FLORA OF CENTRAL AMERICA 
247 
are either absent or very rare in the western United States of 
North America, while abundant in the eastern States. Some 
of these bear an impress of their antiquity in their wide and 
discontinuous range. Now Mr. Hemsley enumerates the fol¬ 
lowing genera of trees as occurring in southern Mexico and in 
the Atlantic States of North America, though they are absent 
from the Pacific forests of the western United States: 
Magnolia, Asimina, Tilia, Robinia, Liquidambar, Ilex, Dio- 
spyros, Bumelia, Ulmus, Celtis, Morus, Ostrya, Carpinus and 
Carya. Even species of plants from southern Mexico and the 
Atlantic States of North America are sometimes identical, 
such as Liquidambar styraciflua, Ostrya virginioa and Car¬ 
pinus americana. And yet only four out of the fourteen 
genera referred to extend even to northern Mexico. Of some 
of these we possess fossil evidence that they lived in Europe 
already in early Tertiary times, and we may safely assume that 
the whole group is of great antiquity. The flora of Guatemala 
is essentially of the same composition, according to Mr. 
Hemsley, as that of southern Mexico, though apparently less 
rich in specific diversity. Some of the trees just alluded 
to, such as limes (Tilia) and elms (Ulmus), are unknown 
in Guatemala; others, for instance sweet gums (Liqui¬ 
dambar), mulberries (Morus), lever-wood (Ostrya) and horn¬ 
beams (Carpinus), occur in that country. The southern floral 
province of Mr. Hemsley comprises Nicaragua, Costa Rica 
and Panama; and, as might be expected, these countries ex¬ 
hibit a much closer relationship with the South American 
tropical flora than Guatemala or Mexico do. The endemic 
generic element of the whole of Mexico and Central America 
is rather inconspicuous, but the southern floral province is by 
far the poorest of the three into which the region has been 
divided. One of the most curious features in the constitution 
of the flora of Mexico is one which I have already briefly 
referred to, namely, the presence there and in the extreme 
south of South America of certain northern genera of plants 
which are absent or only represented in a few scattered dis¬ 
tricts in the intermediate region. Mr. Hemsley assumes that 
such plants have spread southward in remote times. There 
are likewise genera of distinctly southern origin with a simi¬ 
larly discontinuous range in a northward direction. I need 
