examples, but they are too numerous and too little known to be used as an illustration here. “This is among the most remarkable instances of what I would call the arbitrary character of geographical distribution. Such facts cannot be explained by any theory of accidental dispersion, for the upper mountain rivulets, in which these great rivers take their rise, have no connection with each other; nor can any local circumstance explain the presence of some species in all the three basins, while others appear only in one, or perhaps in two, and are absent from the third, or the fact that certain species inhabiting the head-waters of these streams are never found in their lower course when the descent would seem so natural and so easy. In the absence of any positive explanation, we are left to assume that the distribution of animal life has primary laws as definite and precise as those which govern anything else in the system of the universe. “It is for the sake of investigations of this kind that I wish our party to divide, in order that we may cover as wide a ground as possible, and compare a greater number of the water-basins of Brazil. I wish the same to be done, as far as may be, for all the classes of Vertebrates, as well as for Mollusks, Articulates, and Radiates. As we have no special botanist in the party, we must be content to make a methodical collection of the most characteristic families of trees, such as the palms and tree ferns. A collection of the stems of these trees would be especially important as a guide to the identification of fossil woods. Much more is known of the geographical distribution of plants than of animals, however, and there is, therefore, less to be done that is new in that direction.