Well had you read the speech in the first place, you would see that Stephens' subject was the confederate constitution and the improvements it had over the U.S. constitution. One of those improvements was the fact that it clearly places the black population in their proper place in southern society, slavery. And, Stephens adds, the question of slavery was the reason for the rebellion to begin with. I can post a link if you are interested in actually reading the speech.
I gave him a link to the speech.