 
potamia, Northern Arabia, and Barbary, as well as Europe, pro- 
perly so called ; the western parts of Asia, and the northern parts 
of Africa being intimately connected by their geological structure 
with the southern parts of Europe; and, 3d, The North American 
realm, which extends as far north as the table-land of Mexico. 
With these qualifications we may proceed to consider the faune 
which characterize these three realms. But, before studying the 
organic characters of this zone, let us glance at its physical consti- 
tution. The most marked character of the temperate zone is found 
in the inequality of the four seasons, which give to the earth a pe- 
culiar aspect in different epochs of the year, and in the gradual, 
though more or less rapid, passage of these seasons into each other. 
The vegetation particularly undergoes marked modifications ; com- 
pletely arrested, or merely suspended, for a longer or shorter time, 
according to the proximity of the arctic or the tropical zone. We 
find it by turns in a prolonged lethargy, or in a state of energetic 
and sustained development. But in this respect there is a decided 
contrast between the cold and warm portions of the temperate zone. 
Though they are both characterized by the predominance of the 
same families of plants, and in particular by the presence of numerous 
species of the coniferous and amentaceous plants, yet the periodical 
sleep which deprives the middle latitudes of their verdure is more 
complete in the colder regions than in the warmer, which is already 
enriched by some southern forms of vegetation, and where a part 
of the trees remain green all the year. The succession of the sea- 
sons produces, moreover, such considerable changes in the climatic 
conditions in this zone, that all the animals belonging to it cannot 
sustain them equally well. Hence a large number of them migrate 
at different seasons from one extremity of the zone to the other, 
especially certain families of birds. It is known to all the world that 
the birds of Northern Europe and America leave their ungenial 
climate in the winter, seeking warmer regions as far as the Gulf of 
Mexico and the Mediterranean ; the shores of which, even those of 
the African coasts, make a part of the temperate zone. Analogous 
migrations take place also in the north of Asia. Such migrations 
are not, however, limited to the temperate zone; a number of 
species from the arctic regions go for the winter into the temperate 
zone, and the limits of their migrations may aid us in tracing the 
natural limits of the faunez, which thus link themselves to each other, 
as the human races are connected by civilization. 
