The tropical and boreal forests have their greatest extension from east to west or with the parallels of latitude, and are remarkable for their uniformity in general characteristics, the reason being that climatic conditions, and especially the temperature in summer, change less rapidly along east and west than along north and south lines. The Atlantic and Pacific forests, on the other hand, have their greatest extension across the parallels of latitude, and hence experience marked changes from locality to locality in both temperature and precipitation, and are characterized by conspicuous changes from one locality to another in the genera and species of trees of which they are composed. In each of the areas occupied by the major divisions of the encircling continental forest belt there are marked variations in elevation, which are accompanied by corresponding climatic changes, and hence by modifications in the forest growths. Of all portions of the continental forest belt, variation in elevation is least marked in the forests of Canada, and for this reason, in part, we there find the most uniform and most monotonous of all the forests of the continent. The influence of elevation, however, on climate and on both plant and animal life is greater for a given measure, as for 1,000 feet, in the torrid than in the cool or cold zone, for the reason that the possible range in climatic conditions is much greater at the south than at the north. _The Tropical Forest._--There are great areas in southern Mexico which are clothed with a typical tropical forest; while other similar forests cover nearly all of the lower portions of Central America, the larger or more rugged West India islands, and the southern extremity of Florida. Throughout this vast region, within the influence of the trade-winds and of the equatorial rains, the forests are luxuriant and beautiful, except on lowlands not adjacent to the windward side of mountains. The characteristic trees of the hot, humid lowlands extend up the mountain to an elevation of some 4,000 or 5,000 feet, where a change to the aspects familiar in the lowlands of the temperate zone begins, and palms give place to oaks and pines. Of the many features of the tropical forests which impress a traveller from colder regions, none excite greater wonder than the large number of vegetable species growing in close proximity. It has been said, and apparently the statement cannot be successfully challenged, that a greater variety of plants may be collected on 100 square yards of surface in the humid, tropical lowlands than can be found on 100 square miles in the forest of central Canada. It is probably safe to extend this striking contrast by saying that of the trees on a typical area in the tropical forest of the size mentioned, there are in many localities more species than in the whole of the subarctic forest.