What are Douglass' observations about the class of mulatto slaves in relation to the south, and the American idea of the correctness of slavery?

From the text:

Every year brings with it multitudes of this class of slaves. It was doubtless in consequence of a knowledge of this fact, that one great statesman of the south predicted the downfall of slavery by the inevitable laws of population. Whether this prophecy is ever fulfilled or not, it is nevertheless plain that a very different-looking class of people are springing up at the south, and are now held in slavery, from those originally brought to this country from Africa; and if their increase do no other good, it will do away the force of the argument, that God cursed Ham, and therefore American slavery is right. If the lineal descendants of Ham are alone to be scripturally enslaved, it is certain that slavery at the south must soon become unscriptural; for thousands are ushered into the world, annually, who, like myself, owe their existence to white fathers, and those fathers most frequently their own masters.