I haven't read the entire treatise and the author certainly redeems himself in parts:

“Should we admit all that is claimed by certain writers concerning superior and inferior races, it would not follow that we must also accept the logic by which they undertake to show that inferiority has deprived the black race of all right to any position in the human family above that of serfdom. Rights cannot properly be made to depend on discrimination of this kind. In a justly-organized state, the most imperial intellect can have no more rights before the law than the humblest. There is a duty of the strong to the weak which requires something very different from the pretensions of mastership and the parade of superiority, and there is some great deficiency in the civilization of that race or community by which it is not properly recognized.

The logic of superiority that sets aside this duty would justify the enslavement of the weak, humble, and unenlightened classes, in every state where they exist, without regard to race, and show that neither slavery, serfdom, nor any condition of abasement under domination of privileged classes should be abolished anywhere.

But difference in faculty does not necessarily imply inferiority. We are accustomed to assume, without hesitation, that our race is of course superior to all other races that differ from it in faculty or development; in this, however, there may be quite as much egotism as reason.

On what grounds can we safely assume that any race, no matter how powerful it may seem to be, comprehends in the highest degree all the possibilities of human nature? Probably we shall find it necessary to revise our conception of what constitutes the superiority of a race.

Wonderful force to attack, subdue, and sway other peoples, distinguishes the faculty of our race, and this force is celebrated in song and romance. But is this really the highest and most admirable development of human nature? We cannot reflect seriously without feeling that there is something nobler, something more beautiful, something more richly fraught with blessings that increases the possibilities and heightens the charm of human existence, something that must necessarily revise our ideal of what is superior in peoples and races. Reason requires us to believe that each race and each distinct family of mankind has some peculiar gift of its own, in which it is superior to others, and that the All-wise Creator may have designed that each race and family shall bring its own peculiar contribution to the final completeness of civilization.

A race or family is not necessarily inferior to others because it comes into the history of civilization latest, nor superior because it appears there first. It was not our proud Aryan race that created the great civilizations of Arabia, Chaldea, and the Old Monarchy of Egypt. Our race was preceded in development by others, and it was in times quite modern that our own family of the race took its place among the foremost."

However, like the majority of those of his era (incl. Massey) his premise is jaded.