arborescent vegetation as the prairie, and the interesting herbaceous plants found among them are only a portion of this flora. If their latitude generally is considered, they will be found incapable of possessing any important alpine vegetation, and even around their bases the snow will lie long and perseveringly on the ground.

RELATIONS.-Towards the south this region becomes gradually blended with the California and Chihuahua regions: an analogy with the pampas is established through the numerous gramineae; and with the northern regions by numerous cruciferæ and umbelliferæ, but those of the prairie are nearly all peculiar. The more interesting relation exists with the Steppes of Tartary, with which it has many points in common. The chief relation is that of affinity, the same genera being represented in both by different species; among these are artemisia, astragalus, thermopsis, sophora, glycyrrhiza, fritillaria, and diotis; and rheum is replaced by the analogous genus eriogonum.

VII. THE CHIHUAHUA REGION.

EXTENT. This name is pronounced Chi-wah-wah, and though the designation may appear somewhat novel, on the spot it is in extensive use; but almost equally little is known of the inhabitants, productions, and flora. On account of the barrenness of information respecting the latter, we can hardly more than indicate this region. Though an important portion of Mexico, it differs from it in many respects, and it is necessary to draw a strong line of demarcation, since the very name of Mexico is apt to convey to the mind of the botanist an association of characters certainly not pertaining to this part of the

republic. On the north it has the Prairie Region, sweeping round it even to the shores of the Gulf of Mexico, and meeting the Central America Region, thus completely separating it from the Atlantic. To the south, it ceases about the limit of the tropic, and on the west it has the Gulf of California, and the Rio Colorado.

PHYSICAL CHARACTERS.-Bold and mountainous.

CLIMATE.-Cold for the latitude, and apparently liable to vicissitudes.

FLORA. A thin forest occasionally covers part of the surface; at other times the vegetation is lowly, and there are broad spots entirely without any. The general character is rugged and austere, the land rising rapidly to a moderate elevation. Hence the climate is cool for the latitude, and the productions those of ten or fifteen degrees farther north. Steep precipices, and narrow passes abound, with the customary attendants of stern mountain scenery. Between the various ranges are fertile plains well adapted to agriculture, and the valleys are often very productive. Nitre and common salt are sometimes mixed copiously with the soil, depriving it of fertility. Compositæ are numerous; some are shrubby, but the tribe coreopsideæ more particularly prevails. Cacteæ are common; a few amaryllideæ, some showy and interesting spieces of labiatæ, and perhaps also of scrophularineæ and boragineæ.

RELATIONS.-Unknown.

VIII. THE CENTRAL AMERICA REGION.

EXTENT. The southern portion of the republic of Mexico, the whole of the Federal States of Central America, and a portion of New Granada; it thus extends

from the north tropic to the Gulf of St. Michael in the Bay of Panama, but sends a tongue to meet the Prairie Region, on the Mexican Sea. In elevation it attains. 4,500 feet, or the commencement of the cultivation of wheat; and the lowland cultivation ceases about this, which is inconsiderable for the latitude. Humboldt's warm region ceases at 600 metres, or 1,968 feet, but this is no limit to either the introduced or natural productions. PHYSICAL CHARACTERS. That part comprising the Mexican States, and the Upper States of Central America, rises rapidly from the shores of both seas to the elevated and peculiar table-lands of this part of America. Near both shores the soil is productive, if not abundant, but on leaving them the surface is usually rugged, and broken by huge masses of granite, porphyry, serpentine, or bazalt. This part of the region is composed almost entirely of these primary rocks, very few of secondary formation being known to exist. More to the south, and near the Isthmus of Panama, the country is far more even, the continuity of the Andes being completely broken, and in the vicinity of the lakes of Leon and Nicaragua is so even that no perceptible inequality can be noticed on traversing it, and the greatest difference is nowhere more than a few yards. Here the soil is rich and abundant, very productive, and capable of yielding many successive crops.

CLIMATE. The seasons are tropical, the rains commencing from April to June, according to the latitude, and lasting five months. During the rest of the year a hot sun and clear sky prevail.

FLORA. This region belongs to that variety of tropical vegetation where leguminosæ, &c., prevail, and hence we infer a certain aridity of soil and atmosphere. In this respect it yields greatly to the Oronoco Region, and

though Schouw combines them in his anomalous region of Cacteæ and Piperaceæ, I venture to separate them, after some practical acquaintance with both. Everywhere a forest exists, but it is usually a thin open forest: the trees are not distinguished either for stature or bulk, and there is a scarcity of undergrowth. In this latter respect there is a very remarkable difference between it and the NorthWest America Region. Nor is the variety of the forest trees great; hæmatoxylon campechianum is common; swietenia mahagoni and cedrela odorata are gregarious in the neighbourhood of the lakes, and very numerous as individuals. Mimoseæ are particularly abundant on the summits and sides of the hills, where there is any exposure, and the larger kinds convey a particularly airy and picturesque effect. Bauhinia, hymenæa, and schrankia, have several species. Ficus is also numerous, and from the manner of growth is highly distinctive; one species has a strange partiality for encasing the trunk of the chamærops palmetto, of which instances are numerous. Tropical endogenæ are not frequent, a few scitamineæ, musaceæ, and commelineæ appearing only in the wet season; passiflora, piper, melastoma, and ferns, are not common; cacteæ are spread over the region, but are not in such vigorous existence as elsewhere. Agave americana, salvia, hyptis, asclepias, viscum, loranthus, mikania, cordia, geraschanthus, heliotropium, tournefortia, quassia, datura, and solanum, are most frequent in the vegetation. Palmæ are almost comprised in chamærops palmetto, bactris minor, cocos nucifera, a licuala, and a phoenix.

RELATIONS. The extensive existence of ficus is a source of resemblance with the Indian forest. Cacteæ are very generally diffused, though never in any intensity, and through them a general character is maintained

VOL. II.

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with all America, subject to a warm or even temperate climate. It yields to the West India Region in the number, variety, and luxuriance of its vegetation, but its closest connexion is here; also to the South American tropical regions; and, considering its situation, is far from being rich or productive. Within it, it must be remembered, is an alpine region, and the celebrity the flora has enjoyed is shared between them. On the west coast, in 190 N. latitude, I saw a solitary tree of metrosideros glomulifera, which conveys an interesting relation with New Holland.

IX. THE MEXICO ALPINE REGION.

EXTENT. The Mexican mountains, above 4,500 feet, between 120 and 22° north latitude. This height is the lower boundary of the cultivation of wheat, and on the elevated plains it thrives admirably, when fed by regular irrigation. Since latitude within the tropics has such a trifling influence on climate, the difference in the alpine range of growth of vegetation on the mountains of Mexico and the Ecuador is surprising. In the latter, Quercus is not seen lower than 5,800 feet, but in Mexico it commences suddenly at 2,700 feet.

PHYSICAL CHARACTERS.- The distribution of the Mexican highlands is remarkable. Instead of rising gradually to a lofty sierra or ridge, as in the Andes of South America, the ascent suddenly ceases in a broad expanded tableland with an elevation from 4,000 to 8,000 feet. On this are placed many active volcanos, and it is likewise diversified by ridges of low hills and numerous lakes, whence mountain streams take their origin.

CLIMATE. The mean heat is perhaps lower than might be expected, and in the less elevated situations thin ice is