“Any one coming from the North to the Tropics, if he has been in the habit of observing the vegetation about him, even without having made botany a special study, is, in a measure, prepared to appreciate the resemblances and the differences between plants of the tropical and those of the temperate regions. An acquaintance with the Robinia (Locust-trees), for instance, or with the large shrub-like Lotus, and other woody Leguminosæ, will enable him to recognize the numerous representatives of that family, forming so large a part of the equatorial vegetation; and, even should he never have seen specimens of the Mimosa in gardens or hot-houses, their delicate, susceptible foliage will make them known to him; he cannot fail to be struck with the inexhaustible combinations and forms of their pinnate leaves, as well as with the variety in their tints of green, the diversity in their clusters of leaves and in their pods and seeds. But there are families with which he fancies himself equally familiar, the tropical representatives of which will never seem to him like old acquaintances. Thus the tree which furnishes the Indian rubber belongs to the Milk-weed family. Every one knows the Milk-weeds of the North, to be seen, as humble herbs, all along the roadsides, on the edges of our woods and in the sands of our beaches. Yet on the Amazons, the Euphorbiaceæ, so small and unobtrusive with us, assume the form of colossal trees, constituting a considerable part of its strange and luxuriant forest-growth. The giant of the Amazonian woods, whose majestic flat crown towers over all other trees, while its white trunk stands out in striking relief from the surrounding mass of green (the Sumauméra), is allied to our mallows. Some of the most characteristic trees of the river-shore belong to these two families. Our paleontologists who attempt to restore the forests of older geological times should keep in mind this fact of the striking contrasts presented under different latitudes by the same families. Of course the equatorial regions teem with plants and trees belonging to families either entirely unknown or but poorly represented in more temperate latitudes; and these distinct groups naturally arrest the attention of the botanist, and perhaps awaken his interest more than those with which he is already familiar under other forms. But, while these different families are recognized as distinct, and no doubt deserve to be considered by themselves as natural groups, I believe that much might be learned of the deeper relations of plants by studying, not only the representatives of the same families in different latitudes, such as the Mimosas and the Milk-weeds, but also what I may call botanical equivalents,—groups which balance each other in the different climatic zones. This idea is suggested to me by my zoölogical studies in the Amazons, which have led me to perceive new relations between the animals of the temperate and the tropical zone: it seems probable that