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 virginica 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 



