It is the belief of some very intelligent persons that the black population of the United States will gradually move towards the south-west, along with the cotton culture, and be finally absorbed in the mixed races of Central America, and that thus Slavery will cease. Mr. RIVES, of Virginia, advanced some such idea as this in the Senate of the United States, a year or so ago. But it seems clear to my mind that the white master will go as fast in that direction as the negro laborer, and wherever both are found together, one must be a slave. There is no spot on this continent where the negro can be put so as to be removed from the domination of the white man; no remote spot which the negro will reach unless the white man carries him thither. The colored race in this country can never exert their energies in an independent way; they are and must be under the overshadowing influence of a controlling race. What they may become in Africa, their native home, carrying with them to those shores, the vigorous elements imbibed during their apprenticeship of servitude here, other generations yet to come will know better than we of the present. The part which the African is to perform in the progress of civilization, and the development of the entire character of humanity, is a problem which has begun to attract the attention of enlightened men. Mr. KINMONT, whose discourses on the Natural History of Man show so large and comprehensive a mind, dwells with much interest upon the characteristics of the African race. A portion of his remarks, so beautiful, so humane, I can not but quote: