I know further, sir, that we have never dreamt of incorporating into our Union any but the Caucasian race—the free white race. To incorporate Mexico, would be the very first instance of the kind of incorporating an Indian race; for more than half of the Mexicans are Indians, and the other is composed chiefly of mixed tribes. I protest against such a union as that! Ours, sir, is the Government of a white race. The greatest misfortunes of Spanish America are to be traced to the fatal error of placing these colored races on an equality with the white race. That error destroyed the social arrangement which formed the basis of society. The Portuguese and ourselves have escaped—the Portuguese at least to some extent—and we are the only people on this continent which have made revolutions without being followed by anarchy. And yet it is professed and talked about to erect these Mexicans into a Territorial Government, and place them on an equality with the people of the United States. I protest utterly against such a project.

Sir, it is a remarkable fact, that in the whole history of man, as far as my knowledge extends, there is no instance whatever of any civilized colored races being found equal to the establishment of free popular government, although by far the largest portion of the human family is composed of these races. Are we to overlook this fact? Are we to associate with ourselves as equals, companions, and fellow-citizens, the Indians and mixed race of Mexico? Sir, I should consider such a thing as fatal to our institutions.

(http://www.autoadmit.com/thread.php?thread_id=3867126&forum_id=2#35208427)